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An analysis is presented here of intense convection affecting the Friuli Venezia Giulia region (FVG, northeastern Italy) during the Intensive Observation Period 2b (IOP2b) in the first Special Observation Period (SOP1) of the HyMeX (HYdrological cycle in Mediterranean EXperiment). The present study focuses on the first of three severe-convection episodes that affected FVG on the morning of 12 September 2012. In the first episode, a supercell, which produced hail and severe damage to trees and buildings, was observed on the plain of FVG. The available observations are analyzed together with a high-resolution mesoscale model, in order to identify the relevant mechanisms for the formation and development of the cell. Six different simulations were performed starting at three different initial times, using respectively two different analysis/forecasts as initial/boundary conditions. A large spread in forecast precipitation is found among the six simulations. Only a few of the simulations were able to reproduce intense rainfall on the plain of FVG during the morning, although with significant differences in the rainfall distribution among them. One of the six simulations well reproduces the observed elongated distribution of the intense rainfall maximum; the characteristics of the cell responsible for this distribution are consistent with those expected for a supercell and its simulated evolution near the Adriatic coast agrees well with the other observations. Some instability parameters over the FVG plain and offshore (over the northern Adriatic Sea) are analyzed every 5 minutes, showing that during this event the potential instability varies significantly over small space and time intervals and among the simulations. The best simulations have the best match to the observed potential instability calculated using the mean characteristics of the lowest 500 m layer.
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Characteristics and Predictability of a Supercell during 1"
HyMeX SOP1 2"
Mario Marcello Miglietta1 3"
ISAC-CNR, Lecce, Italy 4"
5"
Agostino Manzato 6"
OSMER, ARPA Friuli Venezia Giulia, Visco, Italy 7"
8"
Richard Rotunno 9"
NCAR*, Boulder, USA 10"
11"
12"
*The National Center for Atmospheric Research is sponsored by 13"
the National Science Foundation. 14"
15"
"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
1Corresponding+ author+ address:+ Mario" Marcello" Miglietta," CNR-ISAC," strada" provinciale" Lecce-Monteroni," 73100"
Lecce,"Italy."E-mail:"m.miglietta@isac.cnr.it""
Abstract 16"
An analysis is presented here of intense convection affecting the Friuli-Venezia Giulia region 17"
(FVG, northeastern Italy) during the Intensive Observation Period 2b (IOP2b) in the first Special 18"
Observation Period (SOP1) of the HyMeX (HYdrological cycle in Mediterranean EXperiment). The 19"
present study focuses on the first of three severe-convection episodes that affected FVG on the 20"
morning of 12 September 2012. In the first episode, a supercell, which produced hail and severe 21"
damage to trees and buildings, was observed on the plain of FVG. The available observations are 22"
analyzed together with a high-resolution mesoscale model, in order to identify the relevant 23"
mechanisms for the formation and development of the cell. Six different simulations were 24"
performed starting at three different initial times, using respectively two different analysis/forecasts 25"
as initial/boundary conditions. A large spread in forecast precipitation is found among the six 26"
simulations. Only a few of the simulations were able to reproduce intense rainfall on the plain of 27"
FVG during the morning, although with significant differences in the rainfall distribution among 28"
them. One of the six simulations well reproduces the observed elongated distribution of the intense 29"
rainfall maximum; the characteristics of the cell responsible for this distribution are consistent with 30"
those expected for a supercell and its simulated evolution near the Adriatic coast agrees well with 31"
the other observations. Some instability parameters over the FVG plain and offshore (over the 32"
northern Adriatic Sea) are analyzed which showed that during this event the potential instability 33"
varies significantly over small space and time intervals and among the simulations. The best 34"
simulations have the best match to the observed potential instability calculated using the mean 35"
characteristics of the lowest 500 m layer. 36"
37"
Keywords: 38"
HyMeX; Supercells; convection; predictability; instability indices; limited area model 39"
1. Introduction 40"
Recent events have made it clear that severe convection is not rare in the Mediterranean region and 41"
may sometimes produce significant damage and casualties. For example, on 8 July 2015, an EF4 42"
tornado struck the area west of Venice, causing one death and 72 injured, while a multi-vortex EF3 43"
tornado affected southeastern Italy on 28 November 2012, causing one casualty and estimated 44"
damage of 60 M to the largest steel plant in Europe (Miglietta and Rotunno, 2016). The 45"
monitoring and prediction of such severe localized convective events requires a deeper 46"
understanding of the relevant mechanisms necessary for their development. The present paper is a 47"
contribution towards this goal. 48"
Routine short-term (0-36 h) numerical weather forecasts of deep convection have existed for about 49"
a decade (Weisman et al., 2008). These forecasts strongly rely on mesoscale [O(100-1000 km)] 50"
features in the initial condition to predict the location and timing of areas of convection, as well as 51"
the type of convection (supercells, squall lines, etc.). The state-of-the-art practice is to use 52"
ensembles of such forecasts generated by diverse initial conditions in order to estimate the forecast 53"
uncertainty (Schwartz et al., 2015). As the predictability limit for convective-scale elements is at 54"
most a few hours (Lilly, 1990), the precise location, timing and type of convection within the 55"
mesoscale-model-predicted area is of course not possible. The possibility of making short-term (0-56"
60 min) forecasts using cloud-scale models of severe convection is described in Stensrud et al. 57"
(2009). 58"
As the studies quoted above are in the context of the physical geography of the US, the experience 59"
gained from them cannot be simply applied to the Mediterranean basin, where the different physical 60"
geography (e.g., the presence of complex orography, coastlines, land-sea gradients of temperature 61"
and surface drag) may affect the conditions for the initiation and development of convection. 62"
The Friuli-Venezia Giulia region (FVG, northeastern Italy) has a high incidence of deep convection 63"
and thus is a natural laboratory for the analysis of such events. Together with the peak in the 64"
average yearly rainfall in the Alpine region (Frei and Schär, 1998, Isotta et al., 2014), a high 65"
frequency of thunderstorms (Feudale and Manzato, 2014), hailstorms (Manzato, 2012), tornadoes 66"
and waterspouts (Giaiotti et al., 2007) have been identified in the climatology of FVG; other high 67"
impact events, such as bow-echoes (Pucillo and Manzato, 2010) and heavy rain episodes (Davolio 68"
et al., 2016) have also been observed. The frequent occurrence of these events is due to the interplay 69"
between frontal systems, orography, and the relatively warm and shallow-water bathymetry 70"
Adriatic Sea that borders FVG. 71"
For these reasons, FVG was included as a target area both in the Mesoscale Alpine Programme 72"
(Bougeault et al., 2001) and in the Hydrological cycle in the Mediterranean Experiment (HyMeX, 73"
http://www.hymex.org), first Special Observation Period (SOP1; Ducrocq et al., 2014). In the latter 74"
campaign, three severe-convection episodes affected FVG during Intensive Observation Period 2b 75"
(IOP2b), on the morning of 12 September 2012. In the first of these episodes, a supercell formed on 76"
the plain of FVG and produced hail and severe damage to trees and buildings near the coast. This 77"
case is investigated here with a high-resolution numerical weather prediction model to explore the 78"
characteristics of the event and the sensitivity of precipitation and of the supercell features to 79"
different initial and boundary conditions. Although the physics parameterizations can also influence 80"
the simulations, here the schemes are kept fixed for simulations (although some preliminary 81"
sensitivity tests have been performed; see Section 3). 82"
