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A climatology of mesoscale convective systems over Europe using satellite infrared imagery. II: Characteristics of European mesoscale convective systems
Quarterly Journal of the Royal Meteorological Society
Abstract
An automated method for mesoscale convective system (MCS) identification and tracking (described in part I) is applied in order to derive a sound European MCS database using Meteosat infrared channel (IR10.8) images centred over Europe, the western Mediterranean and north Africa. The database covers five warm seasons, from April to September, for the years 1993 to 1997 and includes more than 6000 MCSs reaching at least an area of 10 000 km ² .
First results of the derived climatology of European MCSs are presented. They mainly address the MCS geographical location, general MCS characteristics (maximum extent, eccentricity, duration) and the diurnal cycle of the MCS.
MCSs are shown to be mainly continental, but some MCS triggering is observed during the second half of August and September over the western Mediterranean Sea. Furthermore, MCS triggering is strongly correlated with orography and local maxima of MCS triggering are observed near all mountain ranges. Regions near the Alps which favour MCS triggering are described in detail. The monthly distributions of occurrence of warm‐season European MCSs are also presented.
Distributions of maximum extent, eccentricity, direction of propagation, life duration and triggering and dissipation times are also derived. On average, a theoretical ‘typical European MCS’ moves to the east‐north‐east, triggers near 3 p.m. Local Solar Time (LST), lasts around 5.5 hours and dissipates near 9 p.m. LST. It typically has an eccentricity at the time of maximum extent of 0.53.
The diurnal cycle is also studied and proved to be in phase with the diurnal radiative heating, except for around 20% of the MCSs.
A detailed analysis of the maximum‐extent distribution shows that it can be fitted by a log‐normal distribution which leads to an average value of the MCS maximum extent of around 9000 km ² . This approximation is statistically meaningful and it is independent of the choice to study only MCSs reaching at least 10 000 km ² .
Finally, a section of this article is devoted to indirect verifications of the discrimination method (see part I) for the whole geographical domain. Copyright © 2002 Royal Meteorological Society
... Choosing 0600 UTC as starting time allows us to include the full climatological life cycle of the majority of MCSs that occur, as they tend to develop between late morning and late afternoon and dissipate during the evening, sometimes even in the second half of the night (Morel & Senesi, 2002b). Some MCSs may initiate before the beginning of the simulation or dissipate after the end. ...
... This section focuses on characterising the spatio-temporal distribution and life cycles of the MCSs that have been tracked and selected in both the AROME simulations and observations. In this context, we will refer regularly to the European climatologies produced by Morel and Senesi (2002b) and Da Silva and Haerter (2023) from observed satellite images only, respectively the BT measured by the SEVIRI radiometer on board MSG and the Integrated Multi-Satellite Retrievals (IMERG) precipitation product. ...
... There is good agreement between the observed and simulated outputs in terms of density of tracks that cover the entire study domain. The majority of tracks, associated with Std-MCSs (bluish lines), are located in a wide continental band oriented from southwest to northeast France, in rough agreement with the density map of Morel and Senesi (2002b). This preferred direction is consistent with the southwestern dynamical synoptic flow that supports deep convection in midlatitudes by advecting low-level subtropical warm and moist air ahead of an approaching front or cut-off low (Trapp, 2013). ...
An object‐based methodology is used to study the mesoscale convective systems (MCSs) developing in France and the interactions they have with their environment during their life cycle. MCSs are detected from images of radar reflectivity and satellite 10.8‐μm brightness temperature with a deep‐learning segmentation method. The ability of the Application of Research to Operations at Mesoscale (AROME‐France) numerical weather prediction model in representing MCSs is first assessed by comparing statistically the attributes of objects tracked in simulated and observed images of severe convective situations between 2018 and 2022. The MCS environment is then studied by averaging ambient variables simulated by AROME‐France within a 100‐km wide area and calculating original ring‐shaped composite maps adapted to the geometry of each object. Based on this methodology, it is shown that MCSs generally develop in unstable areas with low‐level convergence and strong wind aloft. Ambient instability correlates best with MCS severity and maintenance. It undergoes the greatest decrease throughout the life cycle, in relation with low‐level cooling and drying. Further comparisons are made between quasi‐stationary Mediterranean and other MCSs, and particular attention is paid to the impact of the system on its own environment. We finally emphasise three relevant predictors for MCS nowcasting in France: (1) the trend in system size and convective activity, (2) the ambient instability, and (3) the midlevel wind to anticipate MCS movement.
... Climatologies focusing on MCS occurrence have been widely created for the United States (Geerts 1998;Ashley 2018a,b, 2019;Cheeks et al. 2020) and in lesser extent for parts of Mexico (Ramos-Pérez et al. 2022), subtropical South America (Salio et al. 2007), China (Yang et al. 2015), and Europe (Morel and Senesi 2002a;Rigo and Llasat 2007;Kolios and Feidas 2010;Lewis and Gray 2010;Surowiecki and Taszarek 2020). Some parts of literature also addressed climatologies of derechoes, which are a particularly dangerous type of a QLCS (Johns and Hirt 1987;Bentley and Mote 1998;Bentley and Sparks 2003;Celiński-Mysław and Matuszko 2014;Guastini and Bosart 2016;Gatzen et al. 2020;Fery and Faranda 2023;Squitieri et al. 2023a,b). ...
... While MCS climatologies can be constructed using satellite data, which offers good spatiotemporal homogeneity (Morel and Senesi 2002a;Kolios and Feidas 2010;Lewis and Gray 2010), the same does not apply to QLCS events, which require radar data for their proper identification (Jirak et al. 2003). Considerable improvements in terms of coverage and quality of radar data, observed in recent years across Europe, have opened an opportunity to use such data in studying convective hazards. ...
... France, up to northern Germany, Hungary, and portions of neighboring countries (Fig. 6a). The spatial distribution of marginal and moderate QLCSs (Fig. 6b,c) was similar to all QLCSs (albeit with lower frequencies), with a spatial frequency maximum in southern France, Romania, and parts of the Mediterranean coastline, consistent with Kolios and Feidas (2010) and Morel and Senesi (2002a). A derecho corridor (1-2 cases per year) appears across portions of France, eastward to Czechia and Poland, with a derecho frequency maximum noted in Germany (Fig. 6d), similarly found by Celinski-Myslaw and Matuszko (2014), Gatzen et al. (2020), and Surowiecki and Taszarek (2020). ...
In this work, we use 8 years (2014–2021) of OPERA radar data, ESWD severe weather reports, and ATDnet lightning detection data to create a climatology of quasi-linear convective systems (QLCS) across Europe. In the first step, 15-minute radar scans were used to identify 1475 QLCS polygons. Severe weather reports, lightning data, and morphological properties were used to classify QLCSs according to their intensity into 1151 marginal (78.0%), 272 moderate (18.5%), and 52 derecho (3.5%) events. Manual evaluation led to the recognition of QLCS morphological and precipitation archetypes, areal extent, duration, speed, forward motion, width, length, accompanying hazards, injuries and fatalities. Results indicate that QLCSs are the most frequent during summer in central Europe, while in southern Europe their occurrence is extended to late autumn. A bow echo feature occurred in around 29% of QLCS cases, while a mesoscale convective vortex in almost 9%. Among precipitation modes, trailing and embedded stratiform types accounted for around 50% of QLCSs. The most frequent hazard accompanying QLCSs was lightning (taking up on average 94.4% of the area impacted by QLCS), followed by severe winds gusts (7.9%), excessive precipitation (6.1%), large hail (2.9%), and tornadoes (0.5%). Derechoes had the largest coverage of severe wind reports (49.8%), while back-building QLCSs were the most prone for excessive precipitation events (13.5%). QLCSs caused 104 fatalities and 886 injuries. Severe wind gusts were responsible for 87.6% of fatalities and 73.6% of injuries. Nearly half of all fatalities and injuries were associated with only the 10 most impactful QLCS events, mostly warm-season derechoes.
... Another quantity derived from dynamical systems theory is the persistence of a given configuration (Faranda et al., 2017a). Persistence provides an estimate of the number of days we are likely to encounter a map that is an analogue of the one under consideration (Moloney et al., 2019). As with Q and D, we compute the two values of persistence for the extreme event in the factual and counterfactual world and the corresponding distributions of the persistence for the analogues. ...
... Notably, the northeast of France exhibits the highest frequency of events, peaking at 0.65 derechos per year, while Brittany, the westernmost part of France, and Côte d'Azur in the southeast record no events. After closely examining the spatial distribution of observed derecho frequencies in Fig. 1b, we find a partial alignment with the European climatology of lightning in summer, as presented in Taszarek et al. (2020a) and with the density map of MCSs in Europe from Morel and Senesi (2002). Notably, high activity is observed near mountain ranges such as the Pyrenees and the Alps, extending roughly to the southern and eastern regions of France, with relatively less activity in Brittany and Normandy in the northwest. ...
... If this discrepancy were to be validated with larger sample sizes, it could potentially be attributed to the proximity of the Mediterranean Sea. Its warm waters continue to serve as abundant sources of moisture and heat late in the season, fostering significant instability and the development of severe convective storms (Taszarek et al., 2020a;Morel and Senesi, 2002). Notably, southern France is renowned for experiencing extreme convective rainfall episodes during the fall season (Fumière et al., 2020;Ribes et al., 2019;Taszarek et al., 2019). ...
Derechos are severe convective storms known for producing widespread damaging winds. While less frequent than in the United States of America (USA), derechos also occur in Europe. The notable European event on 18 August 2022 exhibited gusts exceeding 200 kmh-1, spanning 1500 km in 12 h. This study presents a first climatology of warm-season derechos in France, identifying 38 events between 2000 and 2022. Typically associated with a southwesterly mid-level circulation, warm-season derechos in France generally initiate in the afternoon and exhibit peak activity in July, with comparable frequencies in June and August. Predominantly impacting the northeast of France, these events exhibit a maximum observed frequency of 0.65 events per year, on average, within a 200 km by 200 km square region. These characteristics are similar to those observed in Germany, with notable differences seen in the USA, where frequencies can attain significantly higher values. The study also examines synoptic and environmental changes linked with analogues of the 500 hPa geopotential height patterns associated with past warm-season derechos, comparing analogues from a relatively distant past (1950–1980) with a recent period (1992–2022). For most events, a notable increase in convective available potential energy (CAPE) is observed, aligning with trends identified in previous studies for southern Europe. However, no consistent change in 0–6 km vertical wind shear is observed in the recent period. These environmental shifts align with higher near-surface temperatures, altered mid-level atmospheric flow patterns and often increased rainfall. The role of anthropogenic climate change in these changes remains uncertain, given potential influences of natural variability factors such as the El Niño–Southern Oscillation (ENSO) or the Atlantic Multidecadal Oscillation (AMO).
... Several previous studies that examined MCSs over Europe focus on a restricted part of Europe (García-Herrera et al., 2005;Punkka & Bister, 2015;Rigo et al., 2019;Surowiecki & Taszarek, 2020), investigate only one season (Kolios & Feidas, 2010;Morel & Senesi, 2002), or use a limited time record to assess climatological properties (García-Herrera et al., 2005;Kolios & Feidas, 2010;Morel & Senesi, 2002). Using 5 years of infrared (IR) satellite data, Morel and Senesi (2002) found that summer MCSs (April-September) are more common over land than sea and are triggered near mountainous areas (Pyrenees, Alps, Carpathians) during the afternoon, a general characteristic also found for the CONUS. ...
... Several previous studies that examined MCSs over Europe focus on a restricted part of Europe (García-Herrera et al., 2005;Punkka & Bister, 2015;Rigo et al., 2019;Surowiecki & Taszarek, 2020), investigate only one season (Kolios & Feidas, 2010;Morel & Senesi, 2002), or use a limited time record to assess climatological properties (García-Herrera et al., 2005;Kolios & Feidas, 2010;Morel & Senesi, 2002). Using 5 years of infrared (IR) satellite data, Morel and Senesi (2002) found that summer MCSs (April-September) are more common over land than sea and are triggered near mountainous areas (Pyrenees, Alps, Carpathians) during the afternoon, a general characteristic also found for the CONUS. ...
... Several previous studies that examined MCSs over Europe focus on a restricted part of Europe (García-Herrera et al., 2005;Punkka & Bister, 2015;Rigo et al., 2019;Surowiecki & Taszarek, 2020), investigate only one season (Kolios & Feidas, 2010;Morel & Senesi, 2002), or use a limited time record to assess climatological properties (García-Herrera et al., 2005;Kolios & Feidas, 2010;Morel & Senesi, 2002). Using 5 years of infrared (IR) satellite data, Morel and Senesi (2002) found that summer MCSs (April-September) are more common over land than sea and are triggered near mountainous areas (Pyrenees, Alps, Carpathians) during the afternoon, a general characteristic also found for the CONUS. ...
Mesoscale convective systems (MCSs) are common over Europe and can produce severe weather, including extreme precipitation that leads to flash floods. The few studies analyzing the climatological characteristics of MCSs over Europe are either focusing on only few years of data or on limited subareas. Using the recent Integrated MultisatellitE Retrievals for Global Precipitation Measurement (IMERG) satellite precipitation climatology, we identify and track MCSs for 16 years over Europe. The tracking algorithm relies on the overlap of precipitation features between consecutive time steps and, unlike previous studies, uses lightning data to distinguish convective from stratiform rain patches, which can reduce potential identification errors. We analyze this new European MCS climatology to characterize MCS precipitation properties and conclude the following results: MCSs overall occur most frequently over the Mediterranean and Atlantic during fall and winter, whereas during summer, they concentrate over the continent. Typically, more than a third of seasonal precipitation can be attributed to MCSs, and their contribution to extreme precipitation is even greater, often exceeding 60%. MCSs over the continent display a clear diurnal cycle peak during the afternoon, and some continental areas also show a second, nocturnal peak. The MCS diurnal cycle for coastal and oceanic regions is more variable. We find that the spatiotemporal distribution of MCS precipitation can be attributed to specific environmental variables, namely (sea) surface temperature, fronts occurrence and convective instability. While inland MCS precipitation is mostly constrained by thermodynamics, for the coastal MCSs the atmospheric dynamics plays an important role as well.
