Article

ELEVATED COLD-SECTOR SEVERE THUNDERSTORMS: A PRELIMINARY STUDY

Authors:
To read the full-text of this research, you can request a copy directly from the author.

Abstract

A preliminary study of atmospheric conditions in the vicinity of severe thunderstorms that occurred in the cold sector, north of east-west frontal boundaries, is presented. Upper-air sound-ings, suiface data and PCGRIDDS data were collected and analyzed from a total of eleven cases from April 1992 through April 1994. The selection criteria necessitated that a report occur at least fifty statute miles north of a well-defined frontal boundary. A brief climatology showed that the vast majority of reports noted large hail (diameter: 1.00-1.75 in.) and that the first report of severe thunderstorms occurred on an average of 150 miles north of the frontal boundary. Data from 22 proximity soundings from these cases revealed a strong baro-clinic environment with strong vertical wind shear and warm air advection from just above the suiface through 500 mb. This advection was reasoned to provide a constant source for destabilization from lifting above the frontal inversion. Convective instability was noted in all cases above the bound-ary layer with stability indices revealing the most unstable parcel located near the 850-mb layer. Despite quite cool and stable suiface conditions, CAPE, best lifted index and total-totals index values suggested at least a marginal degree of instability was required for cold-sector, severe thunderstorm development. After examining the proximity soundings, PCGRIDDS data were then analyzed to determine whichfore-cast fields from the ETA model best delineated cold-sector, severe thunderstorm development. Reports of severe weather occurred VelY near the ETA model forecast, 850-mb, warm air advection maximum. In addition, a majority of reports occurred along the axis of strongest 850-mb theta-e advection. Con-structed cross-sections normal to isotherms or thickness con-tours showed where areas of elevated or slightly sloped theta-e sUlfaces were located above the frontal suiface. These areas of potential convective instability combined with upward verti-cal velocity fields correlated well with the location of subse-quent severe thunderstorm reports.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the author.

... [1] An early climatology of such thunderstorm events above a frontal surface by [2] showed that such storms typically occurred poleward of a surface boundary (often a warm front). While surface weather induced by elevated convection is most commonly associated with heavy rainfall [3][4][5][6], some studies have indicated that severe hail, winds, and even tornadoes have been observed with elevated thunderstorms [7][8][9]. Grant [7] found 11 cases of elevated convection producing severe weather over a two-year period, while [8] extended this study to five years. Of the 11 cases found in [7], most were hail. ...
... While surface weather induced by elevated convection is most commonly associated with heavy rainfall [3][4][5][6], some studies have indicated that severe hail, winds, and even tornadoes have been observed with elevated thunderstorms [7][8][9]. Grant [7] found 11 cases of elevated convection producing severe weather over a two-year period, while [8] extended this study to five years. Of the 11 cases found in [7], most were hail. ...
... Grant [7] found 11 cases of elevated convection producing severe weather over a two-year period, while [8] extended this study to five years. Of the 11 cases found in [7], most were hail. In comparison, [8] found 129 severe elevated cases in which 59% of the reports were hail, 37% were wind, and 4% were tornadoes. ...
Article
Full-text available
A 10-year study of elevated severe thunderstorms was performed using The National Centers for Environmental Information Storm Events Database. A total of 80 elevated thunderstorm cases were identified, verified, and divided into “Prolific” and “Marginal” classes. These severe cases occurred at least 80 km away from, and on the cold side of, a surface boundary. The downdraft convective available potential energy (DCAPE), downdraft convective inhibition (DCIN), and their ratio are tools to help estimate the potential for a downdraft to penetrate through the depth of a stable surface layer. The hypothesis is that as the DCIN/DCAPE ratio decreases, there exists enhanced possibility of severe surface winds. Using the initial fields from the Rapid Refresh numerical weather prediction model, datasets of DCIN, DCAPE, and their ratio were created. Mann-Whitney U tests on the Prolific versus Marginal case sets were undertaken to determine if the DCAPE and DCIN values come from different populations for the two different case sets. Results show that the Prolific cases have values of DCIN closer to zero, suggesting the downdraft is able to penetrate to the surface causing severe winds. Thus, comparing DCIN and DCAPE is a viable tool in determining if downdrafts will reach the surface from elevated thunderstorms.
... Sometimes elevated convection produces severe weather in the form of large hail, strong winds, and/or tornadoes (e.g., Johns and Doswell 1992). Grant (1995) conducted a preliminary study on elevated severe convection, examining eleven cases over two years. He found convective instability above the shallow, but strong, inversion in the proximity soundings for each event. ...
... He found convective instability above the shallow, but strong, inversion in the proximity soundings for each event. Grant (1995) also noted that the majority of events were large hail-producing storms. ...