The paper is organized as follows. Section 2 provides a synoptic and mesoscale overview of the 83"
event. Section 3 is focused on numerical simulations, including a comparison with the available 84"
data (surface-station measurements, satellite-derived winds and radiosounding profiles) and a 85"
discussion of the modeling results from a predictability perspective. Section 4 presents the features 86"
of the simulated storm, identifying the characteristics typical of supercells. Conclusions are 87"
summarized in Section 5. 88"
2. Synoptic and mesoscale conditions 89"
A detailed description of the synoptic conditions during IOP2b of HyMeX SOP1 is provided in 90"
Manzato et al. (2015; M15); here only a brief summary of the most relevant features is reported. In 91"
Fig. 1a, the 500-hPa geopotential height map shows a diffluent trough, associated with a cold front, 92"
moving across western France, from the north Atlantic southeastward on the morning of 12 Sept. 93"
2012. The trough and cold front reach NE Italy in the late afternoon of the same day. 94"
A closer look at the MSLP field in Fig. 1a shows two small-scale cyclones which help guide warm, 95"
moist air to FVG: an orographic lee cyclone in the Gulf of Genoa and another cyclone over the Po 96"
Valley. The latter pressure minimum is associated with a low-level cyclonic circulation that 97"
straddles the Adriatic coast of FVG. This latter circulation has intense southeasterly wind, which 98"
moves warm and moist air northward along the east side of the Adriatic and a southwesterly wind 99"
that flows downslope across the Apennines. This flow configuration produces an elongated tongue 100"
that brings moist air from the sea inland where it is available to feed convection. Further 101"
verification of this mesoscale flow pattern is found in the present case by the satellite-retrieved 102"
surface wind from ASCAT at 0839 UTC over the Adriatic Sea (Fig. 1b). 103"
Figure 2 shows the 6-h accumulated rainfall from 0600 to 1200 UTC, Sept. 12. The data are 104"
obtained from the 6-h accumulated rain estimated by the Fossalon di Grado radar using the 105"
Marshall-Palmer (1948) equation and corrected with raingauge measurements. Intense and 106"
widespread precipitation affects the region, with a peak of more than 150 mm in 6 h in the western 107"
part of the FVG Prealps, and another band of intense precipitation (estimated in about 75 mm) 108"
generated by the supercell, extending from the Veneto region eastward along the coastal regions 109"
(near Palazzolo in Fig. 2). 110"
The evolution of the atmospheric vertical structure at Udine (46.03°N, 13.18°E), which is near the 111"
center of the FVG plain, is shown in Fig. 3. Several features conducive to intense convection 112"
occurring on the morning of Sept. 12 are identified. First, a southeasterly low-level jet of moist and 113"
warm air between 400 and 1400 m amsl at 0600 UTC, which is responsible for the high values of 114"
equivalent potential temperature (θe > 330 K) in the lower troposphere. The latter feature, combined 115"
with cold-air advection at middle levels (see the slope of the black isotherms between 4 and 7 km 116"
amsl), produce conditions of potential instability, increasing the value of CAPE to approximately 117"
2000 J kg-1. At the same time, weak convective inhibition (CIN) makes the environment favorable 118"
to the triggering of convection. An increase of the vertical wind shear is also apparent, due to the 119"
intensification of the upper-level wind and the rotation of the low-level wind from westerly to 120"
easterly. Such unstable conditions are then quickly eliminated by the entrance of much colder air 121"
associated with a frontal system in the afternoon of Sept. 12. 122"
M15’s Fig. 4 shows images from the EUMETSAT (European Organisation for the Exploitation of 123"
Meteorological Satellites) experimental 2.5 min rapid scans from the High Resolution Visible of the 124"
Meteosat Second Generation, which helps identify the transit of cloud systems across the region and 125"
the triggering of convection. At around 0630 UTC, 12 Sept., M15’s Fig. 4 shows the first 126"
convective cell is triggered in the Alpine foothills, in the western part of the FVG region, where the 127"
6-h accumulated rainfall maximum of 150 mm is observed. The radar data presented here in Fig. 4 128"
shows that in the next 1.5 h, the “northern storm” (as referred to in M15) intensifies approximately 129"
in the same location. In the following hour, a new cell, the “southern storm”, develops and grows 130"
rapidly. The evolution of the two cells is clearly identified in detail in Fig. 4. 131"
The radar reflectivity maps of Fig. 4 indicate the triggering of the southern cell near the Adriatic 132"
Sea and its subsequent northward movement (between 0740 and 0820 UTC). Since the 133"
environmental wind is westerly and this cell moves to the left of the flow, there is a strong 134"
suggestion that it is a left mover of a previous supercell splitting (Weisman and Klemp, 1982). 135"
From the analysis of M15 (their Fig. 13), the northern storm is also very likely a supercell. When 136"
the two (probable) supercells get closer to each other at about 0830 UTC, the southern storm 137"
strengthens, probably because it is able to intercept the moist low-level air (Fig. 1b) that had been 138"
feeding the northern storm, so that the latter suddenly dissipates (Fig. 4). Finally, the remaining cell 139"
deviates eastward along the coast, where it exhibits supercellular features, such as the hail reported 140"
in Latisana and the intense downdraft/outflow in Palazzolo at about 0900 UTC (Fig. 4). Later on, 141"
the system moves along the coast evolving into a bow-echo pattern, as shown in M15’s Fig. 5d. 142"
Fig. 5 shows θe observed at four surface stations, whose locations are shown in Fig. 2. While Udine, 143"
which is north of the area of main convective activity, shows small variations of θe, the other three 144"
stations exhibit very sharp drops corresponding to the storm passage. Such large drops in θe suggest 145"
a strong efficiency of the storm in converting the environmental thermodynamic energy into 146"
precipitation and kinetic energy. 147"
3. Numerical simulations 148"
3.1 Simulations with WRF model 149"
The supercell described above is analyzed here by means of numerical simulations performed with 150"
the Weather Research and Forecasting (WRF) model, version 3.5.1 (see www.wrf-model.org; 151"
Skamarock et al., 2008). WRF is a state-of-the-art weather prediction system that solves the fully 152"
compressible, nonhydrostatic equations of atmospheric motion. Forty terrain-following hydrostatic-153"
pressure vertical levels are used in the present simulations with more closely spaced levels in the 154"
boundary layer. In order to analyze the detailed evolution of the meteorological parameters, model 155"
output is saved every 5 min. 156"
The model is implemented using three different one-way-nested grids, with horizontal spacings 157"
respectively of 9, 3, and 1 km, extending for 190 (in the east–west direction) x 150 (in the north–158"
south direction) grid points in the outer grid, 271 x 163 in the middle grid and 181 x 181 in the 159"
inner grid (Fig. 6). The area of interest is in the center of the inner domain and on the eastern side of 160"
the two outer domains, in order to properly represent the large-scale evolution of the trough, which 161"
propagates from the west. 162"
Since the predictability of the event is the main subject of the present paper, different 163"
initial/boundary conditions are considered to force the simulations. In particular, the European 164"
Centre for Medium-range Weather Forecasts (ECMWF) and the Global Forecasting System (GFS) 165"
analysis are used as initial conditions. The boundary conditions are updated every 3 h with the 166"
ECMWF Integrated Forecasting System (IFS) and GFS forecasts, thus the simulations are 167"
performed in an operational-like configuration. Also, different starting times have been tested to 168"
initialize the model simulations, respectively 0000 and 1200 UTC, 11 Sept. 2012, and 0000 UTC, 169"
12 Sept. 2012. Simulations are named according to the initial time, for example the run forced with 170"
GFS data starting at 11 UTC, 12 Sept., is named as GFS1112. As will be shown, GFS1112 is the 171"
simulation that best reproduces the observed evolution of the supercell and is considered the 172"
“control run” hereafter. 173"
Preliminary experiments were undertaken to identify an optimal set of parameterizations, able to 174"