... Several previous studies that examined MCSs over Europe focus on a restricted part of Europe (García-Herrera et al., 2005;Punkka & Bister, 2015;Rigo et al., 2019;Surowiecki & Taszarek, 2020), investigate only one season (Kolios & Feidas, 2010;Morel & Senesi, 2002), or use a limited time record to assess climatological properties (García-Herrera et al., 2005;Kolios & Feidas, 2010;Morel & Senesi, 2002). Using 5 years of infrared (IR) satellite data, Morel and Senesi (2002) found that summer MCSs (April-September) are more common over land than sea and are triggered near mountainous areas (Pyrenees, Alps, Carpathians) during the afternoon, a general characteristic also found for the CONUS. ...
... Several previous studies that examined MCSs over Europe focus on a restricted part of Europe (García-Herrera et al., 2005;Punkka & Bister, 2015;Rigo et al., 2019;Surowiecki & Taszarek, 2020), investigate only one season (Kolios & Feidas, 2010;Morel & Senesi, 2002), or use a limited time record to assess climatological properties (García-Herrera et al., 2005;Kolios & Feidas, 2010;Morel & Senesi, 2002). Using 5 years of infrared (IR) satellite data, Morel and Senesi (2002) found that summer MCSs (April-September) are more common over land than sea and are triggered near mountainous areas (Pyrenees, Alps, Carpathians) during the afternoon, a general characteristic also found for the CONUS. ...
... Several previous studies that examined MCSs over Europe focus on a restricted part of Europe (García-Herrera et al., 2005;Punkka & Bister, 2015;Rigo et al., 2019;Surowiecki & Taszarek, 2020), investigate only one season (Kolios & Feidas, 2010;Morel & Senesi, 2002), or use a limited time record to assess climatological properties (García-Herrera et al., 2005;Kolios & Feidas, 2010;Morel & Senesi, 2002). Using 5 years of infrared (IR) satellite data, Morel and Senesi (2002) found that summer MCSs (April-September) are more common over land than sea and are triggered near mountainous areas (Pyrenees, Alps, Carpathians) during the afternoon, a general characteristic also found for the CONUS. ...
Mesoscale Convective Systems (MCS) are common over Europe and can produce severe weather, including extreme precipitation, which can lead to flash floods.The few studies analyzing the climatological characteristics of MCS over Europe are either based on only few years of data or focus on limited sub-areas.Using the recent Integrated Multi-satellitE Retrievals for Global Precipitation Measurement (IMERG) satellite precipitation climatology, we identify and track MCS for 16 years over Europe.We devise a spatial filter and track cells according to the overlap of filtered rain patches between consecutive time steps.By fitting an ellipse to these patches, we determine their overall shape and orientation.To distinguish convective rain patches we condition on lightning data, thus reducing potential identification errors.We analyze this new European MCS climatology to characterize MCS rainfall properties:MCS overall occur most frequently over the Mediterranean and Atlantic during fall and winter, whereas during summer, they concentrate over the continent.Typically, more than half of seasonal precipitation can be attributed to MCS, andtheir contribution to extreme precipitation is even greater, often exceeding 70\%.MCS over the continent display a clear diurnal cycle peaking during the afternoon, and some continental areas even show a second, nocturnal peak.The MCS diurnal cycle for coastal and oceanic regions is more variable.Selecting sub-areas, we find that the spatio-temporal distribution of MCS precipitation throughout the year can be well explained by the spatio-temporal distribution of specific environmental variables, namely (sea) surface temperature, fronts occurrence and convective instability.
... Here tracking is done using precipitation, while outgoing longwave radiation (OLR) is another well used method for tracking and detection of especially MCSs (e.g. Morel and Senesi 2002;Crook et al. 2019). OLR is smoother in space which would be likely to reduce the difference in the number of tracked events between the two models, but OLR tracking gives problems with false alarms as OLR is not a direct measurement of precipitation. ...
... This confirms that similar events are sampled in the two models when selecting by Maximum Area, whereas this is not the case when selecting by Maximum Intensity. Analysing characteristics of MCSs over Europe, Morel and Senesi (2002) found a larger density of MCSs over land than sea, with a clear concentration in the eastern part of the case area. This suggests that the representation of tracking location is closer to observations in the CPM2 dataset compared to the RCM12. ...
... This suggests that the representation of tracking location is closer to observations in the CPM2 dataset compared to the RCM12. MCSs in Northern Europe were found to have the highest frequency between May and August (Morel and Senesi 2002) which is in agreement with the seasonal distribution in both models, although more apparent in the CPM2. Morel and Senesi (2002) define MCSs as events reaching an area above 10,000km 2 , while an areal threshold of 288km 2 (excluding single cell events) is used in this study with no attempt to distinguish between MCSs and non-MCSs. ...
High resolution regional climate models are needed to understand how climate change will impact extreme precipitation. Current state-of-the-art climate models are Convection Permitting Models (CPMs) at kilometre scale grid-spacing. CPMs are often used together with convective parameterised Regional Climate Models (RCMs) due to high computational costs of CPMs. This study compares the representation of extreme precipitation events between a 12 km resolution RCM and a 2.2 km resolution CPM. Precipitation events are tracked in both models, and extreme events, identified by peak intensity, are analysed in a Northern European case area. Extreme event tracks show large differences in both location and movement patterns between the CPM and RCM. This indicates that different event types are sampled in the two models, with differences extending to much larger scales. We visualise event-development using area-intensity evolution diagrams. This reveals that for the 100 most extreme events, the RCM data is likely dominated by physically implausible events, so called ‘grid-point storms’, with unrealistically high intensities. For the 1000 and 10,000 most extreme events, intensities are higher for CPM events, while areas are larger for RCM extreme events. Sampling extreme events by season shows that differences between RCM and CPM in intensity and area in the top 100 extreme events are largest in autumn and winter, while for the top 1000 and top 10,000 events differences are largest in summer. Overall this study indicates that extreme precipitation projections from traditional coarse resolution RCMs need to be used with caution, due to the possible influence of grid-point storms.
... Not only do MCSs contribute to comparable fractional total rainfall as those in the tropics, particularly in the Central United States (Feng et al., 2016Fritsch et al., 1986;Haberlie & Ashley, 2019), but they also produce hazardous weather including flooding, large hail, tornadoes, and damaging winds (Ashley & Mote, 2005;Doswell et al., 1996;Hu et al., 2020;B. T. Smith et al., 2012;Stevenson & Schumacher, 2014 Regional studies have documented the characteristics of MCSs in China (Chen et al., 2019; X. R. Yang et al., 2015), central and south America (Machado et al., 1998), west Africa (Klein et al., 2018), and Europe (Morel & Senesi, 2002). ...
... In this study, we have further improved the approach by adding time-dependent tracking to the combination of T b and precipitation. In midlatitudes, the IR-only method has been useful for MCSs during the warm season in the Americas (Fritsch et al., 1986;Jirak et al., 2003;Machado et al., 1998), China (Chen et al., 2019; X. R. Yang et al., 2015) and Europe (Morel & Senesi, 2002). Outside of summer when baroclinic forcing is much stronger in the midlatitudes, large and long-lived cloud systems viewed from IR data alone are often insufficient to identify MCS, as many synoptically forced clouds not associated with MCS bear similarity with large and long-lived MCSs in IR signatures (i.e., large and long-lived). ...
... MCSs account for the smallest total rainfall during the winter season in the respective hemispheres, suggesting non-deep convective synoptic systems are more important in winter when the jet stream moves equatorward and convective instability needed to support MCSs decreases without surface heating and ample moisture. In Europe, while MCSs are less common, they tend to occur more often over the continent in summer and over the Mediterranean Sea during fall and winter, consistent with a previous study that finds a peak in late August to September (Morel & Senesi, 2002). The strong seasonal contrast of MCS occurrences found in this study is quite consistent with previous works that use similar T b and precipitation data to identify mature MCSs (YH10), and those that use spaceborne radar observations to depict organized convection with deep and/or wide convective features and broad stratiform rain regions (Houze et al., 2015). ...
A new methodology is developed to construct a global (60°S–60°N) long‐term (2000–2019) high‐resolution (∼10‐km h) mesoscale convective system (MCS) database by tracking MCS jointly using geostationary satellite infrared brightness temperature (Tb) and precipitation feature (PF) characteristics from the Integrated Multi‐satellitE Retrievals for GPM precipitation data sets. Independent validation shows that the satellite‐based MCS data set is able to reproduce important MCS statistics derived from ground‐based radar network observations in the United States and China. We show that by carefully considering key PF characteristics in addition to Tb signatures, the new method significantly improves upon previous Tb‐only methods in detecting MCSs in the midlatitudes for all seasons. Results show that MCSs account for over 50% of annual total rainfall across most of the tropical belt and in selected regions of the midlatitudes, with a strong seasonality over many regions of the globe. The tracking database allows Lagrangian aspects such as MCS lifetime and translational speed and direction to be analyzed. The longest‐lived MCSs preferentially occur over the subtropical oceans. The land MCSs have higher cloud‐tops associated with more intense convection, and oceanic MCSs have much higher rainfall production. While MCSs are observed in many regions of the globe, there are fundamental differences in their dynamic and thermodynamic structures that warrant a better understanding of processes that control their evolution. This global database provides significant opportunities for observational and modeling studies of MCSs, their characteristics, and roles in regional and global water and energy cycles, as well as their hydrologic and other impacts.
... Much current understanding of how convection organized on the mesoscale should adjust to surface warming comes from idealized simulations or regionally limited data (e.g., Anderson & Arritt, 2001;Feng et al., 2018;Laurent et al., 2002;Morel & Senesi, 2002;Muller, 2013;Prein et al., 2017). Models are natural tools to predict changes in weather system properties, but the combination of large overall extent and smaller embedded elements makes convective organization particularly challenging to parameterize and forecast. ...
... This clustering is done both with a T IR threshold of 245 K or less noted above and also with a more stringent value of 220 K or less. This approach was first documented in Machado et al. (1998) and has been widely used and developed since (e.g., Laurent et al., 2002;Masunaga et al., 2005;Morel & Senesi, 2002). The first set of clusters can be understood as mesoscale anvil cloud, and the latter as the cumulonimbus clouds embedded within the larger system. ...
... Here, we have addressed all of these elements using satellite climatologies. A recurrent theme of previous findings is the importance of interaction with larger-scale circulations, whether they be Amazonian wind shifts or mesoscale vortices (Morel & Senesi, 2002;Feng et al., 2018). In our El Niño-La Niña difference climatologies, we see an imprint of shifts in zonal overturning. ...
Convective organization has a large impact on precipitation and feeds back on larger‐scale circulations in the tropics. The degree of this convective organization changes with modes of climate variability like the El Niño–Southern Oscillation (ENSO), but because organization is not represented in current climate models, a quantitative assessment of these shifts has not been possible. Here, we construct multidecade satellite climatologies of occurrence of tropical convective organization and its properties and assess changes with ENSO phase. The occurrence of organized deep convection becomes more concentrated, increasing threefold in the eastern and central Pacific during El Niño and decreasing twofold outside of these regions. Both horizontal extent of the cold cloud shield and convective depth increase in regions of positive sea surface temperature anomaly (SSTa); however, the regions of greatest convective deepening are those of large‐scale ascent, rather than those of warmest SSTa. Extent decreases with SSTa at a rate of about 20 km/K, while the SSTa dependence of depth is only about 0.2 K/K. We introduce two values to describe convective changes with ENSO more succinctly: (1) an information entropy metric to quantify the clustering of convective system occurrences and (2) a growth metric to quantify deepening relative to spreading over the system lifetime. Finally, with collocated precipitation data, we see that rainfall attributable to convective organization jumps up to 5% with warming. Rain intensity and amount increase for a given system size during El Niño, but a given rain amount may actually fall with higher intensity during La Niña.
... The criteria of keeping a cloud track as a critical one (MCS), were the areal extent to reach more than 1000 km 2 during a three-hourly lifecycle, at least (e.g. Morel and Senesi, 2002). In Fig. 2a, the MCSs in the southern and eastern parts of the study domain, have track routes with a clear northeastern direction while in the western parts of the study domain it can be seen a different direction. ...
... Moreover, the rate of the temperature change of the coldest 10% brightness temperature values in the channel of 10.8 μm varies mainly between -2.5 K/hour and 2.5 K/hour which depicts the abrupt changes that occur inside the well-organized structure of these convective systems. Finally, the total distance traveled is clearly larger than the typical distance an MCS usually covers during its lifecycle in the greater area of the Mediterranean (e.g Morel and Senesi, 2002;Kolios and Feidas 2010). In Fig. 5 the majority of the examined MCSs have a lifecycle duration between 5 to 9 hours with a mean value of 7 hours (long-lasting MCSs). ...