... ----------------------------------------* Corresponding author address: Katherine L. Horgan, 4013 Grand Manor Court, Apartment 302, Raleigh, NC 27612;email: klhorgan@ncsu.edu In addition to Colman (1990a) and Grant (1995), several studies have also been performed on specific events of elevated severe convective storms (e.g., Schmidt and Cotton 1989;Bernardet and Cotton 1998;Banacos and Schultz 2005). However, to date, an in-depth study does not exist that examines when, where, and how often these elevated convective events produce severe weather. ...
Article
Full-text available
A 5-yr climatology of elevated severe convective storms was constructed for 1983-87 east of the Rocky Mountains. Potential cases were selected by finding severe storm reports on the cold side of sur- face fronts. Of the 1826 days during the 5-yr period, 1689 (91%) had surface fronts east of the Rockies. Of the 1689 days with surface fronts, 129 (8%) were associated with elevated severe storm cases. Of the 1066 severe storm reports associated with the 129 elevated severe storm cases, 624 (59%) were hail re- ports, 396 (37%) were wind reports, and 46 (4%) were tornado reports. A maximum of elevated se- vere storm cases occurred in May with a secondary maximum in September. Elevated severe storm cases vary geographically throughout the year, with a maximum over the south-central United States in winter to a central and eastern U.S. maximum in spring and summer. A diurnal maximum of elevated severe storm cases occurred at 2100 UTC, which coincided with the diurnal maximum of hail reports. The wind reports had a broad maximum during the daytime. Because the forecasting of hail from elevated storms typically does not pose as significant a forecast challenge as severe wind for forecasters and torna- does from elevated storms are relatively uncommon, this study focuses on the occurrence of severe wind from elevated storms. Elevated severe storm cases that produce only severe wind reports occurred roughly 5 times a year. To examine the environments associated with cases that produced severe winds only, five cases were examined in more detail. Common elements among the five cases included elevated convective available potential energy, weak surface easterlies, and shallow near-surface stable layers (less than 100 hPa thick).
... Reference [1] used three criteria to discriminate elevated thunderstorm station reports from surface-based thunderstorm station reports: (1) the observation must lie on the cold side of an analyzed front that shows a clear contrast in temperature, dew point temperature, and wind; (2) the station's wind, temperature, and dew point temperature must be qualitatively similar to the immediately surrounding values; and (3) the surface air on the warm side of the analyzed front must have a higher equivalent potential temperature ( ) than the air on the cold side of the front. These three criteria have been incorporated into several studies and climatologies to evaluate elevated thunderstorms (e.g., [3][4][5][6][7]). ...
... The two most common severe weather threats associated with elevated thunderstorms are heavy rainfall leading to flash flooding and hail [3,[8][9][10]. This study focuses on elevated thunderstorms that produce heavy rainfall in the geographic region encompassing the elevated thunderstorm occurrence maximum found by [1], but with a relaxed set of selection criteria as alluded to by [8]. ...
Article
Full-text available
Composite analyses of the atmosphere over the central United States during elevated thunderstorms producing heavy rainfall are presented. Composites were created for five National Weather Service County Warning Areas (CWAs) in the region. Events studied occurred during the warm season (April–September) during 1979–2012. These CWAs encompass the region determined previously to experience the greatest frequency of elevated thunderstorms in the United States. Composited events produced rainfall of >50 mm 24 hr ⁻¹ within the selected CWA. Composites were generated for the 0–3 hr period prior to the heaviest rainfall, 6–9 hours prior to it, and 12–15 hours prior to it. This paper focuses on the Pleasant Hill, Missouri (EAX) composites, as all CWA results were similar; also these analyses focus on the period 0–3 hours prior to event occurrence. These findings corroborate the findings of previous authors. What is offered here that is unique is (1) a measure of the interquartile range within the composite mean fields, allowing for discrimination between variable fields that provided a strong reliable signal, from those that may appear strong but possess large variability, and (2) composite soundings of two subclasses of elevated thunderstorms. Also, a null case (one that fits the composite but failed to produce significant rainfall) is also examined for comparison.
... Various stability parameter fields will also be examined. The authors will show that the thunderstorms of 6 June 1993 were not rooted in the planetary boundary layer, as is usually the case with warm season convection (Colman 1990a,b;Grant 1995), but were associated with an elevated layer of convective instability. The evolution of the MCS will be presented via GOES satellite and WSR-88D imagery. ...
... In a study of elevated severe thunderstorms, Grant (1995) noted that these cool sector storms tend not to be as destructive as those that develop in the warm sector, with large hail being the primary threat associated with these storms. While this may be true for severe thunderstorms, it is indeed possible for elevated convection to result in excessive rainfall and flash flooding, as was the case on 6 June 1993. ...