better reproduce the cell evolution. Following the outcome of these experiments, the model is 175"
implemented with the following schemes: Thompson et al. (2008) microphysics; Rapid Radiative 176"
Transfer Model (RRTM) for longwave radiation based on Mlawer et al. (1997); Dudhia (1989) for 177"
shortwave radiation; the unified Noah land-surface model (Niu et al., 2011); Mellor-Yamada-Janjic 178"
planetary boundary layer (Janjic, 2001). Thus, the schemes are the same as those employed for the 179"
simulations with the WRF model in M15. Since the large-scale forcing is the same, the better result 180"
of the GFS1112 run in reproducing the supercell evolution compared to the simulation in M15 181"
indicates the importance of domain sizes (which are larger in the present study) and, mainly, of the 182"
fine horizontal resolution required for a realistic simulation of meso-γ scale features. 183"
There is no consensus on the use of convection parameterization for grid spacing slightly smaller 184"
than 10 km, as in the outer domain used here. Done et al. (2006) reported on cases of fairly intense 185"
convection with mesoscale organization, showing there is no advantage of using a cumulus 186"
parameterization even for grid spacing slightly larger than 10 km. In the present study, no cumulus 187"
scheme is used in any domain. In order to see how this choice affects the simulation, an additional 188"
run was performed, using the same configuration as GFS1112, but switching on the Kain (2004) 189"
cumulus convection scheme in the outer grid and leaving the explicit treatment of convection only 190"
in the two inner domains. 191"
Additionally, different schemes for the planetary boundary layer (PBL) have been tested: we 192"
believe that the PBL parameterization plays a key role in the present case study by modifying the 193"
characteristics of the atmosphere in the low levels, thus the instability properties and the flow 194"
dynamics. In both cases, differences of the simulations with the GFS1112 run are relatively minor 195"
compared to those emerging among experiments with different initial times and/or large-scale 196"
forcing: they show the same solution characteristics, with only slight modifications in the rainfall 197"
amount and distribution. As a consequence, hereafter the predictability analysis will focus on the 198"
sensitivity to different initial/boundary conditions without considering the role of parameterization 199"
schemes. 200"
3.2 Mesoscale and precipitation patterns 201"
In M15, both models (MOLOCH and WRF) used for the simulation of the event were able to 202"
reproduce the triggering of convection over the foothills of the Alps. Also, they simulated some 203"
mesoscale features that possibly played a key role during this phase, such as the tongue of warm air 204"
advected by an intense low-level jet. However, both models missed the exact timing, location and 205"
movement of the cells. Thus, although both models captured the mesoscale environment fairly well, 206"
they were far from an accurate simulation of the convective system. 207"
In order to better explore this point, six experiments were performed with the WRF model using 208"
different initial and boundary conditions, as discussed in Subsection 3.1. The characteristics of the 209"
low-level inflow of warm, moist air at 0600 UTC are shown in Fig. 7 for all runs. The simulations 210"
show important mesoscale differences near the mountain and over the plain are already present 211"
before the triggering of convection near the foothills. Thus, the discrepancies growing from initial 212"
small-scale differences spreading upscale as a consequence of moist convection (Zhang et al., 2002, 213"
2003) probably have a minor effect here. 214"
The ECMWF runs (Fig. 7, right) all show a similar pattern at 0600 UTC: the warm-air tongue 215"
penetrates inland, although with a different northward extent and intensity (the earlier the starting 216"
time, the cooler the air and the narrower the jet), while the cold air always remains confined near 217"
the mountains and the foothills. The differences among the GFS-forced runs are more apparent (Fig. 218"
7, left). The GFS1200 run shows a peculiar configuration, since it is characterized by a wide area of 219"
very cold, low-level air, extending from the mountains to the sea, while the warm tongue is 220"
confined to a very narrow region close to the coast. 221"
Compared with the ECMWF simulations, in the GFS1100 and GFS1112 simulations the cold air 222"
shows a similar southward extent near the foothills, but is somewhat cooler. Also the inland 223"
penetration of the warm air is much deeper, with values close to 340 K simulated even near the 224"
foothills (in particular in GFS1112), and shifted farther to the west than in the other runs. The 225"
westward deflection of the warm air in its northern tip can be probably attributed to the cyclonic 226"
circulation around the pressure meso-low in the Po valley (see Fig. 1), centered near Venice, which 227"
appears slightly deeper in GFS1112 run than in the other runs. This feature plays an important role 228"
in favoring supercell development since it prevents the warm tongue from moving eastward, forcing 229"
it to remain in the vicinity of the foothills. Between the two experiments, GFS1112 has a much 230"
deeper penetration inland, and a stronger and more extensive low-level jet than GFS1100. 231"
As a consequence of the variety in the simulated mesoscale patterns, there is a large variation in the 232"
way the flow interacts with the orography and in the simulated precipitation. To summarize the 233"
differences among the simulations, Fig. 8 shows the 6-h accumulated precipitation from 0600 to 234"
1200 UTC. Only the GFS runs and the ECM1200 run are able to reproduce intense rainfall in the 235"
FVG plain and the coastal area, although with significant differences in its distribution. Similarly, 236"
the variation of maximum updraft vertical velocity with time (not shown) is characterized by a 237"
strong variability. 238"
Two experiments shown in Fig. 8, ECM1100 and ECM1112, produce precipitation mainly in the 239"
northeastern part of the region, near the border with Slovenia, in an area where the observed 240"
precipitation is much smaller (cf. Fig. 8 with Fig. 2), while no precipitation is simulated near the 241"
coast. In these two cases, the warm tongue is advected too far north at later times, while the cold air 242"
remains confined very close to the mountains (not shown); thus the direct orographic uplift is 243"
mainly responsible for rainfall in these runs. 244"
The other experiment forced with ECMWF data (Fig. 8f) reproduces an intense rainfall peak in the 245"
area affected by the supercell, but the simulated precipitation is about 200 mm, about twice that 246"
observed (and about the sum of the rainfall maximum in the northern and southern storms). The 247"
coastal rainfall is produced –as in the GFS experiments- at the northern terminus of the low-level 248"
warm inflow, which remains quasi-stationary near the coast for several hours (not shown). The peak 249"
in vertical velocity is above 20 m s-1 for a few minutes, but the cell does not show the rotation 250"
typical of supercells. Two minor rainfall peaks are also simulated, one in the northeastern region, 251"
and another one (corresponding to the observed maximum) near the foothills at the border with 252"
Veneto region, but they are significantly underestimated and shifted northward. 253"
The simulated precipitation in the GFS1200 experiment shown in Fig. 8 is also very different from 254"
the observations. Some orographic precipitation is again shown near Slovenia, while a rainband is 255"
elongated from the west side of the region to the east, following the eastward movement of the 256"
northern tip of the warm tongue in the morning of Sept. 12. However, the timing is incorrect, the 257"
precipitation is shifted to the north (cf. Fig. 8 with Fig. 2) and the intensity is significantly 258"
underestimated. 259"
The other two GFS-forced experiments shown in Fig. 8 better simulate the observed precipitation. 260"
Both experiments, in particular GFS1112, show a persistent vertical velocity larger than 20 m s-1 261"
lasting for more than one hour. Also, the two simulations are the only ones that produce some 262"
rainfall in the northern part of the region near the Alps, in agreement with the observations. The 263"
GFS1100 run reproduces fairly well the rainfall amount associated with the supercell near the coast, 264"