The Mediterranean tropical-like cyclones (Medicanes) are exceptional meteorological phenomena observed over the Mediterranean Sea. The occurrence of Medicanes is rather rare but during their development phase, storms are being observed reaching the strength of a hurricane (Category 1) while main societal hazards include destructive winds, heavy precipitation, and flash floods. In late September 2018, one characteristic case of Medicane appeared over the southern Ionian Sea and gradually extended mainly over the Greek peninsula, causing extreme weather conditions and damages in the sea transportation sector as well as in inland infrastructures, especially in the coastal regions. In this study, multispectral Meteosat imagery was used to detect and monitor the evolution of the convective cloud characteristics (e.g. lifecycle duration, cloud top temperatures, areal extent). The first results of our analysis regarding the main characteristics of these cloud patterns reveal-among others-that during such events many well-organized mesoscale convective systems are developing reaching the tropopause, are long-lived (lifecycles larger than 4 hours) and are moving with relatively high speed (mean speed larger 40 km/h).
... A temporal shift in the MCS peak activity with early July over the southeast and late July/early August over the northwest was also found. 6) A majority of MCSs moves from the west, southwest, and south sectors, which is consistent with satellitebased estimates from Morel and Senesi (2002b). Only a minor fraction of MCSs propagate from other directions. ...
... Lifetime of a typical MCS in Poland ranges from 3 to 6 h, with initiation time around the afternoon hours (1200-1400 UTC) and dissipating stage in the evening(1900( -2000. This result is consistent withMorel and Senesi (2002b) who indicated aFIG. 11. (top left) Derecho trajectories and (top right) annual mean number of derechos. ...
In this study a 10-year (2008-2017) radar-based mesoscale convective system (MCS) and derecho climatology for Poland is presented. This is one of the first attempt for a European country to investigate morphological and precipitation archetypes of MCSs as prior studies were mostly based on satellite data. Despite its ubiquity and significance for society, economy, agriculture and water availability, little is known about climatological aspects of MCSs over Central Europe. Our results indicate that MCSs are not rare in Poland as an annual mean of 77 MCSs and 49 days with MCS can be depicted for Poland. Their lifetime ranges typically from 3 to 6 hours with initiation time around afternoon hours (12-14 UTC) and dissipation stage in the evening (19-20 UTC). Most frequent morphological type of MCSs is a broken line (58% of cases), then areal / cluster (25%) and quasi-linear convective system (QLCS; 17%), usually associated with a bow-echo (72% of QLCS). QLCS feature also with the longest lifecycle. Among precipitation archetypes of linear MCSs trailing stratiform (73%) and parallel stratiform (25%) are the most common. MCSs are usually observed from April to September with a peak in mid-July. A majority of MCSs travels from W, SW and S sectors. A total of 16 derecho events were identified (1.5% of all MCS and 9.1% of all QLCS), the majority of them were produced by a warm-season QLCS, while only 4 by a cold season narrow cold-front rainbands. Warm season derechos produced a bigger impact compared to cold season events, even though their damage paths were shorter.
... 2001;Morel and Senesi 2002a). Over land, they tend to form around mountains (e.g., Machado et al. 1998;Laurent et al. 2002;Morel and Senesi 2002b), which show clear diurnal variation with the highest frequency during the daytime (Morel and Senesi 2002b;Huang et al. 2018). Convective activities have been investigated over the vast continental climate zone in East Asia (Sato et al. 2007;Iwasaki et al. 2008). ...
... 2001;Morel and Senesi 2002a). Over land, they tend to form around mountains (e.g., Machado et al. 1998;Laurent et al. 2002;Morel and Senesi 2002b), which show clear diurnal variation with the highest frequency during the daytime (Morel and Senesi 2002b;Huang et al. 2018). Convective activities have been investigated over the vast continental climate zone in East Asia (Sato et al. 2007;Iwasaki et al. 2008). ...
This study investigated the impact of land surface heterogeneity on Mesoscale Convective System (MCS) initiations in East Asia, using geostationary satellite data during June–August from 1996 through 2018. The detected MCSs over land exhibited clear diurnal variation with the lowest existence frequency at 10:00 and highest initiation frequency during 12:00–17:00 local time. To quantify land surface heterogeneity, the spatial standard deviation of equivalent Black-Body Temperature (TBB) within a cloud-free 0.35° × 0.35° box (σLSTBB: Land Surface TBB) was computed for 10:00 each day. A comparison of the σLSTBB and MCS databases revealed that the probability of MCS initiations increased with increasing σLSTBB in East Mongolia while the probability was not sensitive to σLSTBB in East China. This indicated that MCSs tend to form over heterogeneous land surface conditions in the semiarid region. We found that the impact of land surface heterogeneity on MCS initiations was highest over flat terrain in East Mongolia, where the convection trigger due to topographically-induced circulation was absent. These results suggest that the impact of land surface heterogeneity on MCS initiations during the warm season varies with climate zones and terrain complexities in East Asia, with strongest impact in semiarid and flat regions.
... Galanaki et al. (2018) also found that the earlier a thunderstorm in the Mediterranean is triggered, the longer its duration tends to be. Morel and Senesi (2002) showed that most short-duration MCSs initiate late in the afternoon when solar heating is decreasing, leaving insufficient time for them to develop into stronger MCSs that can survive longer in the evening when atmospheric instability decreases. ...
Plain Language Summary
Thunderstorms are known as a type of weather system that is typically accompanied by the presence of lighting and other hazardous weather (high winds, heavy rain, hail and tornadoes). Cloud‐to‐ground (CG) lighting produced by thunderstorms is a highly dangerous weather phenomenon that occurs between a thundercloud and the ground and often causes wildfires, explosions and severe damage to buildings. The middle reaches of the Yangtze River Basin in China are a transition zone between plateaus and plains, with dense urban agglomerations, rivers and lakes. However, thunderstorm activity in such complex underlying surfaces is poorly understood. Based on ground‐based radar and lightning observations, the statistical characteristics of thunderstorm activity in this region during the warm seasons (May to September) of 2016–2020 are analyzed using a lightning clustering method. The CG lighting number, area and displacement of thunderstorms increase with thunderstorm duration. Thunderstorms that last longer are mostly triggered near the mountains and often start earlier in the afternoon and end later in the evening. In addition, CG lighting produced by thunderstorms is associated with high radar echo intensity. These findings are useful for improving the nowcasting of lightning and other hazardous weather caused by thunderstorms.
... 3.5. Furthermore, Morel and Senesi (2002) found that MCSs dissipate most commonly at 21:00 local solar time (LST) on average in Europe. Surowiecki and Taszarek (2020) found a similar result, showing that the majority of MCSs dissipate at around 19:00-20:00 UTC in Poland, although squall line/bow echo MCSs most frequently dissipate at midnight. ...
Cold fronts provide an environment particularly favourable for convective initiation in the mid-latitudes and can also be associated with convective hazards such as flooding, wind, hail and lightning. We build a climatology of cold-frontal convective cells between 2007–2016 for April–September in a cell-front distance framework by combining a radar-based cell detection and tracking dataset and automatic front detection methods applied to reanalysis data. We find that on average around twice as many cells develop on cold-frontal cell days compared to non-cold-frontal cell days. Using the 700 hPa level as a reference point, we show the maximum cell frequency is 350–400 km ahead of the 700 hPa front, which is marginally ahead of the typical surface front location. The 700 hPa front location marks the minimum cell frequency and a clear shift in regime between cells, with a weakened diurnal cycle on the warm side of the 700 hPa cold front and strongly diurnally driven cells on the cold side of the 700 hPa front. High cell frequency is found several hundreds of kilometres ahead of the surface front, and cells in this region are most likely to be associated with mesocyclones, intense convective cores and lightning. Namely, mesocyclones were detected in around 5.0 % of pre-surface-frontal cells compared to only 1.5 % of non-cold-frontal cells. The findings in this study are an important step towards a better understanding of cold-frontal convection climatology and links between cold fronts and convective hazards.
... DCC can be dangerous, and severe weather conditions are often accompanied, such as flash floods, tornadoes and hail that cause significant damage (Morel & Senesi, 2002a). Deep convective clouds occur mainly near coasts, on the lee side of mountain ranges, and near areas where long-wave radiation gradients occur (Morel & Senesi, 2002b). It is possible to characterize clouds through remote sensing in complex terrain, as in NWM. ...
... However, some authors have proposed more relaxed MCC criteria. For example, in Morel and Senesi 2002 [17,18], the authors proposed using the gradient of the cloud-top temperature for MCC identification. Feidas and Cartalis (2001) [67] used the threshold values in some spectral bands. ...
Mesoscale convective systems (MCSs) and associated hazardous meteorological phenomena cause considerable economic damage and even loss of lives in the mid-latitudes. The mechanisms behind the formation and intensification of MCSs are still not well understood due to limited observational data and inaccurate climate models. Improving the prediction and understanding of MCSs is a high-priority area in hydrometeorology. One may study MCSs either employing high-resolution atmospheric modeling or through the analysis of remote sensing images which are known to reflect some of the characteristics of MCSs, including high temperature gradients of cloud-top, specific spatial shapes of temperature patterns, etc. However, research on MCSs using remote sensing data is limited by inadequate (in size) databases of satellite-identified MCSs and poorly equipped automated tools for MCS identification and tracking. In this study, we present (a) the GeoAnnotateAssisted tool for fast and convenient visual identification of MCSs in satellite imagery, which is capable of providing AI-generated suggestions of MCS labels; (b) the Dataset of Mesoscale Convective Systems over the European Territory of Russia (DaMesCoS-ETR), which we created using this tool, and (c) the Deep Convolutional Neural Network for the Identification of Mesoscale Convective Systems (MesCoSNet), constructed following the RetinaNet architecture, which is capable of identifying MCSs in Meteosat MSG/SEVIRI data. We demonstrate that our neural network, optimized in terms of its hyperparameters, provides high MCS identification quality (mAP=0.75, true positive rate TPR=0.61) and a well-specified detection uncertainty (false alarm ratio FAR=0.36). Additionally, we demonstrate potential applications of the GeoAnnotateAssisted labelling tool, the DaMesCoS-ETR dataset, and the MesCoSNet neural network in addressing MCS research challenges. Specifically, we present the climatology of axisymmetric MCSs over the European territory of Russia from 2014 to 2020 during summer seasons (May to September), obtained using MesCoSNet with Meteosat MSG/SEVIRI data. The automated identification of MCSs by the MesCoSNet artificial neural network opens up new avenues for previously unattainable MCS research topics.
... In this study, we defined the oceanic (continental) MCCs as a system that reaches the maximum extent while positioned over the ocean (land) as the following method by Morel and Senesi [25] and Blamey and Reason [23]. This research follows the method by Ogino [26] and Mori [27] to define the coastal region. ...
One of the atmospheric phenomena that significantly affects Indonesian rainfall is MJO, and the other phenomenon with a different time scale with MJO is MCC which rainfall until causing storms. This study was conducted to the analysis of MCC during MJO. MCCs were identified and tracked for 15 years (2001–2015) over IMC by infrared satellite imagery using an algorithm that combines criteria of cloud coverage, eccentricity, and cloud lifetime. The identification of the MJO phase is carried out using the MJO index from RMM1 and RMM2. The results showed that the existence of MCC can strengthen the influence of the MJO on increasing rainfall in Indonesia. MCC is more common in areas that have a high convective activity, which indicates the presence of MJO. The existence of MCC which occurs concurrently with MJO will increase its influence on the distribution of rainfall and does not even rule out the possibility of causing extreme rainfall in Indonesia. The effect of active MJO followed by the presence of MCC on rainfall in Indonesia is higher when compared to when MJO is not active. Monthly, in phases 2, 3, and 4, MCC mostly appeared in November, while in phases 1 and 5, MCC mainly occurred in March. MCC often appears in January when the MJO is in phases 6 and 7, and MCC is more commonly found in February when the MJO is in phase 8. The largest MCC cloud core size during the mature phase is found in phase 5. MCC with large average size is found in Region A in almost all phases. The average MCC size found has a cloud core size ranging from 100,000 to 200,000 km2, but the largest sizes above 800,000 km2 are found during phases 3 and 4 of the MJO. The most extended average duration of about 12.1 was found in MCC, which occurred during phase 2 of the MJO. Most of the MCCs that occurred were found on the continents for each MJO phase.
... Such examples can be also found in many high mountain areas such as the Andes Frontiers in Earth Science 13 frontiersin.org (e.g., Zuluaga and Houze, 2015;Warner et al., 2003), the Rocky Mountains (e.g., Houze Jr, 2012) and the Alpine (e.g., Morel and Senesi, 2002). ...
The climate system of the Third Pole region, including the (TP) and its surroundings, is highly sensitive to global warming. Mesoscale convective systems (MCSs) are understood to be a vital component of this climate system. Driven by the monsoon circulation, surface heating, and large-scale and local moisture supply, they frequently occur during summer and mostly over the central and eastern TP as well as in the downstream regions. Further, MCSs have been highlighted as important contributors to total precipitation as they are efficient rain producers affecting water availability (seasonal precipitation) and potential flood risk (extreme precipitation) in the densely populated downstream regions. The availability of multi-decadal satellite observations and high-resolution climate model datasets has made it possible to study the role of MCSs in the under-observed TP water balance. However, the usage of different methods for MCS identification and the different focuses on specific subregions currently hamper a systematic and consistent assessment of the role played by MCSs and their impact on precipitation over the TP headwaters and its downstream regions. Here, we review observational and model studies of MCSs in the TP region within a common framework to elucidate their main characteristics, underlying mechanisms, and impact on seasonal and extreme precipitation. We also identify major knowledge gaps and provide suggestions on how these can be addressed using recently published high-resolution model datasets. Three important identified knowledge gaps are 1) the feedback of MCSs to other components of the TP climate system, 2) the impact of the changing climate on future MCS characteristics, and 3) the basin-scale assessment of flood and drought risks associated with changes in MCS frequency and intensity. A particularly promising tool to address these knowledge gaps are convection-permitting climate simulations. Therefore, the systematic evaluation of existing historical convection-permitting climate simulations over the TP is an urgent requirement for reliable future climate change assessments.