Article
Full-text available
A mesoscale convective system (MCS) developed during the morning hours of 6 June 1993 and moved across northern and central Missouri, resulting in a narrow swath of excessive rainfall (.150 mm). The MCS developed well north of a surface warm front above a cool, stable boundary layer and moved east-southeast across the state. Although some features of the synoptic environment agree with the frontal flash flood composite model, predicting the elevated thunderstorms that composed the MCS posed a unique forecasting challenge. This paper first describes the diagnostic parameters of the prestorm environment that would have been helpful to predict the initiation of the MCS and the resultant locally excessive precipitation. Attention is then drawn to the MCS itself via IR satellite and WSR-88D imagery. Finally, the similarities and differences of this episode to previous studies of flash flooding and elevated thunderstorms are noted, and a summary of key parameters useful in the anticipation of this type of convection and associated heavy rainfall are offered.
... 714 and 816). Deep elevated convection, generally in the form of thunderstorms, can produce excessive rainfall, hail, and occasionally damaging surface winds and tornadoes in areas well removed from strong surface-based instability (e.g., Branick et al. 1988;Schmidt and Cotton 1989;Colman 1990a,b;Neiman et al. 1993;Grant 1995;Bernardet and Cotton 1998;Moore et al. 1998Moore et al. , 2003Banacos and Schultz 2005;Goss et al. 2006;Colby and Walker 2007;Horgan et al. 2007), as well as lightninginitiated wildfires in the western United States (Tardy 2007). ...
... Areas near warm and stationary fronts are frequent sites of deep elevated convection. As previously noted, such convection is of considerable interest given its potential to produce severe weather, even well within the cold air (e.g., Neiman et al. 1993;Grant 1995;Trapp et al. 2001;Horgan et al. 2007). Figure 2c shows elevated thunderstorms forming about 150 km north of a slowly moving warm front. ...
Article
Full-text available
The term elevated convection is used to describe convection where the constituent air parcels originate from a layer above the planetary boundary layer. Because elevated convection can produce severe hail, damaging surface wind, and excessive rainfall in places well removed from strong surface-based instability, situations with elevated storms can be challenging for forecasters. Furthermore, determining the source of air parcels in a given convective cloud using a proximity sounding to ascertain whether the cloud is elevated or surface based would appear to be trivial. In practice, however, this is often not the case. Compounding the challenges in understanding elevated convection is that some meteorologists refer to a cloud formation known as castellanus synonymously as a form of elevated convection. Two different definitions of castellanus exist in the literature-one is morphologically based (cloud formations that develop turreted or cumuliform shapes on their upper surfaces) and the other is physically based (inferring the turrets result from the release of conditional instability). The terms elevated convection and castellanus are not synonymous, because castellanus can arise from surface-based convection and elevated convection exists that does not feature castellanus cloud formations. Therefore, the purpose of this paper is to clarify the definitions of elevated convection and castellanus, fostering a better understanding of the relevant physical processes. Specifically, the present paper advocates the physically based definition of castellanus and recommends eliminating the synonymity between the terms castellanus and elevated convection.
... 714 and 816). Deep elevated convection, generally in the form of thunderstorms, can produce excessive rainfall, hail, and occasionally damaging surface winds and tornadoes in areas well removed from strong surface-based instability (e.g., Branick et al. 1988;Schmidt and Cotton 1989;Colman 1990a,b;Neiman et al. 1993;Grant 1995;Bernardet and Cotton 1998;Moore et al. 1998Moore et al. , 2003Banacos and Schultz 2005;Goss et al. 2006;Colby and Walker 2007;Horgan et al. 2007), as well as lightninginitiated wildfires in the western United States (Tardy 2007). ...
... Areas near warm and stationary fronts are frequent sites of deep elevated convection. As previously noted, such convection is of considerable interest given its potential to produce severe weather, even well within the cold air (e.g., Neiman et al. 1993;Grant 1995;Trapp et al. 2001;Horgan et al. 2007). Figure 2c shows elevated thunderstorms forming about 150 km north of a slowly moving warm front. ...
... These environments in Europe are often associated with "Spanish plume" synoptic setups (Carlson and Ludlam 1968;Van Delden 2001, Mathias et al. 2017). On the other hand, stronger convective inhibition during extreme thunderstorms is also pronounced at night (medians around -20 J kg -1 ), which may correspond to well organised, elevated thunderstorms (Colman 1990;Grant 1995). These thunderstorms initiate above a stable layer without being influenced by strong mixed-layer or surface based convective inhibition. ...