although the affected region has a shorter east-west extent than that observed, due to the earlier 265"
weakening of the supercell. Finally, the GFS1112 run reproduces well the observed elongation of 266"
the intense rainfall maximum toward Slovenia, and the precipitation amount is close to the 267"
observed. The presence of the mountains in Slovenia seems to prevent a longer duration of the 268"
supercell, which lasts for about 1.5-2 hours (in agreement with the analysis in M15’s Section 5). 269"
The observed maximum in the foothills is well captured in the simulation, although separated into 270"
two distinct and weaker maxima. 271"
Apparently, the successful simulation required a deep inland penetration and a cyclonic rotation of 272"
very warm, moist air on the west side of the FVG plain, which is accomplished only in the 273"
GFS1100 and GFS1112 experiments (Fig. 7). Small-scale variations in θe are known to affect the 274"
potential instability of parcels in the layer where convection originates (Done et al., 2011). The 275"
presence of high-θe values in the low levels is required to make the atmosphere more unstable and 276"
allow for the triggering of convection near the foothills, which, as shown in Section 4, leads to the 277"
cold-air outflow that plays an important role in the later evolution of the storms. (Note that the 278"
strong gradient of θe corresponds to an area of low-level convergence between the southerly inflow 279"
and a northeasterly barrier wind from the Alps, which affects the pre-Alpine region and the northern 280"
part of the Po valley as discussed in M15 and Davolio et al., 2016 and favors convective triggering.) 281"
Figure 9 shows that intense precipitation (larger than 40 mm h-1) in the GFS1112 run is initially 282"
triggered near the rainfall maximum observed in the foothills (Fig. 2). This is an area where CIN is 283"
low (below 20 J kg-1) and bordered on its southern side by a band of high CAPE (above 1500 J kg-
284"
1), which was advected northward on the morning of Sept. 12. The simulated soundings near the 285"
foothills show that the advection of low-level moisture dramatically increases the instability of the 286"
environment in that period. For example, at the point (46.0°N, 12.75°E), the CAPE is about 350 J 287"
kg-1 and CIN is -25 J kg-1 at 0300 UTC; after 3 hours, an increase in the mixing ratio of about 2 g 288"
kg-1 at the level of the most unstable parcel (950 hPa), produces an increase in θe from 329 K to 336 289"
K and reduces the inhibition while the CAPE increases up to about 1400 J kg-1. Changes in the 290"
upper-level profiles, due to the incoming trough, appear as relatively minor in this stage. 291"
3.3 Simulated and observed vertical profile and instability indices 292"
Figure 10 shows the time evolution of the atmospheric profiles simulated at the grid point closest to 293"
Udine in the GFS1112 run. Since the time resolution of the model output shown in Fig. 10 is 1 h, 294"
the rapid changes in the meteorological fields can be detected with greater detail than in the 295"
observed time evolution (Fig. 3). 296"
Figure 10 shows that the vertical structure of the equivalent potential temperature at Udine, 297"
simulated by the model, appears consistent with the observed evolution. In particular, the GFS1112 298"
run correctly represents the arrival of cold, dry air at midlevels, the presence of low-level high-θe 299"
air, the large instability (high CAPE) in the early morning of Sept. 12, the rotation of the wind 300"
vector in the low levels along with the intensification of the wind speed in the upper levels and the 301"
transit of the cold front in the evening. However, Fig. 10 shows that the evolution in the simulation 302"
occurs a few hours earlier than in the observations. The more-frequent model output in Fig. 10 303"
shows the presence of high-θe air extending from the ground to the upper troposphere 304"
corresponding to the development of deep convection, simulated both in the morning and afternoon 305"
of Sept. 12. 306"
One of the main aspects that influence the predictability of this event is the model ability to 307"
simulate the vertical structure of the temperature, humidity and wind, which determine potential 308"
instability and type of convection (e.g. supercells, multicells, etc.). In order to compare the potential 309"
instability and wind profiles among the simulations as well as with the observed values, sounding-310"
derived and model-derived indices are calculated (Manzato, 2008), before and during the 311"
convection. Four instability indices are shown in Fig. 11 for all six runs, to represent the evolution 312"
on the morning of Sept. 12. On the right side of Fig. 11 both observed values (cross marks) derived 313"
from the Udine radiosoundings (46.03°N, 13.18 °E) and simulated values every 5 min are shown. 314"
On the left side of Fig. 11, the simulated indices are shown at a grid point located offshore over the 315"
Adriatic Sea (45.4°N, 13.0 °E). All these indices are computed with the Tv method” described in 316"
Manzato and Morgan (2003), which uses also the “virtual correction” suggested by Doswell and 317"
Rasmussen (1994). Moreover, a centered moving average of 3 points (10 min of time interval) has 318"
been applied to smooth the fast varying indices: even so, the time evolution is very fast, with the 319"
indices over land having much sharper fluctuations than those offshore. 320"
Figure 11a shows the classical Lifted Index (LI; Galway, 1956), which uses as the initial parcel the 321"
mean air properties in the lowest 500 m. Between 0500 and 0800 UTC the potential instability is 322"
much larger offshore (LI in the range -5 to -8 °C) than over land (LI from 0 to -4 °C). In the GFS 323"
runs, the model-derived LI is closer to the observed values in Udine, in particular at 0535 UTC. 324"
However, the Lifted Index using the most unstable parcel method (MUP, not shown), in which the 325"
initial parcel corresponds to the maximum θe in the lowest 250 hPa, shows that ECMWF runs have 326"
a better estimate of LI using MUP, due to its better estimation of the maximum θe in the lowest 250 327"
hPa (Figure 11b), which is located above 500 m. In conclusion, it seems that potential instability 328"
based on the lowest 500 m is better described by GFS, while that based on the most unstable parcel 329"
(located at higher levels) is better described by ECMWF. The fact that GFS1112 better reproduces 330"
the observed dynamics and the precipitation indicates that the potential instability based on the very 331"
low levels (lowest 500 m) is the most important to be well predicted in this case. 332"
Figure 11b also shows that the sounding-derived maximum θe at 0535 UTC is higher than the 333"
highest simulated θe, while the observed maximum θe at 1105 UTC is slightly lower than the lowest 334"
simulated θe. This means that the drop of almost 20 K in air mass between 0535 and 1105 UTC is 335"
underestimated by all six models. Consistent with the LI, Fig. 11b shows the simulated maxima of θe 336"
have much higher values offshore than inland. This very strong north-south gradient of θe across the 337"
coast is very significant considering that the two locations are only 70 km apart. The feature of the 338"
simulations corresponds well with the surface observations, as the value of θe in Udine is on 339"
average 10 K lower than it is in Lignano before storm passage (Fig. 5). This observation means that 340"
the very warm, moist air remains mainly near the coast in the west part of FVG. 341"
The fast variations in the simulated Udine indices can be attributed to sudden changes in the 342"
northern extent of the low-level jet, probably associated with the movement of the convective cells 343"
along the region, which may temporarily block the southerly inflow of warm and moist air. Figure 344"
11c shows the meridional component of the mean wind in the lowest 500 m (LLWv, with positive 345"
values indicating southerly flow). We see that, while a weak southerly component (from about 0 to 346"
4 m s-1) is simulated for most of the time in Udine, for a short period the wind becomes northerly in 347"
some runs (GFS1100, GFS1200, ECM1200), due to the outflow associated with the northern storm 348"
(Fig. 4). 349"
Fig. 11c shows that LLWv in the point offshore is southerly for all time and all runs, apart from 350"
ECMWF1100, and shows a progressive intensification of its magnitude until about 9 m s-1, 351"
followed by a sudden drop (occurring between 0730 or 0830 UTC, depending on the model), as to 352"