... A similar weakening of the diurnal cycle was observed for mesocyclones compared to lightning in Germany(Wapler et al., 2016). Furthermore,Morel and Senesi (2002) found that mesoscale convective systems (MCSs) dissipate most commonly at 21 Local Solar Time (LST) on average inSurowiecki and Taszarek (2020) found a similar result showing the majority of MCSs dissipate around 19-20UTC in Poland, though squall line/bow echo MCSs most frequently dissipate at midnight. MCSs typically live longer than individual ...
Cold fronts provide an environment particularly favourable for convective initiation in the mid-latitudes and can also be associated with convective hazards such as wind, rain and hail. We build a climatology of cold-frontal convective cells between 2007–2016 for April–September in a cell-front distance framework by combining a radar-based cell detection and tracking dataset and automatic front detection methods applied to reanalysis data. We find that on average around twice as many cells develop on cold-frontal cell days compared to non-cold-frontal cell days. Using the 700 hPa level as a reference point we show the maximum cell frequency is 350–400 km ahead of the 700 hPa front which is marginally ahead of the mean surface front location. The 700 hPa front location marks the minimum cell frequency and a clear shift in regime between cells with a weakened diurnal cycle on the warm-side of the 700 hPa cold front and strongly diurnally driven cells on the cold-side of the 700 hPa front. High cell frequencies are found several hundreds of kilometres ahead of the surface front and cells in this region are most likely to be associated with mesocyclones, intense convective cores and lightning. These results are an important step towards a better understanding of cold-frontal convection climatology and links between cold fronts and convective hazards.
... Definition of MCS. The definition here is inspired by previous work 17,32 . We require MCSs to have maximum contiguous area of interest exceeding 1000 km 2 , but they can have much greater maximum areas (100,000+ km 2 ) 32 . ...
Larger organised convective storms (mesoscale-convective systems) can lead to major flood events in Europe. Here we assess end-of-century changes to their characteristics in two convection-permitting climate simulations from the UK Met Office and ETH-Zürich that both use the high Representative Concentration Pathway 8.5 scenario but different approaches to represent atmospheric changes with global warming and different models. The UK Met Office projections indicate more frequent, smaller, and slower-moving storms, while ETH-Zürich projections show fewer, larger, and faster-moving storms. However, both simulations show increases to peak precipitation intensity, total precipitation volume, and temporal clustering, suggesting increasing risks from mesoscale-convective systems in the future. Importantly, the largest storms that pose increased flood risks are projected to increase in frequency and intensity. These results highlight that understanding large-scale dynamical drivers as well as the thermodynamical response of storms is essential for accurate projections of changes to storm hazards, needed for future climate adaptation.
... In particular, Ruiz-Leo et al. (2013) suggested that this threshold should be fixed when the convective component represents 60% of the total cumulative precipitation since Houze (1993) found that this percentage can be considered a characteristic of the convective rainfall in a Mesoscale Convective System (MCS). Nevertheless, it has been noticed that in Europe these organized complexes of cumulonimbus are characterized by different features (e.g., size and duration) compared to other parts of the world (Rigo and Llasat, 2007;Kolios and Feidas, 2010;Michaelides et al., 2018) and are mainly continental (Morel and Senesi, 2002;Kolios and Feidas, 2010), suggesting that also other types of a thunderstorm (e.g., single-and multi-cell storms) need to be considered in deriving I cr for Sicily. Feloni et al. (2019), instead, shifted the previous threshold from 60 to 50% of the total cumulative precipitation. ...
Heavy rainfall events are likely to become more frequent and severe in the next future, under the effects of a changing climate. In this context, the scientific literature is characterized by a relevant number of studies trying to identify the convective component of precipitation, since this kind of phenomena, due to their short duration and high intensity, may lead to an increased risk of flash floods or debris flows and, consequently, human life losses and economic damages. In this study, a separation between the convective and stratiform precipitation, starting from the subhourly precipitation time series recorded over Sicily (Italy), is provided on a monthly scale. Results show that the percentage of convective precipitation increases as the associated intensity increases, following what is generally known about convective events. Moreover, during the summer months, higher percentages of convective rainfall are generally reached for lower values of intensity, since this season favours the occurrence of these phenomena due to high temperature and relative humidity. In addition to the regional analysis, even an at‐site analysis has been carried out, through the definition of a critical intensity threshold to identify the predominant convective component for each rain gauge. This analysis has pointed out that such convective events have occurred mainly in the east part of the island mainly due to the complex orography of this area, which favours the occurrence not only of such phenomena but of flash floods and debris flows as well.
... Therefore, in this region characterized by its complex orography, PECS which also typically initiate of the SMO entirely dominate the morphology classification. This PECS domination associated with the complex terrain and with the orographic uplift is similar to what occurs in the mountain ranges of central Europe (e.g., the Pyrenees and the Alps) [129,130] and the Himalayas [41]. These results are also consistent with those of Jirak et al. [40], who found that in the relatively flat topography of the central Plains MCC and PECS have similar frequency, supporting the argument that complex topography strongly impacts MCS morphology. ...
Mesoscale Convective Systems (MCS) may vary greatly with respect to their morphology, propagation mechanism, intensity, and under which synoptic-scale conditions as a function of topographic complexity. In this study, we develop a long-term climatology of MCS during the North American Monsoon focusing on MCS morphology, lifecycle, and intensity as well as possible propagation mechanisms. We employ an MCS tracking and classification technique based on 23 years (1995 to 2017) of GOES IR satellite data. MCS intensity is also gauged with 7 years (2011 to 2017) of Vaisala GLD360 lightning data and, finally, monthly and interannual variability in synoptic conditions are examined with ERA5 reanalysis data. Our results based on 1594 identified MCS reveal that 98% are morphologically classified as Persistent Elongated Convective Systems. During the 23 summers (June through September) observed, the number of MCS varied considerably, averaging 70 MCS with minimum of 41 and maximum of 94. MCS typically have an average duration of around 8 h ± with a 2 h standard deviation. Propagation speeds, estimated with Hovmöller diagrams in addition to MCS centroid initial and final position, vary slightly depending on the trajectory. A notable result suggests that MCS propagation speeds are more consistent density currents or cold pools and not gravity waves nor steering-level winds. The results of this study could also provide a dataset for examining larger-scale controls on MCS frequency in addition to assesing convective parameterization and convective-resolving models in regions of complex topography.
... Previous studies have made significant advances in MCS tracking and analysis using either satellite data or radar measurements. Studies based on satellite data usually use the extent of the cold cloud-top obtained from satellite imageries to identify MCSs (Maddox 1980;Laing and Fritsch 1997;Ma et al. 1997;Anderson and Arritt 1998;Machado et al. 1998;Morel and Senesi 2002;Zheng et al. 2008;Yang et al. 2015;Ai et al. 2016). Satellites with high time and spatial resolution can reveal the life cycle of MCS, but cold cloud-top coverage does not necessarily overlap with heavy precipitation areas (Houze 1993). ...
Mesoscale convective systems (MCSs) are important warm-season precipitation systems in eastern China. However, our knowledge of their climatology and capability in their simulation is still insufficient. This paper examines their characteristics over the 2008–2017 warm seasons using convection-permitting climate simulations (CPCSs) with a 3-km grid spacing that explicitly resolves MCSs, as well as a high-resolution gauge-satellite merged precipitation product. An object-based tracking algorithm is applied to identify MCSs. Results indicate that the MCS genesis and occurrence are closely related to the progression of the East Asian monsoon and are modulated by the underlying topography. On average, about 243 MCSs are observed each season and contribute 19% and 47% to total and extreme warm-season precipitation. The climatological attributes and variabilities are reasonably reproduced in the CPCS. The major model deficiencies are excessive small MCS occurrence and overmuch MCS rainfall, consequently overestimating the precipitation contributions, whereas observational uncertainties may play a role too. Both the observed and simulated MCS precipitation feature a nocturnal or morning maximum and an eastward delayed diurnal peak east of the Tibetan Plateau, in contrast to the dominant afternoon peak of non-MCS precipitation. The favorable comparison with observations demonstrates the capability of CPCSs in simulating MCSs in the Asian monsoon climate, and its usefulness in projecting the future changes of MCSs under global warming. The finding that non-MCS precipitation is responsible for the high biased afternoon precipitation provides helpful guidance for further model improvement.
... Convective storms produce most short-duration (1-3 h) heavy precipitation events (Chappell, 1986); with MCSs predominating in the mid-latitudes (Jiang et al., 2006;Morel & Senesi, 2002;Schumacher & Rasmussen, 2020). The "ingredients-based approach" used by contemporary meteorologists for forecasting convective storms provides a physical basis for factors required for an event, for example, moisture, instability, and lift for thunderstorms. ...
Under climate change, increases in precipitation extremes are expected due to higher atmospheric moisture. However, the total precipitation in an event also depends on the condensation rate, precipitation efficiency, and duration. Here, a new approach following an ‘ingredients-based methodology’ from severe weather forecasting identifies important aspects of the heavy precipitation response to climate change, relevant from an impacts perspective and hitherto largely neglected. Using 2.2km climate simulations, we show that a future increase in precipitation extremes across Europe occurs, not only because of higher moisture and updraft velocities, but also due to slower storm movement, increasing local duration. Environments with extreme precipitation potential are 7x more frequent than today by 2100, whilst the figure for quasi-stationary ones is 11x (14x for land). We find that a future reduction in storm speeds, possibly through Arctic Amplification, could enhance event accumulations and flood risk beyond expectations from studies focusing on precipitation rates.
... Object-oriented tools have been used for many years in nowcasting. In CNRM, the ISIS (Instrument de Suivi dans l'Imagerie Satellitaire) tracking algorithm developed by Morel and Senesi (2002a) and initially based on satellite infrared imagery to characterise European MCSs (Morel and Senesi 2002b) is still applied to 5 min radar reflectivities to monitor precipitating active areas in storms. Brousseau et al. (2016) use this algorithm to characterise improvements in the convective cell simulation of AROME-simulated radar reflectivities between 2.5 and 1.3 km resolutions. ...
Modelling the rare but high-impact Mediterranean Heavy Precipitation Events (HPEs) at climate scale remains a largely open scientific challenge. The issue is adressed here by running a 38-year-long continuous simulation of the CNRM-AROME Convection-Permitting Regional Climate Model (CP-RCM) at a 2.5 km horizontal resolution and over a large pan-Alpine domain. First, the simulation is evaluated through a basic Eulerian statistical approach via a comparison with selected high spatial and temporal resolution observational datasets. Northwestern Mediterranean fall extreme precipitation is correctly represented by CNRM-AROME at a daily scale and even better at an hourly scale, in terms of location, intensity, frequency and interannual variability, despite an underestimation of daily and hourly highest intensities above 200 mm/day and 40 mm/h, respectively. A comparison of the CP-RCM with its forcing convection-parameterised 12.5 km Regional Climate Model (RCM) demonstrates a clear added value for the CP-RCM, confirming previous studies. Secondly, an object-oriented Lagrangian approach is proposed with the implementation of a precipitating system detection and tracking algorithm, applied to the model and the reference COMEPHORE precipitation dataset for twenty fall seasons. Using French Mediterranean HPEs as objects, CNRM-AROME’s ability to represent the main characteristics of fall convective systems and tracks is highlighted in terms of number, intensity, area, duration, velocity and severity. Further, the model is able to simulate long-lasting and severe extreme fall events similar to observations. However, it fails to reproduce the precipitating systems and tracks with the highest intensities (maximum intensities above 40 mm/h) well, and the model’s tendency to overestimate the cell size increases with intensity.
... Infrared satellite images can reveal the structure of the front and are used to track the MCS [43,44]. The satellite images during this Mei-Yu event revealed that the front is a complex system, composed of several convection systems (e.g., Chimney cloud with convective hot spots). ...
Radiative transfer model can be used to convert the geophysical variables (e.g., atmospheric thermodynamic state) to the radiation field. In this study, the Community Radiative Transfer Model (CRTM) is used to connect regional Weather Research and Forecasting (WRF) model outputs and satellite observations. A heavy rainfall event caused by the Mei-Yu front on the June 1, 2017, in the vicinity of Taiwan, was chosen as a case study. The simulated cloud performance of WRF with four microphysics schemes (i.e., Goddard (GCE), WRF single-moment 6 class (WSM), WRF double-moment 6 class (WDM), and Morrison (MOR) schemes) was investigated objectively using multichannel observed satellite radiances from a Japanese geostationary satellite Himawari-8. The results over the East Asia domain (9 km) illustrate that all four microphysics schemes overestimate cloudy pixels, in particular, the high cloud of simulation with MOR when comparing with satellite data. Sensitivity tests reveal that the excess condensation of ice at ≥14 km with MOR might be associated with the overestimated high cloud cover. However, GCE displayed an improved performance on water vapor channel in clear skies. When focusing on Taiwan using a higher (3 km) model resolution, each scheme displayed a decent performance on cloudy pixels. In the grid-by-grid skill score analysis, the distribution of high clouds was the most accurate among the three cloud types. The results also suggested that all schemes required a longer simulation time to describe the low cloud horizontal extend.