Article
The relationship between convective parameters derived from ERA5 and cloud-to-ground (CG) lightning flashes from the PERUN network in Poland was evaluated. All flashes detected between 2002–2019 were divided into intensity categories based on a peak 1-minute CG lightning flash rate and were collocated with proximal profiles from ERA5 to assess their climatological variability. Thunderstorms in Poland are the most frequent in July, between 1400–1600 UTC and over the southeastern parts of the country. The highest median of MUCAPE for CG lightning flash events is from June to August, between 1400–1600 UTC (around 900 J kg ⁻¹ ), whereas patterns in 0–6 km wind shear (DLS) are reversed, with the highest median values during winter and night (around 25 m s ⁻¹ ). The best overlap of MUCAPE and DLS (MUWMAXSHEAR parameter) is in July–August, typically between 1400–2000 UTC with median values of around 850 m ² s ⁻² . Thunderstorms in Poland are the most frequent in MUCAPE below 1000 J kg ⁻¹ , and DLS between 8–15 m s ⁻¹ . Along with increasing MUCAPE and DLS, peak CG lightning flash rates increase as well. Compared to DLS, MUCAPE is a more important parameter in forecasting any lightning activity, but when these two are combined together (MUWMAXSHEAR) they are more reliable in distinguishing between thunderstorms producing small and high CG lightning flash rates. Our results also indicate that higher CG lightning flash rates result in thunderstorms more frequently associated with severe weather reports (hail, tornado, wind).
... For example, so-called elevated supercells are widely assumed to pose a greatly diminished tornado threat compared to surface-based supercells (Davies 2004). [An elevated storm (e.g., Colman 1990a,b;Grant 1995;Moore et al. 1998Moore et al. , 2003 is defined herein as one that draws its inflow from a layer not in contact with the surface, at least not in close proximity to the storm's updraft, because of the presence of an underlying, surfacebased, cold air mass that is independent of the storm's precipitation region.] Supercell environments characterized by relatively large surface-based CIN are conducive to elevated supercells because in such cases the dynamic vertical pressure gradient may not be able to lift negatively buoyant air parcels from the surface to their level of free convection (LFC), if the parcels near the surface even have an LFC. ...
Article
This paper uses idealized numerical simulations to investigate the dynamical influences of stable boundary layers on the morphology of supercell thunderstorms, especially the development of low-level rotation. Simulations are initialized in a horizontally homogeneous environment with a surface-based stable layer similar to that found within a nocturnal boundary layer or a mesoscale cold pool. The depth and lapse rate of the imposed stable boundary layer, which together control the convective inhibition (CIN), are varied in a suite of experiments. When compared with a control simulation having little surface-based CIN, each supercell simulated in an environment having a stable boundary layer develops weaker rotation, updrafts, and downdrafts at low levels; in general, low-level vertical vorticity and vertical velocity magnitude decrease as initial CIN increases (changes in CIN are due only to variations in the imposed stable boundary layer). Though the presence of a stable boundary layer decreases low-level updraft strength, all supercells except those initiated over the most stable boundary layers had at least some updraft parcels with near-surface origins. Furthermore, the existence of a stable boundary layer only prohibits downdraft parcels from reaching the lowest grid level in the most stable cases. Trajectory and circulation analyses indicate that weaker near-surface rotation in the stable-layer scenarios is a result of the decreased generation of circulation coupled with decreased convergence of the near-surface circulation by weaker low-level updrafts. These results may also suggest a reason why tornadogenesis is less likely to occur in so-called elevated supercell thunderstorms than in surface-based supercells.
... Storms that occur at this time are dynamically distinct from the lessfrequent nocturnal events, which tend to occur in association with elevated convection and lesser buoyancy (Wallace 1975;Colman 1990). Because of the suboptimal sampling interval of radiosondes (1200 UTC is after the peak time of such storms) and the lesser frequency of severe convection at these times (Grant 1995;Branick 2005;Horgan et al. 2007), only afternoon-evening storms associated with the 0000 UTC sounding environments are considered in this study. As such, only surface-based CAPE is considered. ...
Article
To date, neither observational studies nor direct climate model simulations have been able to document trends in the frequency or severity of deep moist convection associated with global climate change. The lack of such evidence is not unexpected as the observational record is insufficiently long and computational limitations prevent modeling at the scales necessary to simulate explicitly such phenomena. Nonetheless, severe deep moist convection represents an important aspect of regional climate, particularly in the central United States, where damage, injuries, and fatalities are a frequent result of such phenomena. Accordingly, any comprehensive assessment of the regional effects of climate change must account for these effects. In this work, the authors present a “perfect prog” approach to estimating the potential for surface-based convective initiation and severity based upon the large-scale variables well resolved by climate model simulations. This approach allows for the development of a stable estimation scheme that can be applied to any climate model simulation, presently and into the future. The scheme is applied for the contiguous United States using the output from the Parallel Climate Model, with the Intergovernmental Panel on Climate Change third assessment A2 (business as usual) as input. For this run, relative to interannual variability, the potential frequency of deep moist convection does not change, but the potential for severe convection is found to increase east of the Rocky Mountains and most notably in the “tornado alley” region of the U.S. Midwest. This increase in severe potential is mostly tied to increases in thermodynamic instability as a result of ongoing warm season surface warming and moistening. Finally, approaches toward improving such estimation methods are briefly discussed.