track the passage of a low-level jet. This is in agreement with the presence of a southerly low-level 353"
jet over the Adriatic Sea, observed at 0839 UTC in Fig. 1b. The LLWv drop is less pronounced in 354"
the three GFS runs than in the remaining two ECMWF simulations, hence GFS runs are more 355"
efficient in pushing the high-θe air of the lowest 500 m from the sea inland. 356"
Lastly, Figure 11d shows that the storm-relative helicity, which is calculated considering the 357"
simulated eastward storm speed of 7 m s-1, is in good agreement with observations, in particular for 358"
GFS1112 and GFS1200 (note also that the values for the GFS1112 run are the highest in the point 359"
offshore). The parameter shows intense fluctuations inland; offshore the peak occurs a couple of 360"
hours earlier (between 0700 and 0800 UTC, depending on the model run). In contrast with most of 361"
the other indices, the peaks of this parameter are much higher inland than offshore, probably due to 362"
the much stronger wind shear brought about by larger drag over land, in particular in the presence 363"
of complex orography. Simulated values of helicity larger than 100 m2 s-2 in Udine occur in all 364"
models, denoting a larger potential for cyclonic updraft rotation inland. The increase in this index 365"
reflects the larger vorticity advection associated with the cold front and the upper-level trough 366"
approaching from the northwest. 367"
In conclusion, from this analysis we have learned that, at least in this case, a more realistic 368"
simulation of the lowest 500 m (both in terms of θe and wind structure) seems to have a strong 369"
influence on the better predictions of some forecasts with respect to others. 370"
4. Supercell features 371"
In the present Section, the characteristics of the cell generated by the merging of the northern and 372"
southern storm are analyzed and compared with the classical supercell conceptual model. Thus, the 373"
3D structure of the flow around the simulated supercell is investigated more deeply. In the 374"
following, only the GFS1112 run, which reproduces better the observed 6-h accumulated rainfall, is 375"
considered. 376"
Following Rotunno and Klemp (1985), two distinctive features are recognized as hallmarks of 377"
supercell thunderstorms: the propagation to the right of the mean tropospheric wind shear (apart 378"
from the left movers, as in Fig. 4) and a significant degree of organized rotation around the updraft 379"
that persists for tens of minutes (Doswell and Burgess, 1993; Thompson, 1998). For the former, the 380"
simulated cell movement is from west-northwest to east-southeast, approximately coincident with 381"
the rainband elongation from Veneto toward Slovenia (Fig. 2), and agrees well with the 382"
observations (cf. with M15’s Fig. 5); thus, it is rightward of the average tropospheric wind shear, 383"
which in the morning of Sept. 12 is approximately west-south-westerly (Figs. 3 and 10). M15’s Fig. 384"
13 confirms that the northern storm and the merged cell, as simulated in that paper, have the 385"
rightward propagation typical of supercells. For the latter feature, Fig. 12a shows the vertical 386"
velocity and the vertical component of vorticity at 5500 m at 0905 UTC, Sept. 12. The near 387"
superposition of the maxima of the two fields is a typical feature of supercellular systems. Also, on 388"
the northern side of the cell, a small area of anticyclonic circulation is present, which can be 389"
associated with the splitting of the original cell (generated on the Alpine foothills at about 0600 390"
UTC) induced by the downdraft, according to the conceptual model discussed in Rotunno (1981, his 391"
Fig. 3) and Rotunno and Klemp (1982). 392"
Figure 12b shows the structure of the cell at lower levels and suggests that the intense rainfall 393"
induces strong evaporation contributing to the formation of a cold pool on the rear flank, which is 394"
crucial for the baroclinic generation of vorticity along the cold-air boundary and the low-level 395"
rotation of the system (Rotunno and Klemp, 1985). Also, the inflow of moist, potentially unstable 396"
low-level air from the Adriatic Sea that feeds the updraft is necessary to continuously trigger 397"
convection above the surface gust front. Such features are consistent with the conceptual model in 398"
Markowski and Richardson (2010)’s Fig. 8.20. However, in the present case the supercell moves in 399"
synchronicity with the high-θe tongue; thus the evolution of the synoptic and mesoscale features 400"
appears to control its displacement. (In particular, its eastward movement follows the mesocyclone, 401"
while the southward movement corresponds to the intrinsic dynamics of a supercell.) While sharing 402"
a similarity with supercells forming on and moving with a dryline (Bluestein et al., 2015), in the 403"
present case there is also the low-level jet coming from the Adriatic Sea. As discussed in Feudale 404"
and Manzato (2014), this jet can be considered as occurring in a “channel” formed by the Dinaric 405"
Alps and the Apennines; the jet is influenced by the Alpine barrier on its northern side, causing 406"
local convergence and inhomogeneity in the high-θe tongue, and thereby influencing the evolution 407"
of the supercell in way that appears qualitatively different from supercells over the U.S. Great 408"
Plains. 409"
Figure 13 shows a 3D view during the mature stage of the cell. The intense updraft (of more than 30 410"
m s-1) is generated where the cold pool in the rear of the cell and the warm air inflow meet, a few 411"
km from the coast. The outflow associated with the downdraft is apparent, as well as the low-level 412"
rotation below the updraft. The high rainwater content assumes a bow-echo pattern, induced by the 413"
downward movement of the potentially cold mid-level air, which is further cooled down by the 414"
rainfall evaporation and moves underneath the low level inflow. 415"
The low-level rotation is more apparent in Fig. 14, where two specific trajectories are shown. In 416"
agreement with the Browning (1964)’s supercell model and with Klemp et al. (1982), the rear-flank 417"
updraft is created by ambient air entering the storm along the right (south) flank and then wrapped 418"
around the rear flank by the strengthening mesocyclone, while the forward-flank downdraft is 419"
wrapped around the north side of the updraft. 420"
5. Conclusions 421"
The present paper focuses on a severe-convection episode occurring in Friuli Venezia Giulia (FVG, 422"
northeastern Italy) on the morning of 12 September 2012, during the Intensive Observation Period 423"
2b (IOP2b) in the first Special Observation Period (SOP1) of the HyMeX campaign. One supercell, 424"
which produced hail and severe damage to trees and buildings, developed on the plain of FVG and 425"
was generated by the interaction between two previously existing cells. 426"
Observations are analyzed together with high-resolution WRF model simulations to identify the 427"
mechanisms responsible for the formation and development of the cell. Among six runs, starting 428"
from different large scale forcing at different times, the simulation initialized at 1200 UTC, 11 429"
September 2012 forced with GFS analysis/forecasts (GFS1112) is the one that best reproduces the 430"
observations which included a supercell thunderstorm. 431"
The mesoscale features responsible for the event are well identified. Warm, moist air, mainly 432"
confined near the coast, is advected by a low-level jet toward the Prealps (Fig. 2), producing large 433"
instability (although smaller than offshore) by increasing the local value of water-vapor mixing 434"
ratio, θe and CAPE; at the same time, an area of low-level convergence between the southerly 435"
inflow and a northeasterly barrier wind from the Alps favors convective triggering in the foothills; 436"
finally, cold air advection in the middle levels enhances potential instability. 437"
A strong sensitivity to the initial and boundary conditions is demonstrated: only a two of six 438"
simulations (GFS112 and GFS1100) were able to reproduce a persistent updraft rotation and the 439"
rightward movement typical of supercells; in two runs (ECMWF1100 and ECMWF1112), no 440"
precipitation is simulated along the coast, with rainfall generated by direct orographic uplift only 441"
near the Alps; in the other two runs (GFS1200 and ECMWF1200), the timing and the intensity of 442"
rainfall is far from that observed (the triggering of the cells occurs early in the simulations, and may 443"