... Geostationary satellites with orbital periods that match the Earth's rotation cycle are particularly useful for continuous 24-h observation of specific locations. Geostationary satellites are thus very useful for monitoring precipitation systems and tracking the movement of convective storms (Bley et al. 2016;Senf et al. 2015;Morel and Senesi 2002b). Localized heavy rainfall and floods in Korea have become more frequent in recent years, which has resulted in an increase in property damage (Han et al. 2015). ...
New technologies for the classification of convective cloud lifecycles and the prediction of their movements are needed to detect severe convective weather and to support objective cloud guidance. Satellites enable earlier detection of severe weather over larger coverage areas than ground-based observations or radar. The use of satellite observations for nowcasting is thus likely. In this study, convective initiation (CI) data are paired with a modified rapid-development thunderstorm (RDT) algorithm for the analysis of new data from the Geostationary Korea Multi-Purpose Satellite-2A (GEO-KOMPSAT-2A, GK2A). The RDT algorithm is further modified to accommodate the additional GK2A satellite channels, and new satellite data are used to continuously analyze thunderstorms associated with severe weather in Korea. The logistic regression (LR) machine learning approach is used to optimize the criteria of interest fields and weighting coefficients of the RDT algorithm for convective detection. In addition, auxiliary data (cloud type, convective rainfall rate, and cloud top temperature/height) calculated from RDT sub-module is replaced with GK2A derived products. The fully modified RDT algorithm (K-RDT) is quantitatively verified using lightning data from summer convection cases. The probability of detection (POD) for convective clouds is increased by 30–40%, and the threat score (TS) for average lightning activity is improved by 10–30%. The channel properties of Japan Himawari-8 satellite are similar to those of the GK2A satellite. Due to the lack of GK2A satellite data during the development period, CI data from the Himawari-8 satellite are used as proxies.
... MCCs are important because they produce a large fraction of warm-season rainfall [18] and often are associated with severe weather [19,20]. They commonly occur over land in the lee of major mountain ranges and association with low-level jets (LLJs), and likely make significant contributions to local and global hydrologic budgets [21]. The MCC is typically formed in association with a weak midtropospheric shortwave trough and a weak surface front or outflow boundary. ...
This study analyzed the large-scale meteorological condition during Mesoscale Convective Complexes (MCCs) over the Indian Ocean. The MCCs were identified by infrared satellite imagery using an algorithm that combined information about cloud coverage, eccentricity, and the cloud lifetimes of MCCs. The data used were a combination of satellite data and reanalysis data, while the Brightness temperature (T BB ) was obtained from the MTSAT IR1 satellite data. The results of this study showed that, in general, the initial stage of the MCC was characterized by strong low-level convergence and vertical convection, and largely driven by the convergence of the moisture flux in the lower troposphere. The mature stage of the MCC was characterized by weak surface convergence, strong upper-level divergence, and a shortwave ridge in the mid- and upper levels. Where there was strong surface divergence, the decay and dissipation stages were very similar, and the surface convergence left the system. The movement of most MCCs resulted from the combined contributions of advection and the propagation of surface convergence. Results from this research showed that these large convective systems tended to be formed in the vicinity of the terminus of a low-level jet that transported moist and warm air to the originating regions of the MCCs. Shortwave troughs and baroclinic zones were associated with MCC development.
... Identification of MCSs has been traditionally carried out by tracking convective elements in geosynchronous satellite or ground-based radar data, which provide temporally continuous spatial data. For example, satellite imagery that provides cloud radiative properties such as brightness temperature at high temporal resolution has been used to track deep convective clouds at a regional to global scale (Schmetz et al. 1993;Machado et al. 1998;Morel and Senesi 2002;Héas and Mémin 2008;Roca et al. 2014). However, the incapacity of visible and infrared satellite data to reveal the structure of the clouds in the lower troposphere leaves large uncertainties in satellite-based MCS tracking. ...
The Global Precipitation Measurement (GPM) core observatory satellite launched in 2014 features more extended latitudinal coverage (65°S-65°N) than its predecessor Tropical Rainfall Measuring Mission (TRMM, 35°S-35°N). The Ku-band radar onboard of the GPM is known to be capable of characterizing the 3D structure of deep convection globally. In this study, GPM’s capability for detecting mesoscale convective systems (MCSs) is evaluated. Extreme convective echoes seen by GPM are compared against an MCS database that tracks convective entities over the contiguous US. The tracking is based on geostationary satellite and ground-based Next Generation Radar (NEXRAD) network data obtained during the 2014-2016 warm seasons. Results show that more than 70% of the GPM-detected Deep-Wide Convective Core (DWC) and Wide Convective Core (WCC) objects are part of NEXRAD identified MCSs, indicating that GPM-classified DWCs and WCCs correlate well with typical MCSs containing large convective features. By applying this method to the rest of the world, a global view of MCS distribution is obtained. This work reveals GPM’s potential in MCS detection at the global scale, particularly over remote regions without dense observation network.
... With respect to blocking over the Baltic Sea, the correlation to thunderstorm occurrence seems to be more relevant in regions where convection is often related to synoptic forcing (e.g., cold fronts), such as central France, northern Germany or south of the Alps in Switzerland (Schemm et al., 2016;Kunz et al., 2019), as compared to regions with local-scale forcing dominated by orographic effects (Morel and Senesi, 2002;Barthlott et al., 2011), such as southern Germany or eastern Austria. ...
A statistically significant link is presented between atmospheric blocking located over the eastern North Atlantic and northern Europe and warm‐season thunderstorm activity over western and central Europe. Lightning data from 2001 to 2014 were used to identify thunderstorm days and blocking events were extracted from the ERA‐Interim reanalysis using an objective identification algorithm. The statistical link between the two phenomena is established through odds ratio analysis. Two areas – one over the eastern part of the North Atlantic and one over the Baltic Sea – were identified as locations where blocking influences the occurrence of deep moist convection in parts of western and central Europe. Based on the mean ambient conditions on days with blocking in these two areas, well‐known dynamic and thermodynamic mechanisms supporting or suppressing the development of thunderstorms were confirmed. The anticyclonic circulation of a block over the eastern part of the North Atlantic leads to a northerly to northwesterly advection of dry and stable air masses into Europe on the eastern flank of the block. In addition, these environmental conditions are on average associated with large‐scale subsidence of air masses (convection‐inhibiting conditions). In contrast, the southerly to southwesterly advection of warm, moist and unstable air masses on the western flank of a block over the Baltic Sea results in convection‐favouring conditions over western and central Europe. Both blocking situations are on average associated with weak wind speeds at mid‐tropospheric levels and with weak wind shear. As a consequence, thunderstorms related to atmospheric blocking over the Baltic Sea tend to be on average less organised.
... Besides, the polar-orbiting satellites (e.g., Yuan & Houze, 2010), the Tropical Rainfall Measuring Mission (e.g., Choudhury et al., 2016;Jaramillo et al., 2017), and the active remote sensing satellites such as Cloudsat (e.g., Yuan et al., 2011) also provide extra sources to examine the MCSs. Among ground-based observations, considerable collocated ground-based weather radar measurements have been used to verify the MCSs as identified and tracked by the satellites (e.g., Kolios & Feidas, 2010;Morel & Senesi, 2002a, 2002bXia et al., 2018). ...
The knowledge of mesoscale convective system (MCS) in the Asian monsoon region remains still deficient due to the limited available data and less powerful algorithms. Here, using the data from Advanced Himawari Imager onboard Himawari‐8 (HW8), an improved algorithm combining the area overlapping with the Kalman filter is developed, which captures much smaller MCSs that are unavailable otherwise. Several influential factors like the overlapping rate and splitting/merging in the area overlapping method, and the initial state variable in the Kalman filter method, all of which were less appreciated, are handled explicitly. The occurrence frequency, and moving trajectory of two types of MCS, including the ordinary MCS and superconvective system, has been comprehensively examined in the Asian monsoon region for the warm season (April to September) of 2016. Comparison analyses with ground precipitation and radar measurements confirm the good performance of our algorithm. In particular, the moving direction of MCS strongly depends on latitudes, so does the horizontal velocity. Compared with over ocean, the frequency of MCSs dominates over land or along coasts in the tropics, where strong moisture flux convergence is frequently observed in the low troposphere. In addition, the MCSs detected in eastern China can roughly capture the meridional propagation over time, which corresponds well to the precipitation belts linked to Meiyu front systems. The superconvective systems dominate over the Bay of Bengal and South China Sea due to the large‐scale circulation. Our findings provide new insights to spatiotemporal patterns of MCSs during warm season in the Asian monsoon region.
This study employs 3D idealized numerical experiments to investigate the physical processes associated with coastal convection initiation (CI) as an offshore-moving squall line traverses a mountainous coastal region. A squall line can propagate discretely as convection initiates over the lee slope downstream of the primary storm as the cold pool collides with a sea breeze. Intensity of the initiating convection, thus the downstream squall line, is sensitive to the sea breeze numerical initialization method, since it influences sea breeze and cold pool characteristics, instability and vertical wind shear in the sea breeze environment, and ultimately the vertical acceleration of air parcels during CI. Here, sea breezes are generated through four commonly used numerical methods: a cold-block marine atmospheric boundary layer (MABL), prescribed surface sensible heat flux function, prescribed surface sensible plus latent heat flux functions, and radiation plus surface-layer parameterization schemes. For MABL-initialized sea breezes, shallow weak sea breeze flow in a relatively low instability environment results in weak CI. For the remainder, deeper stronger sea breeze flow in an environment of enhanced instability supports more robust CI. In a subset of experiments, however, the vertical trajectory of air parcels is suppressed leading to weaker convection. Downward acceleration forms due to the horizontal rotation of the sea breeze flow. Accurate simulations of coastal convective storms rely on an accurate representation of sea breezes. For idealized experiments such as the present simulations, a combination of initialization methods likely produces a more realistic representation of sea breeze and the associated physical processes.
Ni-based catalysts are an alternative to precious metal catalysts. However, monometallic Ni catalysts still have weaknesses, such as being easily deactivated and being able to agglomerate if the Ni concentration is too high. This can be overcome by adding a second metal (bimetallic catalyst) in which the structure and morphology of the bimetallic catalyst are strongly influenced by the method used. The wet-impregnation method is used to obtain a supported bimetallic catalyst where the order of metal carrying will affect the structure and morphology and is specific for each material. On the Ni-Zn/ZrO2 catalyst, which has been synthesized by co-impregnation (Cat-1) and sequentially (Cat-2) method, XRD results show that both catalysts have specific peaks of ZrO2, NiO, and ZnO. However, the Cat-2 catalyst has lower crystallinity and smaller NiO crystallite size (14.79 nm) than Cat-1 (19.45 nm). Micrograph results show that both catalysts have irregular particle shapes. Cat-1 has a smaller particle size and a more homogeneous metal dispersion than Cat-2. The elemental analysis shows that metal concentration at the surface is close to the theoretical (10%). Further research with further characterization and application to a specific chemical reaction is needed to determine which of the two catalysts perform better. This is because each reaction has specific requirements, and each catalyst will produce different results in each reaction even though they have the same properties.
Dynamical physical processes associated with an onshore moving marine atmospheric boundary layer (MABL, i.e., sea breeze) over sloping terrain, sensitivity of these processes to MABL characteristics, and flow modifications induced by an offshore‐moving squall line are investigated using idealized simulations. The moving MABL gradually advances inland, exhibiting farther advancement and greater upslope wind speed for deeper and cooler MABLs. The local acceleration is primarily driven by a MABL‐generated perturbation pressure gradient force (PPGF). As the moving MABL air accumulates onshore over time, an opposing force associated with the increasing negative buoyancy eventually balances the PPGF and results in a quasi‐steady upslope flow. The approaching squall line disrupts this flow in two distinct ways; Initially the storm's cold pool enhances the ambient downslope winds which diminishes the upslope wind speeds, and subsequently the storm‐generated high‐frequency waves and the associated surface pressure low enhances the upslope‐directed PPGF which reintensifies the upslope flows.
We here inter-compare four different tracking algorithms by applying them onto the precipitation fields of an ensemble of convection-permitting regional climate models (cpRCMs) and on high-resolution observational datasets of precipitation. The domain covers the Alps and the northern Mediterranean and thus we here analyse heavy precipitation events, that are renowned for causing hydrological hazards. In this way, this study is both, an inter-comparison of tracking algorithms as well as an evaluation study of cpRCMs in the Lagrangian frame of reference. The tracker inter-comparison is performed by comparison of two case studies as well as of climatologies of cpRCMs and observations. We find that that all of the trackers produce qualitatively equal results concerning characteristic track properties. This means that, despite of quantitative differences, equivalent scientific conclusions would be drawn. This result suggests that all trackers investigated are reliable analysis tools of atmospheric research. With respect to the model ensemble evaluation, we find an encouraging performance of cpRCMs in comparison to radar-based observations. In particular prominent hotspots of heavy precipitation events are well-reproduced by the models. In general most characteristic properties of precipitation events have positive biases. Assuming the under-catchment of precipitation in observations in a domain of such complex orography, this result is to be expected. Only the mean area of tracks is underestimated, while their duration is overestimated. Mean precipitation rate is estimated well, while maximum precipitation rate is overestimated. Furthermore, geometrical and rain volume are overestimated. We find that models overestimate the occurrence of precipitation events over all mountain chains, whereas over plain terrain in summer precipitation events are seen underestimated. This suggests that, despite the convection-permitting resolution, thermally driven thunderstorms are either not triggered or their dynamics still under-resolved. Eventually we find that biases in the spatio-temporal properties of precipitation events appear reduced when evaluating cpRCMs against Doppler radar-based and rain gauge-adjusted observational datasets of comparable spatial resolution, strengthening their role in evaluation studies.