... Galway and Pearson ( 1981 ) focused on winter tornadoes in the central United States that usually had widespread blizzards, heavy snow, and / or extensive glazing on the cold side of the weather systems. Grant ( 1995 ) studied severe spring thunderstorms in the cold sector north of fronts without coincident frozen precipitation at the ground. Marwitz and Toth ( 1993 ) determined the synoptic-scale kinematic fields and radar structure of a warm-frontal snowband over Oklahoma ahead of a surface cold front; thunder was observed but was not the focus of the study. ...
Article
Full-text available
Network-detected cloud-to-ground lightning coincident with mainly frozen precipitation (freezing rain, sleet, snow) was studied over the central United States during two outbreaks of arctic air in January 1994. During the first event, the ratio of positive to total flashes was 59%, flashes were few and disorganized in area, and no surface observer reported thunder. For the other event the ratio was 52% during the first few hours in subfreezing surface air, then decreased when flashes formed in the nearby region above freezing. Also, flashes in this case were linearly aligned and coincided with conditional symmetric instability; thunder was heard infrequently by surface observers. On radar, reflectivity cores grew from weak to moderate intensity within a few hours of the lightning during both cases. Echo area increased greatly before flashes in one case, while the area increase coincided with flashes in the other. Some base-scan reflectivities were strong in both thunderstorm regions due to the radar beam intersecting the melting level. Regions with lightning often could be identified better by high echo tops than reflectivity. Analyses on the scale of one or two states diagnosed the strength of low-level warming that contributed to formation of thunderstorms and significant frozen precipitation. Quasigeostrophic analyses showed that 850-mb temperature advection and 850-500-mb differential vorticity advection were similar in magnitude in the lightning area during both events. Once convection formed, lightning and echo-top information identified downstream regions with a potential for subsequent frozen precipitation.
... Among challenging problem areas investigated were tornadoes associated with tropical cyclones (Weiss 1985;Ostby and Weiss 1993;Vescio et al. 1996), northwest flow and derechos (Johns , 1984Johns and O S T B Y FIG. 9. A convective outlook from 1996. Contrast the detailed reasoning contained herein with that of Fig. 8. Hirt 1987), bow echoes , winter tornadoes (Galway and Pearson 1981), cold-sector severe thunderstorms (Grant 1995), storm-relative winds and helicity (Kerr and Darkow 1996;Thompson 1998), and tornadoes associated with boundaries (Rogash 1995). Regional studies to improve understanding and prediction of severe weather in various geographical areas included the following: Los Angeles Basin (Hales 1985), North Dakota (Hirt 1985), the Northwest (Evenson and Johns 1995), the Southeast (Anthony 1988), and the Northeast (Johns and Dorr 1996). ...
Article
The purpose of this paper is to review the large strides made in tornado and severe thunderstorm forecasting by the Severe Local Storms Unit (SELS) of the National Severe Storms Forecast Center during the last 25 years or so of its existence. The author compares and illustrates the tools available to the SELS forecasters in the early 1970s versus those of the 1990s. Also discussed is the transition over the years from a largely empirical forecast approach to an approach based strongly on physical reasoning. The evolution of the computer systems employed at SELS and their impact on the forecast operation are traced. With the advent of interactive computer processing capability, SELS forecasters were able to assess the potential for severe convection with much greater precision than ever before. Noteworthy was the improvement brought about by the automation of largely clerical tasks, allowing the forecasters more time to focus on the forecast problem at hand. In addition, the forecast staff was able to devote more time to relevant research projects and benefit from the significant advances taking place in improved understanding of mesoscale processes. Verification results are shown to validate the notion that these advances led to better predictions. For example, the watch accuracy in terms of percent of severe weather watches verified rose from 63% in 1975 to 90% in 1996. Finally, information is given showing important milestone in the history of SELS and a list of the lead forecasters whose experience, judgment, and forecast skill brought about these improvements.
... In fact, he recorded over 725 reports of elevated thunderstorms for April– September for this 4-yr period. Thus, many warm-season MCSs are composed of elevated thunderstorms, which can produce heavy rainfall and even large hail (Grant 1995). Junker et al. (1995) also note that the majority of the summer-1993 heavy-rainfall events had many similarities to elevated thunderstorms, as described by Colman (1990a,b). ...