be possibly affected by the model spin up). Considering that all above experiments are undertaken 444"
in an operational-like mode, this result clearly shows that an ensemble approach (even a “poor man” 445"
ensemble, as the one shown here) appears absolutely necessary to provide some indication on the 446"
risk of localized severe convective weather. From the preliminary experiments performed to 447"
identify an “optimal” setup, the sensitivity to physics appears minor compared to that due to 448"
different larger-scale forcing and initial starting times, at least for this case. 449"
Apparently, the successful simulation requires a deeper low in the Po Valley giving a farther inland 450"
penetration of deep (~500m) low-level high-θe air, especially on the west side of the FVG plain, a 451"
weak (but nonzero) convective inhibition, necessary to confine the release of convection near the 452"
foothills, where the cold-air outflow plays an important role in the later evolution of the storm and, 453"
finally, low-level cold air confined near the mountains and the foothills. The latter point is very 454"
tricky, since cold air generally remains confined mainly in the narrow Alpine valleys, which are 455"
well below the resolution of a large-scale model. Thus, the presence of a cold-air damming may 456"
easily be missed or misrepresented in the initial and boundary conditions. The significant 457"
climatological underestimation of the rainfall simulated by ECMWF forecasts in the FVG plain and 458"
coastal area during summer (Manzato et al., 2016) is probably also a consequence of such a 459"
limitation. 460"
The analysis of some instability parameters over the FVG plain and offshore (over the northern 461"
Adriatic Sea) before and during the event reveals significant small-scale variations in space and 462"
time, mainly as a consequence of the variations in the low-level θe. In particular, the sudden 463"
variations simulated in Udine are probably associated with the movement of the convective cells, 464"
which may temporary limit the tongue of warm air more to the south. 465"
Lastly, supercell features emerging from the best simulation are consistent with the classical 466"
supercell model developed in Rotunno and Klemp (1985) mainly for US Plain supercells. However, 467"
while in the latter case the pattern of θe is generally homogeneous and stationary, in the present case 468"
the synchronous movement of the high θe tongue with the cell is a distinctive feature, which 469"
appears to be controlled mainly by mesoscale features. Also, the interaction of the moist and warm 470"
low-level jet with the Alps causes local convergence and inhomogeneity in the high-θe tongue (as 471"
shown also in Figure 1b), influencing the evolution of the supercell. The generality of these results 472"
should be tested extending a similar analysis to other Mediterranean events. 473"
For future work, we plan to simulate the environment conducive to the present supercell in idealized 474"
conditions. In this way, we can systematically analyze the sensitivity of the solution to a range of 475"
values or to small perturbations added to the relevant parameters, in order to better understand the 476"
mechanisms that may have affected the triggering and development of the supercell, making 477"
apparent the differences with respect to the US supercell environment. Also, since the 478"
characteristics of the present supercell appear to have survived only for a short period (a few tens of 479"
minutes), the reason for such a short lifetime need to be analyzed and discussed, possibly 480"
considering the role of the orography. 481"
ACKNOWLEDGEMENTS 482"
This work is a contribution to the HyMeX program. Arturo Pucillo is gratefully acknowledged for 483"
Fig. 1a, Stefano Zecchetto for Fig. 1b. Figures 7, 8, 12-15 were produced with VAPOR 484"
(www.vapor.ucar.edu). Figures 3, 4 and 5 are made with the NCAR Zebra software (Corbet et al. 485"
1994). The stay of R. Rotunno in Lecce was supported by CNR short-term mobility program; the 486"
stay of M. Miglietta in Boulder was supported by NCAR. 487"
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593"
594"
595"
FIGURE CAPTIONS: 596"
Figure 1: a) 500 hPa geopotential height (violet lines) and mean sea level pressure (black lines) at 597"
0600 UTC, 12 Sept. 2012, from the ECMWF forecast (initial time 0000, 12 Sept 2012) (Courtesy of 598"
Arturo Pucillo, OSMER ARPA FVG); b) wind data from MetOP-ASCATa scatterometer (12.5 599"
km horizontal resolution) at 0839 UTC, 12 September (Courtesy of Stefano Zecchetto, ISAC-600"
CNR). The location of the places mentioned in the text is shown in a) and the box in a) corresponds 601"
to the domain shown in b). 602"
Figure 2: FVG 6-h accumulated rainfall in the period 06-12 UTC, 12 Sept. 2012 (6-level radar data 603"
corrected with raingauge observations). 604"
Figure 3: Vertical-time series in the layer 1000 400 hPa (left scale in hPa) of soundings at the 605"
location of Udine sounding station (46.03°N, 13.18°E) from 1200 UTC, 11 Sept. to 0000 UTC, 13 606"
Sept., with horizontal winds (red barbs) and θe (colorbar scale). Superimposed are estimates of 607"
CAPE (red line) and CIN (blue line), LFC (Level of Free Convection) (“+” signs), and temperature 608"
(black contours). Observed soundings are reported every 6 h between 0000 UTC of Sept. 12 and 609"
0000 UTC of Sept. 13, due to the request of two additional soundings at 0600 and 1800 UTC (note 610"
that the wind at 1800 UTC is not shown because it was not recorded correctly). 611"
Figure 4: Vertical Maximum Intensity (VMI) of the reflectivity measured by the Fossalon di Grado 612"
radar at 0810, 0820, 0830, 0840, 0850 and 0900 UTC, 12 Sept., with equivalent potential 613"
temperature and 10m wind observed by surface stations 5 minutes later and CESI (Centro 614"
Elettrotecnico Sperimentale Italiano) cloud-to-ground lightning ±6 minutes around the nominal 615"
time. 616"
Figure 5: Equivalent potential temperature (θe) every 5 min observed in San Vito (cyan), Palazzolo 617"
(red), Lignano (blue) and Udine (green) between 0300 and 1500 UTC, 12 Sept. 2012. 618"
Figure 6: Model grids and topography. 619"
Figure 7: Wind vectors at 350 m height (white arrows), θe at 300 m (shaded, no data between 332 620"
and 335 K) at 0600 UTC, 12 Sept. 2012, from GFS runs initialized at a) 0000 UTC, 11 Sept., b) 621"
1200 UTC, 11 Sept., c) 0000 UTC, 12 Sept and from ECMWF runs initialized at d) 0000 UTC, 11 622"
Sept., e) 1200 UTC, 11 Sept., f) 0000 UTC, 12 Sept. 623"
Figure 8: 6 h rainfall simulated (WRF inner grid) from 0600 UTC to 1200 UTC, 12 Sept. 2012, 624"
from GFS runs initialized at a) 0000 UTC, 11 Sept., b) 1200 UTC, 11 Sept., c) 0000 UTC, 12 Sept 625"
and from ECMWF runs initialized at d) 0000 UTC, 11 Sept., e) 1200 UTC, 11 Sept., f) 0000 UTC, 626"
12 Sept. 627"
Figure 9: CAPE (green shading) and CIN (red contour = -20 J kg-1) at 0600 UTC, 12 Sept.; hourly 628"
rainfall (blue-red shading) at 0700 UTC. The orography is in grey tones. 629"
Figure 10: As Fig. 3, but for GFS1112 simulated soundings at the location of Udine sounding 630"
station (46.03°N, 13.18°E). Temperature and θe are shown every hour, while horizontal winds every 631"
6 hours. 632"
Figure 11. From top to bottom: 10-min moving average of a) Lifted Index, b) θe of the most 633"
unstable parcel, c) Low level v-wind component and d) Storm Relative helicity, offshore over the 634"
Adriatic sea (45.4°N, 13.0 °E) (left) and in the grid point closer to Udine (46.03°N, 13.18 °E) 635"
(right) between 0500 UTC and 1200 UTC of Sept. 12. Note that the 0600 (1200) UTC Udine 636"
(WMO16044) sounding has been launched at 0526 (1059) UTC and has reached 500 hPa at 0545 637"
(1116) UTC, so that the corresponding indices are plotted at 0535 (1105) UTC. 638"
Figure 12: a) Vertical velocity (shading; value between –1 and 1 m s-1 are not shown), vertical 639"
component of vorticity (c.i. = 0.005 s-1; black for negative values, white for positive, 0 not shown), 640"
and wind vectors at 5500 m, b) Maximum reflectivity (shading; values below 30 dbZ are not 641"
shown), θe (c.i. = 3 K; yellow contours) and wind vectors at 100 m, simulated by the WRF model 642"
inner grid (GFS1112 run) at 0905 UTC, Sept. 12. 643"
Figure 13: as Figure 7, but for GFS1112 run at 0915 UTC, Sept. 12. The isosurface of w = 12 m s-1 644"
(orange) and rainwater content at 1000 m height (c.i. = 1 g kg-1) are also shown. 645"
Figure 14: Two different trajectories wrapping around due to supercell rotation. The colors along 646"
the trajectories represent θe (in K). 647"
648"
649"
650"