This chapter provides an overview of the characteristics of the climate over the Mediterranean region and of the processes that determine its evolution. It describes how numerical models are used for simulating the climate at the regional Mediterranean scale, the heat, and moisture balance at Mediterranean regional scale, their relation to surface climate. The evolution of the regional climate conditions is described since the times when the Mediterranean present morphology emerged as a remnant of the Tethys Ocean. Finally, the impacts of the ongoing anthropogenic climate change are described.
Research on Mesoscale Convective Systems (MCS) in Indonesia has been studied to identify the characteristics and distribution of MCS. Persistent Elongated Convective System (PECS) is one of the MCS types that can cause extreme weather. However, identifying the spatial and temporal variations of PECS in Indonesia has not been studied further. This study examines the Persistent Elongated Convective System (PECS) in Maritime Continent from 2010-2014. MERG Dataset with brightness temperature is used to identify PECS and input Grab ‘Em Tag ‘Em Graph ‘Em (GTG) tracking algorithm to gain a better understanding of spatiotemporal variation in Indonesia. Flood data from BNPB in Indonesia is used to find out the correlation between flood and PECS events in Indonesia. PECS events in 2010-2014 are 5574 events, which the transitional period (MAM and SON) has the most PECS events and longest duration of life. The PECS spread in the early morning on land and sea, while on day and nighttime, it is concentrated on land for the area of Java, Papua, and Southern Kalimantan. Java and Papua have the largest concentrated PECS events. The life phase of the PECS has two peaks or semidiurnal cycles, in the morning and nighttime. There are differences in peak times for each phase of the PECS on land and sea. In Indonesia, PECS has the smallest average maximum area than in the USA and China but has the longest duration and largest eccentricity. Based on flood data from BNPB, during 2010-2014, Java has the most flood. According to flood data and PECS events in Java, there is 6.72% of flood coincided with PECS events during the period.
Mesoscale convective systems (MCSs) have been identified as an important source of precipitation in the Tibetan Plateau (TP) region. However, the characteristics and structure of MCS‐induced precipitation are not well understood in this location. Infrared (IR) satellite imagery has been used for MCS tracking, but cirrus clouds or cold surfaces can lead to false MCS classification over mountain regions. Here, we combine brightness temperatures from IR imagery with satellite precipitation estimates from GPM IMERG and track MCSs over the TP, at the boundary of the TP (TPB), and in the surrounding lower‐elevation plains (LE), between 2000 and 2019. In most parts of LE and TPB, MCSs produced 50%–80% of the total summer precipitation (60%–90% of summer heavy precipitation), whereas MCSs over the TP account for below 10% of the total summer precipitation (10%–30% of summer heavy precipitation). Our results also show that MCSs that produce the largest amounts of heavy precipitation are characterized by longevity and large extents rather than by high intensities. These are mainly located in the populous areas south and east of the TP. A tracking of meso‐β convective systems over the TP shows that small‐scale convection makes a large contribution to total and heavy precipitation. This suggests that more localized convective systems are important for the regional water cycle over the higher terrain and highlights the importance of convective‐scale modeling to improve our understanding of precipitation dynamics in the TP region.
This chapter presents an integrated platform to simulate and predict flash floods. The platform consists of various state-of-the-art components, including remote sensing data assimilation and advanced atmospheric and hydrological models. The Local Analysis and Prediction System (LAPS) and the Chemical Hydrological Atmospheric Ocean wave System (CHAOS), including the Advanced Weather Research and Forecasting (WRF-ARW) model and the WRF-Hydro hydrological model, have been utilized. The efficiency of the platform was assessed in a flash flood event affected the suburban area of Mandra in western Attica, Greece. WRF-Hydro simulated the flood using forcing data, including four different precipitation estimations, obtained from CHAOS forecasts (CHAOS-hydro), the National Observatory of Athens X-band dual-polarization (XPOL) weather radar (XPOL-hydro), the Global Precipitation Measurement (GMP)/Integrated Multi-satellitE Retrievals GPM/IMERG-hydro, and LAPS nowcasts (LAPS-hydro). Comparisons with site-detailed postsurveys of flood extent revealed that XPOL-hydro well predicted flooded area with a 68% probability of detection (POD) while CHAOS-hydro and GPM/IMERG-hydro presented smaller POD, 43% and 30%, respectively. The nowcasting approach assessment showcased that 3 h before the maximum flood extent, LAPS-hydro yielded a 49% POD while it showed 61% and 67% POD 2 and 1 h before, respectively, comparable to POD resulted by XPOL-hydro.
The frequency of European convective windstorms, environments in which they form, and their convective organizational modes remain largely unknown. A climatology is produced using 10 233 severe convective-wind reports from the European Severe Weather Database between 2009–2018. Severe convective-wind days have increased from 50 days yr–1 in 2009 to 117 days yr–1 in 2018, largely because of an increase in reporting. The highest frequency of reports occurred across central Europe, particularly Poland. Reporting was most frequent in summer, when a severe convective windstorm occurred every other day on average. The preconvective environment was assessed using 361 proximity soundings from 45 stations between 2006–2018, and a clustering technique was used to distinguish different environments from nine variables. Two environments for severe convective storms occurred: Type 1, generally low-shear–high-CAPE (mostly in the warm season) and Type 2, generally high-shear–low-CAPE (convective available potential energy; mostly in the cold season). Because convective mode often relates to the type of weather hazard, convective organizational mode was studied from 185 windstorms that occurred between 2013–2018. In Type-1 environments, the most frequent convective mode was cells, accounting for 58.5% of events, followed by linear modes (29%) and the nonlinear noncellular mode (12.5%). In Type-2 environments, the most frequent convective mode was linear modes (55%), followed by cells (36%) and the nonlinear noncellular mode (9%). Only 10% of windstorms were associated with bow echoes, a much lower percentage than other studies, suggesting that forecasters should not necessarily wait to see a bow echo before issuing a warning for strong winds.
Idealized 3D numerical simulations are used to quantify the impact of moving marine atmospheric boundary layers (MABLs) on squall lines in an environment representative of the U.S. mid-Atlantic coastal plain. Characteristics of the MABL, including depth and potential temperature, are varied. Squall lines are most intense while moving over the deepest MABLs, while the storm encountering no MABL is the weakest. Storm intensity is only sensitive to MABL temperature when the MABL is sufficiently deep.
Collisions between the storm cold pools and MABLs transition storm lift from surface-based cold pools to wave-like features, with the resulting ascent mechanism dependent on MABL density, not depth. Bores form when the MABL is denser than the cold pool and hybrid cold pool-bores form when the densities are similar. While these features support storms over the MABL, the type of lifting mechanism does not control storm intensity alone. Storm intensity depends on the amplification and maintenance of these features, which is determined by the ambient conditions.
Isolated convective cells form ahead of squall lines prior to the cold pool-MABL collision, resulting in a rain peak and the eventual discrete propagation of the storms. Cells form as storm-generated high-frequency gravity waves interact with gravity waves generated by the moving marine layers, in the presence of reduced stability by the squall line itself. No cells formed in the presence of the storm nor the MABL alone.
Mesoscale convective systems (MCSs) have been identified as an important source
of precipitation in the Tibetan Plateau (TP) region. However, the characteristics and
structure of MCS-induced precipitation are not well understood. Infrared satellite imagery has been used for MCS tracking, but cirrus clouds or cold surfaces can cause misclassifications of MCS in mountain regions. We therefore combine brightness temperatures from IR imagery with satellite precipitation data from GPM and track MCSs over
the TP, at the boundary of the TP (TPB) and in the surrounding lower-elevation plains
(LE) between 2000 and 2019. We show that MCSs are less frequent over the TP than
earlier studies have suggested and most MCSs over land occur over the Indo-Gangetic
Plain (LE) and the south of the Himalayas (TPB). In the LE and TPB, MCSs have produced 10 % to 55 % of the total summer precipitation (10 % to 70% of summer extreme
precipitation), whereas MCSs over the TP account for only 1 % to 10 % to the total summer precipitation (1 % to 30 % of the total summer extreme precipitation). Our results
also show that MCSs that produce the largest amounts of convective precipitation are
characterized by longevity and large extents rather than by high intensities. These are
mainly located south of the TP, whereas smaller-scale convection makes a greater contribution to total and total extreme precipitation over the TP. These results highlight
the importance of convective scale modeling to improve our understanding of precipitation dynamics over the TP.
As lightning-detection records lengthen and the efficiency of severe weather reporting increases, more accurate climatologies of convective hazards can be constructed. In this study we aggregate flashes from the NLDN and ATDnet lightning-detection networks with severe weather reports from ESWD and SPC Storm Data on a common grid of 0.25° and 1-hour steps. Each year approximately 75–200 thunderstorm hours occur over the southwestern, central and eastern United States, with a peak over Florida (200–250 hours). The activity over the majority of Europe ranges 15–100 hours, with peaks over Italy and mountains (Pyrenees, Alps, Carpathians, Dinaric Alps; 100–150 hours). The highest convective activity over continental Europe occurs during summer and over the Mediterranean during autumn. The United States peak for tornadoes and large hail reports is in spring, preceding the maximum of lightning and severe wind reports by 1–2 months. Convective hazards occur typically in the late afternoon, with the exception of the Midwest and Great Plains, where mesoscale convective systems shift peak lightning threat to the night. The severe wind threat is delayed by 1–2 hours compared to hail and tornadoes. The fraction of nocturnal lightning over land ranges 15%–30% with lowest values observed over Florida and mountains (∼10%). Wintertime lightning shares the highest fraction of severe weather. Compared to Europe, extreme events are considerably more frequent over the United States, with maximum activity over the Great Plains. However, the threat over Europe should not be underestimated, as severe weather outbreaks with damaging winds, very large hail and significant tornadoes occasionally occur over densely populated areas.
Mesoscale convective systems (MCSs) describe organized groupings of thunderstorms in the tropics and mid-latitudes that span thousands of square kilometres. While recognized for over a century, the advent of satellite and radar observations, as well as atmospheric-model simulations, has brought about their increased understanding. In this Review, we synthesize current knowledge on MCS formation, climatological characteristics, hazardous weather, predictive capacity and projected changes with anthropogenic warming. Driven by typical deep moist convective processes (moisture, lift and instability) and vertical wind shear, MCS formation occurs preferentially in locations where these ingredients are present and can be maintained by large-scale ascent and the cold pools that they produce. MCSs also generate hazardous weather, including extreme rainfall, flooding, derechos and, sometimes, tornadoes and hail, all of which have substantial economic and societal impacts. Given that MCSs also produce a large fraction of warm-season rainfall, there is critical need for both short-term forecasts and long-term projections, presently challenged by inadequate model resolution. Yet, with continually improving modelling capabilities, as well as greater theoretical basis, it is suggested that MCSs might increase in frequency and intensity under a warming climate. Further modelling progress, in turn, offers improved understanding of MCS characteristics, from their life cycle through to impacts.
This study analysed some characteristics of local winds, thunderstorms and their perceived effects on some farming activities in Lokoja area. The objectives were to; determine the prevailing local winds, determine some characteristics of local winds and thunderstorms and to assess their perceived effects on some farming activities in Lokoja area. Both primary and secondary data were used. The secondary data was obtained from Nigerian Meteorological Agency by earlier studies, while the primary data was obtained through the administration of 150 questionnaire to farmers in some settlements in Bassa, Lokoja, Ajaokuta, Kogi and Adavi LGAs. The Beaufort Wind scale was used to categorise the local winds. Also Thunderstorm Spectrum was used in classifying the thunderstorms. Results show the wind directions that dominated in Lokoja area as S/W and N/E with mean speed of 1.858 and 1.122 respectively. Results further indicated local winds of Lokoja as calm. It was found out that the thunderstorms fall within range of 1000-2500 J/kg with a mean of 1221.4 J/kg. This places the category of thunderstorms under ordinary with some pulse severe. However the thunderstorms cannot be dissociated from the Multicell types being at the boundary between the two. Some of the perceived effects of local winds include damages to stalks of plants, dropping of unripe fruits amongst others. The advantages of early thunderstorm include, making the soil easy for tillage and ready for seed planting, while its disadvantages include dumping of dried leaves on farm sites, surface run off leading to flash floods and so on.
Extreme precipitation events (EPEs) are meteorological phenomena of major concern for society. They can have different characteristics depending on the physical mechanisms responsible for their generation, which in turn depend on the large and mesoscale conditions. This work provides a systematic classification of EPEs over northern–central Italy, one of the regions in Europe with the highest frequency of these events. The EPE statistics have been deduced using the new high‐resolution precipitation dataset ArCIS (Climatological Archive for Central–Northern Italy), that gathers together a very high number of daily, quality‐controlled and homogenized observations from different networks of 11 Italian regions. Gridded precipitation is aggregated over Italian operational warning‐area units (WA). EPEs are defined as events in which daily average precipitation in at least one of the 94 WAs exceeds the 99th percentile with respect to the climate reference 1979–2015. A list of 887 events is compiled, significantly enlarging the database compared to any previous study of EPEs. EPEs are separated into three different dynamical classes: Cat1, events mainly attributable to frontal/orographic uplift; Cat2, events due to frontal uplift with (equilibrium) deep convection embedded; Cat3, events mainly generated by non‐equilibrium deep convection. A preliminary version of this classification is based on fixed thresholds of environmental parameters, but the final version is obtained using a more robust machine‐learning unsupervised K‐means clustering and random forest algorithm. All events are characterized by anomalously high integrated water vapour transport (IVT). This confirms IVT as an important large‐scale predictor, especially for Cat2 events, which is shown to be the most important category in terms of impacts and EPE area extension. Large IVT values are caused by upper‐level waves associated with remotely triggered Rossby wave packets, as shown for two example Cat2 events.