Article
Twenty-one warm-season heavy-rainfall events in the central United States produced by mesoscale convective systems (MCSs) that developed above and north of a surface boundary are examined to define the environmental conditions and physical processes associated with these phenomena. Storm-relative composites of numerous kinematic and thermodynamic fields are computed by centering on the heavy-rain-producing region of the parent elevated MCS. Results reveal that the heavy-rain region of elevated MCSs is located on average about 160 km north of a quasi-stationary frontal zone, in a region of low-level moisture convergence that is elongated westward on the cool side of the boundary. The MCS is located within the left-exit region of a south-southwesterly low-level jet (LLJ) and the right-entrance region of an upper-level jet positioned well north of the MCS site. The LLJ is directed toward a divergence maximum at 250 hPa that is coincident with the MCS site. Near-surface winds are light and from the southeast within a boundary layer that is statically stable and cool. Winds veer considerably with height (about 140°) from 850 to 250 hPa, a layer associated with warm-air advection. The MCS is located in a maximum of positive equivalent potential temperature θe advection, moisture convergence, and positive thermal advection at 850 hPa. Composite fields at 500 hPa show that the MCS forms in a region of weak anticyclonic curvature in the height field with marginal positive vorticity advection. Even though surface-based stability fields indicate stable low-level air, there is a layer of convectively unstable air with maximum-θe CAPE values of more than 1000 J kg-1 in the vicinity of the MCS site and higher values upstream. Maximum-θe convective inhibition (CIN) values over the MCS centroid site are small (less than 40 J kg-1) while to the south convection is limited by large values of CIN (greater than 60 J kg-1). Surface-to-500-hPa composite average relative humidity values are about 70%, and composite precipitable water values average about 3.18 cm (1.25 in.). The representativeness of the composite analysis is also examined. Last, a schematic conceptual model based upon the composite fields is presented that depicts the typical environment favorable for the development of elevated thunderstorms that lead to heavy rainfall.
... To focus on the environments that are to a greater or lesser extent associated with atmospheric instability, the authors decided to include only soundings with the most unstable lifted index (MU LI) parameter (the measure of the atmospheric instability; Galway, 1956;Grant and Prentice, 1995;Craven et al., 2002;Manzato and Morgan, 2003) amounting b2. The value of 2 was chosen in order to obtain a relatively meaningful number of soundings since the threshold value of 0 would exclude too many cases. ...
Article
While thunderstorms in equatorial and mid-latitudes are well documented, little is known about their presence in high latitudes. There are barely a few studies on this phenomenon analyzing their occurrence in the European Arctic region. In an attempt to rectify this situation authors aim to explain which conditions are conducive to their formation in Bjørnøya, Jan Mayen and Svalbard islands. A total of 41 thunderstorm days derived from SYNOP reports from the period of 1981-2010 were used to define thunderstorm-favorable synoptic conditions from NCEP/NCAR reanalyses and sounding data. In order to underline seasonal variation, anomalies were presented in the polar day and polar night timeframes. As it turned out polar night thunderstorms occur most often in situations with southern warm marine air advections intensified by the positive North Atlantic and Arctic Oscillations. Thunderstorms in this season are characterized by steep vertical lapse rates and occur most likely at the cold fronts. Polar day thunderstorms form when warm air masses move from the continental north-eastern Europe to the Arctic, and create unstable conditions. In this type, thunderstorms are generated by elevated convection and occur most likely in a cyclone’s cool side of warm sector.
... Some other supercells derive a part of their inflow from above the surface, and these are referred to as elevated supercells (e.g., Grant 1995;Calianese et al. 1996;Corfidi et al. 2008). Accordingly, the air near the ground may have little relevance to forecasting the motion of these storms, but not necessarily in all cases (Nowotarski et al 2011). ...
Article
Full-text available
The 0–6-km AGL mean wind has been used widely in operations to predict supercell motion. However, when a supercell is low-topped or elevated, its motion may be poorly predicted with this default mean wind—which itself could be height-based or pressure-weighted. This information suggests that a single, fixed layer is inappropriate for some situations, and thus various mean-wind parameters are explored herein. A dataset of 583 observed and 829 Rapid Update Cycle supercell soundings was assembled. When the mean wind is computed using pressure weighting for an effective inflow-layer as the base, and 65% of the most-unstable equilibrium level height as the top, the result is that better supercell motion predictions can be obtained for low-topped and elevated supercells. Such a mean-wind modification would come at the cost of only a minor increase in mean absolute error for the entire sample of supercell cases considered.