651"
652"
653"
Figure 1: a) 500 hPa geopotential height (violet lines) and mean sea level pressure (black lines) at 654"
0600 UTC, 12 Sept. 2012, from the ECMWF forecast (initial time 0000, 12 Sept 2012) (Courtesy of 655"
Arturo Pucillo, OSMER ARPA FVG); b) wind data from MetOP-ASCATa scatterometer (12.5 656"
km horizontal resolution) at 0839 UTC, 12 September (Courtesy of Stefano Zecchetto, ISAC-657"
CNR). The location of the places mentioned in the text is shown in a) and the box in a) corresponds 658"
to the domain shown in b). 659"
660"
Figure 2: FVG 6-h accumulated rainfall in the period 06-12 UTC, 12 Sept. 2012 (6-level radar data 661"
corrected with raingauge observations). 662"
663"
Figure 3: Vertical-time series in the layer 1000 400 hPa (left scale in hPa) of soundings at the 664"
location of Udine sounding station (46.03°N, 13.18°E) from 1200 UTC, 11 Sept. to 0000 UTC, 13 665"
Sept., with horizontal winds (red barbs) and θe (colorbar scale). Superimposed are estimates of 666"
CAPE (red line) and CIN (blue line), LFC (Level of Free Convection) (“+” signs), and temperature 667"
(black contours). Observed soundings are reported every 6 h between 0000 UTC of Sept. 12 and 668"
0000 UTC of Sept. 13, due to the request of two additional soundings at 0600 and 1800 UTC (note 669"
that the wind at 1800 UTC is not shown because it was not recorded correctly). 670"
671"
Figure 4: Vertical Maximum Intensity (VMI) of the reflectivity measured by the Fossalon di Grado 672"
radar at 0810, 0820, 0830, 0840, 0850 and 0900 UTC, 12 Sept., with equivalent potential 673"
temperature and 10m wind observed by surface stations 5 minutes later and CESI (Centro 674"
Elettrotecnico Sperimentale Italiano) cloud-to-ground lightning ±6 minutes around the nominal 675"
time. 676"
677"
Figure 5: Equivalent potential temperature (θe) every 5 min observed in San Vito (cyan), Palazzolo 678"
(red), Lignano (blue) and Udine (green) between 0300 and 1500 UTC, 12 Sept. 2012. 679"
680"
Figure 6: Model grids and topography. 681"
682"
683"
Figure 7: Wind vectors at 350 m height (white arrows), θe at 300 m (shaded, no data between 332 684"
and 335 K) at 0600 UTC, 12 Sept. 2012, from GFS runs initialized at a) 0000 UTC, 11 Sept., b) 685"
1200 UTC, 11 Sept., c) 0000 UTC, 12 Sept and from ECMWF runs initialized at d) 0000 UTC, 11 686"
Sept., e) 1200 UTC, 11 Sept., f) 0000 UTC, 12 Sept. 687"
688"
Figure 8: 6 h rainfall simulated (WRF inner grid) from 0600 UTC to 1200 UTC, 12 Sept. 2012, 689"
from GFS runs initialized at a) 0000 UTC, 11 Sept., b) 1200 UTC, 11 Sept., c) 0000 UTC, 12 Sept 690"
and from ECMWF runs initialized at d) 0000 UTC, 11 Sept., e) 1200 UTC, 11 Sept., f) 0000 UTC, 691"
12 Sept. 692"
693"
Figure 9: CAPE (green shading) and CIN (red contour = -20 J kg-1) at 0600 UTC, 12 Sept.; hourly 694"
rainfall (blue-red shading) at 0700 UTC. The orography is in grey tones. 695"
696"
Figure 10: As Fig. 3, but for GFS1112 simulated soundings at the location of Udine sounding 697"
station (46.03°N, 13.18°E). Temperature and θe are shown every hour, while horizontal winds every 698"
6 hours. 699"
700"
701"
Figure 11. From top to bottom: 10-min moving average of a) Lifted Index, b) θe of the most 702"
unstable parcel, c) Low level v-wind component and d) Storm Relative helicity, offshore over the 703"
Adriatic sea (45.4°N, 13.0 °E) (left) and in the grid point closer to Udine (46.03°N, 13.18 °E) 704"
(right) between 0500 UTC and 1200 UTC of Sept. 12. Note that the 0600 (1200) UTC Udine 705"
(WMO16044) sounding has been launched at 0526 (1059) UTC and has reached 500 hPa at 0545 706"
(1116) UTC, so that the corresponding indices are plotted at 0535 (1105) UTC. 707"
708"
Figure 12: a) Vertical velocity (shading; value between –1 and 1 m s-1 are not shown), vertical 709"
component of vorticity (c.i. = 0.005 s-1; black for negative values, white for positive, 0 not shown), 710"
and wind vectors at 5500 m, b) Maximum reflectivity (shading; values below 30 dbZ are not 711"
shown), θe (c.i. = 3 K; yellow contours) and wind vectors at 100 m, simulated by the WRF model 712"
inner grid (GFS1112 run) at 0905 UTC, Sept. 12. 713"
714"
Figure 13: as Figure 7, but for GFS1112 run at 0915 UTC, Sept. 12. The isosurface of w = 12 m s-1 715"
(orange) and rainwater content at 1000 m height (c.i. = 1 g kg-1) are also shown. 716"
717"
Figure 14: Two different trajectories wrapping around due to supercell rotation. The colors along 718"
the trajectories represent θe (in K). 719"
... On the one hand, when a blocked-flow situation persists, low-level convergence occurs well upstream of the orography (Fig. 3a). When the incoming marine flow is conditionally unstable, deep convection ("Upstream" HPEs in Davolio et al., 2016), even in the form of mesoscale convective systems (Davolio et al., 2009;Ricchi et al., 2021) and supercells (Manzato et al., 2015;Miglietta et al., 2016), may develop, which produce heavy rainfall and hail over the plain, even close to the coastal areas . In this category of persisting blocked-flow conditions (e.g., IOP18), characterized by low values of the Froude number U/N h (where U is the wind speed, N the upstream Brunt-Väisälä frequency, and h the mountain height; Rotunno and Ferretti, 2001;Rotunno, 2005, 2006), other parameters defined by the vertical profile provide an indication of the possible evolution and severity of the event. ...
... IOP2 allowed a detailed study of two intense convective systems evolving into a supercell Manzato et al., 2015;Miglietta et al., 2016) over the northeastern plain. A synoptic trough determined diffluent flow in the middle troposphere over NEI (a common worldwide condition favoring HPEs; see Pontrelli et al., 1999, andLin et al., 2001) and drove a very warm and moist southeasterly lowlevel jet stream from the Adriatic. ...
... Finally, Miglietta et al. (2016) showed that the low-level cold air trapped in the narrow Alpine valleys can be crucial for limiting the northward extent of the warm-air tongue. Due to their relatively coarse resolution, global analyses/forecasts often miss this feature, possibly leading to the significant climatological underestimation of the rainfall simulated by ECMWF forecasts in the FVG plain and coastal area during summer . ...
Article
Full-text available
The first Special Observation Period (SOP1) of HyMeX (Hydrological cycle in the Mediterranean eXperiment) was held in fall 2012 and focused on heavy precipitation events (HPEs) and floods in the northwestern Mediterranean. Nine intensive observation periods (IOPs) involved three Italian target areas (northeastern Italy, NEI; Liguria and Tuscany, LT; central Italy, CI), enabling an unprecedented analysis of precipitation systems in these regions. In the present work, we highlight the major findings emerging from the HyMeX campaign and in the subsequent research activity over the three target areas by means of conceptual models and through the identification of the relevant recursive mesoscale features. For NEI, two categories of events (Upstream and Alpine HPEs) were identified, which differ mainly in the temporal evolution of the stability of the upstream environment and of the intensity of the impinging flow. The numerical simulation of convection in the Po Valley was found to be very sensitive to small changes in the environmental conditions, especially when they are close to the threshold between “flow-over” and “flow-around” regimes. For LT, HyMeX SOP1 focused on orographically enhanced precipitation over the Apennines and quasi-stationary mesoscale convective systems over the sea or close to the coast. For the latter category of events, associated with the majority of the recent HPEs, local-scale or large-scale convergence lines appear fundamental to trigger and sustain convection. These lines are affected not only by the orography of the region, but also by the perturbations induced by Sardinia and Corsica on the environmental flow, and, at later times, by cold pools formed via evaporation of precipitation. For CI, a high low-level moisture content and marked low-level convergence over the sea were critical to support deep convection in the IOPs affecting the Tyrrhenian coast. For the HPEs affecting the Adriatic regions, a cut-off low over the Tyrrhenian Sea induces intense bora over the Adriatic basin. Low-level convergence triggers convection over the sea, while orographic uplift produces stratiform precipitation. The Adriatic Sea plays a critical role mainly through air–sea exchanges, which modify the characteristics of the flow and in turn the effect of the orographic forcing.