This article studies the life cycle of the well-organised mesoscale convective systems (MCSs)that affect Catalonia and surrounding regions, using a weather radar composite with sophisticated corrections and lightning data over a full period of five years. Nearly 350 MCSs were identified and analysed for the 2012–2016 period after applying size and duration criteria to 438,000 radar composites. MCSs are responsible for the majority of flood events in the region of interest and in many other areas around the world. We have analysed the main radar parameters and lightning properties, looking for differences between the systems depending on the season of the year. Autumn and spring show the highest frequency of MCSs, but there are considerable differences between their properties for the two seasons. More specifically, lightning activity, maximum reflectivity and duration are higher in autumn than in winter, although the total accumulated rainfall may be lower. This higher convective activity is associated with the warmer sea surface temperature of the Mediterranean and a large number of cyclones that affect the region of analysis.
Urban areas often experience high precipitation rates and heights associated with flash flood events. Atmospheric and hydrological models in combination with remote-sensing and surface observations are used to analyze these phenomena. This study aims to conduct a hydrometeorological analysis of a flash flood event that took place in the sub-urban area of Mandra, western Attica, Greece, using remote-sensing observations and the Chemical Hydrological Atmospheric Ocean Wave System (CHAOS) modeling system that includes the Advanced Weather Research Forecasting (WRF-ARW) model and the hydrological model (WRF-Hydro). The flash flood was caused by a severe storm during the morning of 15 November 2017 around Mandra area resulting in extensive damages and 24 fatalities. The X-band dual-polarization (XPOL) weather radar of the National Observatory of Athens (NOA) observed precipitation rates reaching 140 mm/h in the core of the storm. CHAOS simulation unveils the persistent orographic convergence of humid southeasterly airflow over Pateras mountain as the dominant parameter for the evolution of the storm. WRF-Hydro simulated the flood using three different precipitation estimations as forcing data, obtained from the CHAOS simulation (CHAOS-hydro), the XPOL weather radar (XPOL-hydro) and the Global Precipitation Measurement (GMP)/Integrated Multi-satellitE Retrievals for GPM (IMERG) satellite dataset (GPM/IMERG-hydro). The findings indicate that GPM/IMERG-hydro underestimated the flood magnitude. On the other hand, XPOL-hydro simulation resulted to discharge about 115 m3/s and water level exceeding 3 m in Soures and Agia Aikaterini streams, which finally inundated. CHAOS-hydro estimated approximately the half water level and even lower discharge compared to XPOL-hydro simulation. Comparing site-detailed post-surveys of flood extent, XPOL-hydro is characterized by overestimation while CHAOS-hydro and GPM/IMERG-hydro present underestimation. However, CHAOS-hydro shows enough skill to simulate the flooded areas despite the forecast inaccuracies of numerical weather prediction. Overall, the simulation results demonstrate the potential benefit of using high-resolution observations from a X-band dual-polarization radar as an additional forcing component in model precipitation simulations.
Digital GOES infrared imagery is used to document mesoscale convective complexes (MCCs) over the United States during 1985. The warmer threshold area measurement (≤ -32oC) of Maddox's original criteria has been dropped from consideration because its measurement was too subjective, and also was determined to be unnecessary. In 1985, 59 MCCs were identified; this total is approximately 20 to 40 more than in any year since 1978, when these annual summaries began. The monthly distribution and seasonal progression of MCCs in 1985 are similar to those of prior years. The enhanced MCC activity in June 1985 is associated with a persistent favorable quasi-geostrophic forcing during that period. -from Authors
Several annual mesoscale convective complex (MCC) summaries have been compiled since Maddox strictly defined their criteria in 1980. These previous studies have largely been independent of each other and therefore have not established the extended spatial and temporal patterns associated with these large, quasi-circular, and, typically, severe convective systems. This deficiency is primarily due to the difficulty of archiving enough satellite imagery to accurately record each MCC based on Maddox's criteria. Consequently, this study utilizes results from each of the MCC summaries compiled between 1978 and 1999 for the United States in order to develop a more complete climatology, or description of long-term means and interannual variation, of these storms. Within the 22-yr period, MCC summaries were compiled for a total of 15 yr. These 15 yr of MCC data are employed to establish estimated tracks for all MCCs documented and, thereafter, are utilized to determine MCC populations on a monthly, seasonal, annual, and multiyear basis. Subsequent to developing an extended climatology of MCCs, the study ascertains the spatial and temporal patterns of MCC rainfall and determines the precipitation contributions made by MCCs over the central and eastern United States. Results indicate that during the warm season, significant portions of the Great Plains receive, on average, between 8% and 18% of their total precipitation from MCC rainfall. However, there is large yearly and even monthly variability in the location and frequency of MCC events that leads to highly variable precipitation contributions.
A cluster at a given brightness temperature threshold is defined as the area covered by adjacent cloud cells with brightness temperature lower than the threshold. The clusters are classified according to the area they cover and the position of their center of mass. Results show that the convective cluster number can be approximated by a power law of the radius with an exponent around -2. This gives a nearly equal contribution of each cluster size to the mean high cloud cover for a given brightness temperature threshold. Using the visible channel (0.4-1.1μm) of Meteosat, we show that the part of the clusters with reflectance larger than 0.7 also follows a power law. -from Authors
For the first time, the temporal and spatial distributions of lightning in southern Germany are studied. Data is taken from a newly installed lightning location system (LPATS). The data base covers the years 1992-1994. Inside the observational are of 500 km × 430 km a mean value of 634,000 cloud-to-ground lightning strokes per year is detected. This corresponds to a lightning frequency of 1.2 per minute and a spatial density of 2.8 yr-1km-2. Most of the lightning events are confined to the summer months May-August with a mean number of 29 thunderstorm days per year. The diurnal cycle of lightning activity peaks at 1600-1700 local time with a subsequent slow decrease towards the minimum in the morning hours. A secondary maximum is found between 2100 and 2200 which is shown to be a propagation effect from distant thunderstorm source areas. The spatial lightning distribution confirms the well-known preference of certain areas for the development of thunderstorms. Such preferred areas are the leeward side of the Black Forest, the Allgäu and other parts of the Alpine foreland. Here lightning densities of more than 10 yr-1km-2 may occur. A comparison with routinely gathered thunderstorm data yields a good agreement.
A new precipitation climatology covering the European Alps is presented. The analysis covers the entire mountain range including adjacent foreland areas and exhibits a resolution of about 25 km. It is based on observations at one of the densest rain-gauge networks over complex topography world-wide, embracing more than 6600 stations from the high-resolution networks of the Alpine countries. The climatology is determined from daily analyses of bias-uncorrected, quality controlled data for the 20 year period 1971-1990. The daily precipitation fields were produced with an advanced distance-weighting scheme commonly adopted for the analysis of precipitation on a global scale. The paper describes the baseline seasonal means derived from the daily analysis fields. The results depict the mesoscale distribution of the Alpine precipitation climate, its relations to the topography, and its seasonal cycle. Gridded analysis results are also provided in digital form. The most prominent Alpine effects include the enhancement of precipitation along the Alpine foothills, and the shielding of the inner-Alpine valleys. A detailed analysis along a section across the Alps also demonstrates that a simple precipitation-height relationship does not exist on the Alpine scale, because much of the topographic signal is associated with slope and shielding rather than height effects. Although systematic biases associated with the rain-gauge measurement and the topographic clustering of the stations are not corrected for, a qualitative validation of the results, using existing national climatologies shows good agreement on the mesoscale. Furthermore a comparison is made between the present climatology and the Alpine sections of the global climatology of Legates and Willmott and the Greater European climatology from the Climate Research Unit (University of East Anglia). Results indicate that the pattern and magnitude of analysed Alpine precipitation critically depend upon the density of available observations and the analysis procedure adopted. (C) 1998 Royal Meteorological Society.
Infrared imagery from GOES was used to document mesoscale convective complexes (MCCs) over the United States during 1986 and 1987. A near-record 58 MCCs occured in 1986, and 44 occurred in 1987. Although these totals were above average relative to MCC numbers of the 7 years prior to 1985, seasonal distributions for both years were atypical. Results confirmed the primary importance of MCC development of strong low-level thermal forcing, as well as proper vertical phasing of favorable lower- and midtropospheric environments. A cursory survey of MCCs documented outside of the United States reveals that MCCs, or MCC-type storms, are a warm-season phenomenon of midlatitude, subtropical, and low-latitude regions around the globe. -from Authors
A climatological study of mesoscale convective complexes (MCCs) during 1983-1985 over the western Pacific region (WPR), using full-disc, enhanced infrared satellite imagery from the Japanese Geostationary Meteorological Satellite is presented. The systems are nocturnal and tend to form over or in the immediate vicinity of land. Cold-cloud shields in the Americas last for about 10h while WPR shields last about 11h. The cold-cloud-shield size distribution is similar to that of the Americas, with most systems exhibiting areas between 2×10 5 and 3×10 5km 2. The frequency of midlatitude systems peaks in late spring and early summer while low-latitude MCCs are distributed uniformly throughout the warm season. Climatologically, low-level jets of high-θ e air and upper-level diffluence are present in these zones. Tracks of WPR MCCs show that, like American systems, they typically move to the right (left in the Southern Hemisphere) of the climatological mean 700-500mb flow. The deviation from the mean flow is in the direction of the source region of highest-θ e air. A few MCCs that moved over water formed tropical storms. Likewise, a few tropical systems moved over land and formed MCCs. -from Authors
A particular class of weather system, the Mesoscale Convective Complex (MCC) is identified, defined, and contrasted with other types of convective weather systems. It is found that MCC systems frequently occur over the central United States, grow to tremendous areal extent, and often persist for periods exceeding 12 h. In addition to widespread beneficial rains, a wide variety of severe convective weather phenomena attends these systems.
The development and evolution of MCC systems is not explicitly predicted by operational numerical models even though they are shown to be organized in a distinctly non-random mode on scales that cannot be considered subgrid. The MCC is a convectively driven weather system whose physics are not yet understood, much less incorporated into operational parameterization schemes. A preliminary conceptual model of the life cycle of these systems is presented using enhanced, infrared satellite imagery in conjunction with conventional surface and radar data. The outlook for further study and ultimately for the prediction of MCC systems is encouraging since their time and space scales —coupled with their frequent occurrence over the central United States—make them highly amenable to detailed investigation.
An objective algorithm is devised which is capable of locating tropical cloud clusters at the point of initiation, tracking them to the point of dissipation and thereby accumulating statistics on their size distributions and preferred geographical locations and times of occurrence. The technique is able to account for periods of growth, mergers, splits and decay, which take place during the lifetime of an individual cluster. The size distribution of the clusters identified and tracked shows that by far most of the cumulative cloud cover was accounted for by a few very large clusters. This result implies that the vertical distribution of diabatic heating in the region of the experiment was strongly influenced by dynamical and radiative processes in the widespread stratiform cloud and precipitation areas that typify very large clusters. A pronounced diurnal cycle was characterized by a preference for the very large clusters to reach the middle of their lifetime over the sea during morning.-from Authors
Full disk infrared satellite imagery from the Indian National Satellite
System (INSAT) geostationary meteorological satellite was examined to
determine if mesoscale convective complexes (MCCs) frequent the Indian
subcontinent (ISC) region. Using western hemisphere MCC studies as a
guide, MCC criteria were defined for the ISC; 49 systems met the ISC MCC
criteria during the period April to December 1988. From this database,
characteristics of ISC MCCs were calculated and compared to the
characteristics of previously documented MCC populations.The results
indicate that the characteristics of ISC MCCs are very similar to those
of MCCs found in the Americas, China, Australia, and the western Pacific
region. In particular, the typical system is nocturnal, tends to form
over land, has a lifetime of about 9 h, and produces a cold cloud shield
area of about 3 × 105 km2. Furthermore, as
with other MCC populations, some of the systems developed into tropical
depressions. It is concluded that the systems observed over the Indian
subcontinent are essentially the same phenomena as the MCC populations
documented for other areas of the world. Their high frequency and large
rainfall production indicates that they are a fundamental component of
the monsoon and hydrological cycle in the Indian monsoon region.
Enhanced infrared satellite imagery and conventional surface and sounding data are used to document the existence and climatological characteristics of mesoscale convective complexes over midlatitude South America and in the tropical region between North and South America. It is found that MCCs occur with approximately the same frequency in mid-latitude South America as they do in mid-latitude North America and the characteristics of mid-latitude South American MCCs are similar to those of MCCs in the US. The most notable difference is that the South American systems are, on the average, about 60% larger than MCCs in the US. One of the 2 years of data that was investigated was an El Nino year. During the El Nino period the number of mid-latitude South American systems was more than double the number in the non-El Nino year. Moreover, several MCCs formed over the anomalously warm winter that appeared along the Peruvian coast. Thus, on the basis of this very small sample, there may be a direct connection between MCC activity and El Nino. -from Authors
The environmental characteristics associated with 313 significant convective events in the western Mediterranean are investigated using radiosonde ascents made in Mallorca (Spain). The events are separated into five groups, based on the observed event (hail, heavy rain, "dry" storms, storms with heavy rain, and tornadoes). Classic stability indices, as well as values of convective available potential energy and helicity, are considered for each group. These traditional convective indices appear not to provide good guidance for discriminating environments associated with each group of events. In order to classify the environments, each sounding is defined by means of 34 variables that describe the thermal and humidity vertical structure, instability, precipitable water, and helicity. A cluster analysis shows that four different vertical structures appear. Each kind of event shows preference for the environments defined by a cluster. A simple method is presented for sounding classification using the four categories obtained from the cluster analysis. The method looks for the best correlation between a particular sounding and those defined by each cluster.