... The studies of [1,2] defined an elevated thunderstorm via the following selection criteria based on observations from stations reporting a thunderstorm: (1) the observation must lie on the cold side of an analyzed front that shows a clear contrast in temperature, dew point, and wind; (2) the station's wind, temperature, and dew point must be qualitatively similar to the immediately surrounding values; and (3) the surface air on the warm side of the analyzed front must have a higher equivalent potential temperature (θ e ) than the air on the cold side of the front. Similar criteria were also used by [3][4][5]19,20] for studies involving elevated thunderstorms and will be used for this study as well. ...
Article
Full-text available
There are differences in the character of surface-based and elevated convection, and one type may pose a greater threat to life or property. The lightning and rainfall characteristics of eight elevated and eight surface-based thunderstorm cases that occurred between 2007 and 2010 over the central Continental United States were tested for statistical differences. Only events that produced heavy rain (>50.8 mm·day−1) were investigated. The nonparametric Mann–Whitney test was used to determine if the characteristics of elevated thunderstorm events were significantly different than the surface based events. Observations taken from these cases include: rainfall–lightning ratios (RLR) within the heavy rain area, the extent of the heavy rainfall area, cloud-to-ground (CG) lightning flashes, CG flashes·h−1, positive CG flashes, positive CG flashes·h−1, percentage of positive CG flashes within the heavy rainfall area, and maximum and mean rainfall amounts within the heavy rain area. Results show that elevated convection cases produced more rainfall, total CG lightning flashes, and positive CG lightning flashes than surface based thunderstorms. More available moisture and storm morphology explain these differences, suggesting elevated convection is a greater lightning and heavy rainfall threat than surface based convection.
... He further refined this definition to include that observations must lie on the cold side of an analyzed front with clear contrast in the mass and momentum fields with surrounding stations recording similar conditions. Similar criteria were also used by Grant [13], Rochette and Moore [1], Moore et al. [14], Moore et al. [3], and McCoy et al. [4] for studies involving elevated thunderstorms. However, this definition is rather specific, and, while being useful in determining if a given cell is surface-based or elevated by way of synoptic map interrogation, it falls short in defining elevated convection in scenarios where a boundary is not well-defined. ...
Article
Full-text available
The Program for Research on Elevated Convection with Intense Precipitation (PRECIP) field campaign sampled 10 cases of elevated convection during 2014 and 2015. These intense observing periods (IOP) mostly featured well-defined stationary or warm frontal zones, over whose inversion elevated convection would form. However, not all frontal zones translated as expected, with some poleward motions being arrested and even returning equatorward. Prior analyses of the observed data highlighted the downdrafts in these events, especially diagnostics for their behavior: the downdraft convective available potential energy (DCAPE) and the downdraft convective inhibition (DCIN). With the current study, the DCAPE and DCIN are examined for four cases: two where frontal motion proceeded poleward, as expected, and two where the frontal motions were slowed significantly or stalled altogether. Using the Weather Research and Forecasting (WRF) model, a multi-model ensemble was created for each of the four cases, and the best performing members were selected for additional deterministic examination. Analyses of frontal motions and surface cold pools are explored in the context of DCAPE and DCIN. These analyses further establish the DCAPE and DCIN, not only as a means to classify elevated convection, but also to aid in explaining frontal motions in the presence of elevated convection.
... In the United States, elevated thunderstorms are climatologically favoured to occur in the spring and fall with a primary maximum in April and secondary maximum in September; they are more widespread in geographical coverage in the spring (March to June). The significant weather associated with elevated thunderstorms is typically heavy rain (Rochette & Moore, 1996) or heavy snow and often large hail (Grant, 1995). Typically, the statically stable boundary layer precludes the occurrence of tornadoes or strong straight-line winds from downbursts. ...
Article
Two case studies in which elevated thunderstorms played an important role in enhancing precipitation totals are discussed. During the period 20–22 February 1993, elevated thunderstorms over Iowa produced a mesoscale band of snow with amounts in excess of 25 cm across north central Iowa. Diagnosis of the environment revealed an elevated layer of convective instability between 700 and 540 hPa above a well-defined frontal zone. Elevated, upright convection resulted from the release of the convective instability as air parcels, ascending isentropically over the frontal zone, reached saturation. A second case, in which heavy rain (greater than 50 mm in 24 h) fell over parts of Oklahoma, Kansas and Missouri during the period 27–28 April 1994, is also examined. Once again, elevated thunderstorms resulted from the ascent of an elevated layer of convective instability over a strong baroclinic zone. Positive CAPE values are found by lifting air parcels having the maximum equivalent potential temperature in the lower portion of the troposphere, whereas no available energy is diagnosed for surface parcels. While both cases strongly resemble the climatology of elevated thunderstorms, these case studies suggest that convective instability aloft released by isentropic ascent, rather than frontogenetical forcing in the presence of weak symmetric stability, can result in heavy precipitation.
Article
A brief case study is provided of a striking example of vertical and horizontal jet coupling associated with the upper jet level as well as upper- and lower-level jet coupling; these jet interactions also supported the development of elevated convection and led to flash flooding. The case constitutes a faithful, real-world verification of what has been suggested in idealized conceptual models by several investigators for similar situations of elevated convection with excessive precipitation.
Article
Numerical weather prediction models often fail to correctly forecast convection initiation (CI) at night. To improve our understanding of such events, researchers collected a unique dataset of thermodynamic and kinematic remote sensing profilers as part of the Plains Elevated Convection at Night (PECAN) experiment. This study evaluates the impacts made to a nocturnal CI forecast on 26 June 2015 by assimilating a network of atmospheric emitted radiance interferometers (AERIs), Doppler lidars, radio wind profilers, high-frequency rawinsondes, and mobile surface observations using an advanced, ensemble-based data assimilation system. Relative to operational forecasts, assimilating the PECAN dataset improves the timing, location, and orientation of the CI event. Specifically, radio wind profilers and rawinsondes are shown to be the most impactful instrument by enhancing the moisture advection into the region of CI in the forecast. Assimilating thermodynamic profiles collected by the AERIs increases midlevel moisture and improves the ensemble probability of CI in the forecast. The impacts of assimilating the radio wind profilers, AERI retrievals, and rawinsondes remain large throughout forecasting the growth of the CI event into a mesoscale convective system. Assimilating Doppler lidar and surface data only slightly improves the CI forecast by enhancing the convergence along an outflow boundary that partially forces the nocturnal CI event. Our findings suggest that a mesoscale network of profiling and surface instruments has the potential to greatly improve short-term forecasts of nocturnal convection.
Article
Full-text available
This document deals primarily with fundamental aspects of convective meteorology. In order to present storm-scale meteorology, a wide range of topics are considered. Certain subjects, like precipitation physics, are examined at some length in order to provide the basic physical understanding of how thunderstorms work. A classification scheme is introduced for convective storms. This scheme allows the reader to make certain physical distinctions in the way storms are organized and will allow one to infer how the storms behave. Originator-supplied keywords: Meteorology; Forecasting; Analysis; Convective weather; Mesoanalysis; Upper air data; Entrainment; Thermodynamics; Thunderstorms; Wind shear; Severe storm classes; Surface data; Hail; Computer simulations; Severe weather numerical guidance; Precipitation processes; Thunderstorm electricity; Tornadoes.
Article
The second of two papers describing thunderstorms that occur above frontal surfaces, frequently in environments without positive convective available potential energy (CAPE), focuses on an impressive outbreak, of elevated thunderstorms during AVE-SESAME I. It is shown that the thunderstorms occurred in three convective impulses, each of which developed in the warm sector before propagating onto the frontal surface; subsequent thunderstorms developed over the frontal surface. While in the warm sector, the convection was supported by an extremely unstable boundary layer. However, this convective energy quickly diminished above the frontal surface and thunderstorms continued and developed for many hours in an essentially stable hydrostatic environment. During the lifetime of these impulses, mesoscale updrafts developed and moved with the convective areas, maintaining nearly steady-state systems with strong low-level inflow. -from Author
Article
This collection of notes discusses the various types of severe- weather air masses, how severe weather systems form, which parameters best define the existence and intensity of severe weather, and how to use local information to better forecast the occurrence of phenomena at individual stations. Specifically, wind gust and hail size forecasting techniques and the usefulness of various stability indexes are presented. Also, a chapter on severe weather in tropical air masses is included. A number of detailed case studies are in the report to help the reader visualize how forecasting concepts are applied, and to emphasize the importance of forecasting experience. The revised material concentrates on the application of computer-derived aids to severe weather forecasting produced by the Air Force Global Weather Central. Foremost among these aids are analyses and prognoses of the Severe Weather Threat (SWEAT) Index.
Th e SHARP Workstation. A Skew T-HodographAnalysis and Research Program
  • J Korotky
_ _ _ _, and J. Korotky, 1992: Th e SHARP Workstation. A Skew T-HodographAnalysis and Research Program. NOAAINWS Forecast Office, Charleston, WV, 30 pp.
Doswell III , 1992: Severe local storms forecasting
  • R H Johns
Johns, R. H., and C. A. Doswell III, 1992: Severe local storms forecasting. Preprints, Symposium on Weather Forecasting, Atlanta, GA, Amer. Meteor. Soc.
Isentropic Analysis and IlltellJretatiol1
  • J T Moore
Moore, J. T., 1992: Isentropic Analysis and IlltellJretatiol1.(2nd Edition). National Weather Service Training Center, Kansas City, MO.