The typical MCC's 10-12 h evolution displays a fairly consistent sequence of events, including the monotonic areal expansion of its anvil from its formation to its maximum size, followed by the monotonic shrinkage of the colder cloud top areas as the system weakens and dissipates. Primarily within the growth phase of this cycle, a characteristic IR signature reflects the MCC in its most intense, mesoconvective stage, which lasts ~4 h and during which the coldest cloud top area reaches its largest extent. The sample reveals several interesting tendencies: 1) smaller, less-organized systems tended to be "drier' than similar-sized but better-organized MCCs; 2) large systems were "rainier' than smaller ones; 3) large systems tended to be "rainier' in the eastern part of the sample domain than in the western part, but this was not so for small systems; and 4) the eastern systems, had a more coherent and intense core of heavy precipitation through their life cycle than the western systems. -from Authors
Digitized full-disk infrared satellite imagery from the European geostationary satellite (Meteosat) for 1986 and 1987 was used to construct a climatology of mesoscale convective complexes (MCCs) in Africa. One hundred ninety-five systems formed over Africa and its near vicinity during the two-year study period. From this database, characteristics of Africa MCCs were calculated. The results indicate that these MCCs display many of the same characteristics as those found in the Americas, the Indian subcontinent, and the western Pacific region. The systems are predominantly nocturnal and tend to form over or in the immediate vicinity of land. The average lifetime of African MCCs is about 11.5 h. The size distributions of the African systems are also extremely similar to those of the Americas, the Indian subcontinent, and the western Pacific region with most systems exhibiting areas between 2 [times] 10[sup 5] and 3 [times] 10[sup 5] km[sup 2]. The monthly frequency distribution of African systems indicates that peak activity tends to occur during the period of most intense insolation. Like the MCCs in the western Pacific region and the Americas, the African MCCs tend to propagate toward the low-level high-[theta][sub e] air that feeds the convective systems. Systems over northern Africa moved toward the west-southwest, with a few developing into tropical cyclones over the Atlantic. Systems over southeastern Africa generally moved toward the northeast and east. It is concluded that the satellite-observed systems over Africa are essentially the same phenomena as the MCC populations observed over the Americas, the Indian monsoon region, and the western Pacific region. In addition, the large number of MCCs found worldwide (approximately 300-400 per year) indicate that they may be significant contributors to the global tropospheric energy budget and hydrological cycle. 46 refs., 9 figs., 1 tab.
Precipitation from 74 mesoscale convective complexes is examined to
determine the total precipitation, areal extent, and characteristic
precipitation pattern of an average convective complex. The relationship
between the average precipitation pattern and the track of the centroid
of the satellite-observed, cold-cloud shield is determined as an aid to
forecasting. The amount and spatial distribution of precipitation during
each stage (i.e., initiation, maturation and dissipation) of the average
convective system's life cycle are presented, as well as the
precipitation patterns for systems that form in particular synoptic
environments. The precipitation characteristics of MCCs are compared to
those from 32 other convective weather systems that are similar to MCCs
but do not meet all the MCC-definition criteria.
Starting times of thunderstorms for 450 stations in the conterminuos
United States for a 25-year period were analyzed using harmonic analysis
techniques. Diurnal variations were expressed as both the time of
maximum storm occurrence and the concentration of activity around this
time. Distinct seasonal and spatial variations in diurnal activity
occur. Analysis of these variations indicates that the country can be
divided into nine thunderstorm regions. In the central states the
majority of storms occur at night, but storms are frequent at any time.
In both the east and the west there is a marked concentration of storms
in the afternoon. In the west and northeast winter storms are rare,
while along the Pacific Coast summer thunder is uncommon.
A 1961–1990 mean monthly climatology for a ‘greater European’ region extending from 32°W to 66°E and from 25° to 81°N has been constructed at a resolution of 0.5°latitude by 0.5° longitude for a suite of nine surface climate variables: minimum, maximum, and mean air temperature; precipitation totals; sunshine hours; vapour pressure; wind speed; and (ground) frost day and rain day ( > 0.1 mm) frequencies. This climatology has been constructed from observed station data distributed across the region. Station frequencies range from 936 (wind speed) to 3078 (precipitation). Over 95 per cent of these data are based on observations between 1961 and 1990 and over 90 per cent were supplied by individual national meteorological agencies (NMAs) on specific request. For four variables, some standardization of the data had to be performed because different countries supplied data under different definitions. Thus cloud cover had to be converted to sunshine hours, relative humidity to vapour pressure, air frost days to ground frost days and rain days > 1 mm to rain days > 0.1 mm.
The interpolation of the station data to the grid used elevation as one of the predictor variables and thus enabled three climate surfaces to be produced for each variable, reflecting the minimum, mean, and maximum elevation within each 0.5° by 0.5° cell. Subsets of stations were used for the interpolation of each variable, the selection being based on optimizing the spatial distribution, source priority and length of record. The accuracy of the various interpolations was assessed using validation sets of independent station data (i.e. those not used in the interpolation). Estimated mean absolute errors (MAE) ranged from under 4 per cent for vapour pressure to about 10 per cent for precipitation and up to 20 per cent for wind speed. The accuracy of the interpolated surfaces for minimum and maximum temperature was between 0.5°C and 0.8°C.
We believe these results constitute the first climatology that has been constructed for this extensive European region at such a fine spatial resolution (0.5° by 0.5°) from relatively dense station networks, for three different elevation surfaces and for a wide range of surface climate variables, all expressed with respect to a standard 30‐year period. The climatology is already being used by researchers for applications in the areas of ecosystem modelling, climate change impact assessment and climate model validation, and is available from the authors.
This paper provides an eight-year high-resolution climatology of Sahelian mesoscale convective systems (MCSs) during the summer. MCSs are defined as convective cloud clusters larger than 5000 km2. They are extensively tracked from METEOSAT full-resolution infrared images (time resolution 0.5 h and spatial resolution about 5 km). The method enables every MCS to be tracked throughout its entire lifetime. For each time step, the MCS location and its morphological and radiative characteristics are computed for three different brightness temperature thresholds. The methodology is presented, evaluated and compared with previous studies using low-resolution data.
Statistical MCS distributions, diurnal cycle and spatial variability of MCS characteristics are analysed on the basis of this high-resolution tracking. It is shown that a few large and long-lived cloud clusters contribute most of the total cloud cover. Sahelian cloud clusters propagate westward at a greater speed when very deep convection is well developed. The diurnal organization of the convection has been analysed, and has proved that the merging of MCSs is partly explained by the actual merging of independent convective entities, whereas the splitting of MCSs is mostly associated with weakening of convection. The importance of mesoscale convective complexes for the total MCS coverage has also been studied.
A global set of 714 mesoscale convective complexes is compiled and some of the common properties of the convective systems are identified and examined from a global perspective. the data set includes date of occurrence, time of first storms, initiation, maximum extent, termination, duration, cold-cloud shield areas, and tracks from initiation to termination.
It is found that the typical convective complex is nocturnal, generates a cold-cloud shield area of approximately 350 000 km2, and persists for about 10 h. the largest systems and most persistent systems tend to occur near the summer solstices. For the globe, about 400 systems occur each year, primarily over land areas. Most systems develop in favoured zones, although some activity occurs over every continent (except Antarctica) and all major oceans. the concentration of activity into favoured zones indicates that there must be special dynamic and/or thermodynamic conditions necessary for convection to organize into convective complexes.
Activity is strongly tied to the solar day, and shifts from 35°S in early January to about 50°N during the boreal summer and back to 35°S by December. Within the northern hemisphere there is a pronounced poleward migration as the jet stream shifts northward. Relatively little migration occurs in the ocean-dominated southern hemisphere where the subtropical jet remains quasi-stationary over the convective-complex regions.
The nocturnal life cycles, copious rainfall, large cloud shields, and great frequency of mesoscale convective complexes suggest that they may be significant contributors to the global hydrologic cycle and earth-system energy budget.
An automated method aimed at producing a significant European mesoscale convective system (MCS) climatology is presented. It uses Meteosat infrared window channel images and is composed of two main tools: an automated cloud‐shield tracking method and a robust method of discriminating between convective and nonconvective cloud shields.
The automated cloud‐tracking method defines cloud systems as connected sets of pixels, named ‘cells’, after temperature and area thresholding and it is based on the overlapping between cells in successive images. It handles splits and merges of cells and takes cell movement into account. It has three parameters: the temperature and area thresholds and a minimum overlapping threshold. It is concluded that it performs a correct tracking at any temperature threshold between −30 ° C and −55 ° C and for an area threshold greater than 1000 km ² , so that it allows the tracking of MCSs during most of their life cycle.
The automated discrimination between convective and non‐convective cloud shields uses a discrimination parameter based on brightness‐temperature gradients on the edges of cells, because strong values of this gradient are observed at the beginning of the life cycle of MCSs when cold anvils develop. A seasonal study, and the sensitivity of the method to the temperature threshold, are presented. The method shows significant quality during the entire warm season (from April to September): it correctly discriminates 80% of MCSs and more than 90% of the most electrically active ones, while showing a low false‐alarm rate around 8%; therefore the method seems to be useful for climatological purposes. Copyright © 2002 Royal Meteorological Society
The motion of thunderstorms in southern Germany was investigated. The thunderstorms were observed by a lightning position system during the summer months of the years 1992 1996. On average every second day thunderstorms were observed somewhere in southern Germany. In general thunderstorms approached from westerly and south-westerly directions. The average speed was 13 m s[minus sign]1. No significant relation between the occurrence of thunderstorms and the large-scale synoptic pattern described by the Grosswetterlagen (large-scale weather pattern) was found. Thunderstorms were observed during almost all Grosswetterlagen. The reduction to eight weather patterns based on the low-level flow in southern Germany showed that thunderstorms are likely when the flow has a westerly (43%) or easterly direction (20%). Three distinct groups of different lighting patterns could be identified: stationary, moving thunderstorms and thunderstorm lines. The convective available potential energy (CAPE) and the wind shear were retrieved from radio soundings from München and Stuttgart. On average CAPE was 583 J kg[minus sign]1 for stationary thunderstorms, 701 J kg[minus sign]1 for moving thunderstorms and 876 J kg[minus sign]1 for thunderstorm lines. The corresponding average bulk Richardson numbers were 37, 22 and 21. The steering level was found to be at about 6 km m.s.l. However, it should be noted that in most cases the soundings do not completely describe the local environment of thunderstorms, since radio soundings are only available twice a day.
A new precipitation climatology covering the European Alps is presented. The analysis covers the entire mountain range including adjacent foreland areas and exhibits a resolution of about 25 km. It is based on observations at one of the densest rain-gauge networks over complex topography world-wide, embracing more than 6600 stations from the high-resolution networks of the Alpine countries. The climatology is determined from daily analyses of bias-uncorrected, quality controlled data for the 20 year period 1971–1990. The daily precipitation fields were produced with an advanced distance-weighting scheme commonly adopted for the analysis of precipitation on a global scale. The paper describes the baseline seasonal means derived from the daily analysis fields. The results depict the mesoscale distribution of the Alpine precipitation climate, its relations to the topography, and its seasonal cycle. Gridded analysis results are also provided in digital form.
This paper provides for the first time an objective short-term (8
yr) climatology of African convective weather systems based on satellite
imagery. Eight years of infrared International Satellite Cloud Climatology
Project-European Space Agency's Meteorological Satellite (ISCCP-Meteosat)
satellite imagery has been analyzed using objective feature identification,
tracking, and statistical techniques for the July, August, and September
periods and the region of Africa and the adjacent Atlantic ocean.
This allows various diagnostics to be computed and used to study
the distribution of mesoscale and synoptic-scale convective weather
systems from mesoscale cloud clusters and squall lines to tropical
cyclones. An 8-yr seasonal climatology (1983-90) and the seasonal
cycle of this convective activity are presented and discussed. Also
discussed is the dependence of organized convection for this region,
on the orography, convective, and potential instability and vertical
wind shear using European Centre for Medium-Range Weather Forecasts
reanalysis data.
This method taken into account eventual separation or merging of clouds. Results obtained in several cases not only demonstrate the capability of the method to perform correct tracking in different situations, but also show that objective determination of parameters, such as propagation speeds, system area, and volume index, is possible. The analysis of the time evolution of these last parameters allows a clear characterization of the cloud life cycle with its growing and decreasing stages, which may be useful for improving precipitation-estimation methods based only on cold-cloud occurrences or cloud-top temperatures. -from Authors
A study aimed at examining the size distributions of cloud clusters and the temperature distributions within them as functions of location, year, phase of the ISV, and time of day is considered. IR satellite images of tropical convection over the oceanic warm pool were processed to reveal cloud clusters, connected areas with cloud-top temperatures lower than a given threshold value. Results were obtained for a very cold threshold (208 K), corresponding roughly to the radar echo area within Australian monsoon cloud clusters, and for a moderately cold threshold (235 K) frequently used in climatic rainfall estimation.
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