... On the other hand, when the incoming marine flow is conditionally unstable, the blocked flow situation persists, resulting in low-level convergence well upstream of the orography (Fig. 3) that may evolve in the development of deep convection ("Upstream" HPEs in Davolio et al., 2016) or even mesoscale convective systems (Davolio et al., 2009;Ricchi et al., 2021) and supercells (Manzato et al., 2015;Miglietta et al., 2016), which produce heavy rainfall and hail over the plain, even close to the coastal areas . In this category (e.g., IOP18), characterized by high values of the Froude number, 220 other parameters defined by the vertical profile provide an indication of the possible evolution and severity of the event. ...
... IOP2 allowed a detailed study of two intense convective systems evolving into a supercell (Ferretti et al., 2014;Manzato et al., 2015;Miglietta et al., 2016) over the northeastern plain. A synoptic trough determined diffluent flow in the middle troposphere over NEI (a common worldwide condition favoring HPEs; see Pontrelli et al., 1999, Lin et al., 2001) and drove 250 a very warm and moist southeasterly low-level jet (another ingredient favorable to HPEs) from the Adriatic. ...
... Finally, Miglietta et al. (2016) showed that the low-level cold air trapped in the narrow Alpine valleys can be crucial to limit the northward extent of the warm air tongue. Due to their relatively coarse resolution, global analyses/forecasts often miss this feature, possibly leading to the significant climatological underestimation of the rainfall simulated by ECMWF forecasts 290 in the FVG plain and coastal area during summer . ...
Preprint
Full-text available
The first Special Observation Period (SOP1) of HyMeX (Hydrological cycle in the Mediterranean eXperiment) was held in Fall 2012 and focused on heavy precipitation events (HPEs) and floods in the northwestern Mediterranean. Nine intensive observation periods (IOPs) involved the three Italian target areas (north-eastern Italy, NEI; Liguria and Tuscany, LT; central Italy, CI), enabling an unprecedented analysis of precipitation systems in these regions. In the present work, we highlight the major findings emerging from the HyMeX campaign and in the subsequent research activity over the three target areas, by means of conceptual models and through the identification of the relevant recursive mesoscale features. For NEI, two categories of events (Upstream and Alpine HPEs) have been identified, which differ mainly in the temporal evolution of the stability of the upstream environment and of the intensity of the impinging flow (i.e., the Froude number). The numerical simulation of convection in the Po Valley was found very sensitive to small changes in the environmental conditions, especially when they are close to the threshold between “flow-over” and “flow-around” regimes. Some mesoscale features (e.g., the presence of a shallow pressure minimum in the eastern Po Valley) were identified as fundamental to adequately simulate the detailed evolution of severe convective episodes. For LT, HyMeX SOP1 focused on orographically-enhanced precipitation over the Apennines and quasi-stationary mesoscale convective systems over the sea or close to the coast. For the latter category of events, associated with the majority of the recent HPEs in the area, local-scale or large-scale convergence lines appear fundamental to trigger and sustain convection. These lines are affected not only by the orography of the region, but also by perturbations induced by Sardinia and Corsica on the environmental flow. Cold pools formed via evaporation of precipitation also played a major role in determining the position of the trigger at later times. The accurate representation of the moisture structure below 2 km is the key to an accurate simulation of the timing and location of precipitation. For CI, a high low-level moisture content and marked low-level convergence over the sea were critical to support deep convection in IOPs affecting the Tyrrhenian coast. Also, an elevated moisture plume from the Tropics was observed to locally reinforce the intensity of the updrafts. For HPEs affecting the Adriatic regions, generally a cut-off low over the Tyrrhenian Sea induces intense Bora over the Adriatic basin. Low-level convergence triggers convection over the sea, while orographic uplift produces stratiform precipitation. The Adriatic Sea plays a critical role mainly through air-sea exchanges, which modify the characteristics of the flow and in turn the effect of the orographic forcing.
... Punge et al. [11] gave a general perspective about the future occurrence of hail in Europe, identifying the Mediterranean basin as one of the main hotspots. The Italian peninsula, due to its complex morphology, is prone to supercell formation, which sometimes is responsible for hailfall [12]. In fact, the presence of mountains and of coastlines favors low-level horizontal wind shear, a necessary ingredient for supercell development [13]; also, the warm Mediterranean Sea is a source of energy and moisture that may feed the storm development [14]. ...
... The Adriatic basin, especially on its northern side, is strongly affected by hailstorms; in fact, the Apennines, the Dinaric Alps, and the Alps form a natural corridor confined between the Italian peninsula and the Balkans, where southeasterlies (Sirocco) travel nearly undisturbed over the sea, bringing warm and moist air to the northern Adriatic basin, where they may meet cold northeasterlies (Bora), thus favoring windshear and strong instability conditions [12,15,16]. The supercell developed on 10 July 2019 along the Italian Adriatic coast, which is the subject of the present study, represents an event of this kind, with considerable hailstone sizes up to 14 cm at the surface [17]. ...
Article
Full-text available
On 10 July 2019, a giant hail-bearing supercell hit the Adriatic coast of central Italy. Hailstones with a maximum diameter of 14 cm were reported in the city of Pescara between 10:00 and 11:00 UTC. In this work, the main synoptic and mesoscale features, responsible for the triggering and the development of the supercell, are analyzed using the WRF model. The intrusion of Bora wind over the northern and central Adriatic was relevant for two reasons: on the one side, the arrival of low-level cold air produced an uplift of the pre-existing warm air and favored the triggering of convection; on the other side, the strong vertical wind shear, also due to the presence of intense upper-level southwesterlies, created conditions favorable to the formation of the supercell. The predictability of the event is also discussed, comparing simulations starting at different initial times and forced with GFS and IFS forecasts. The model results show that the runs initialized at earlier times reproduced more accurately the track and the time evolution of the supercell. The HAILCAST module of WRF was also used to simulate hailstorm characteristics, such as the average hailstone diameter. WRF-HAILCAST simulations proved to be in fair agreement with the radar reflectivity retrievals and with local reports.
... Valley is characterized by an almost flat terrain surrounded by the Alps in the northern and western sectors and Apennines in the south. The short distance from the Mediterranean Sea to the South, which advects moist and warm air, makes this area particularly complex and prone to severe and organized convection [30][31][32][33]. Thunderstorms are, in most cases, associated with frontal passage (generally cold, [30]) across the Valley and, due to the water vapor and heat accumulation at low levels, are often associated with violent phenomena, such as hail, strong winds, squall lines, supercells and even tornadoes, especially in the summer period [34]. ...
Article
Full-text available
The growth of air transport demand expected over the next decades, along with the increasing frequency and intensity of extreme weather events, such as heavy rainfalls and severe storms due to climate change, will pose a tough challenge for air traffic management systems, with implications for flight safety, delays and passengers. In this context, the Satellite-borne and IN-situ Observations to Predict The Initiation of Convection for ATM (SINOPTICA) project has a dual aim, first to investigate if very short-range high-resolution weather forecast, including data assimilation, can improve the predictive capability of these events, and then to understand if such forecasts can be suitable for air traffic management purposes. The intense squall line that affected Malpensa, the major airport by passenger traffic in northern Italy, on 11 May 2019 is selected as a benchmark. Several numerical experiments are performed with a Weather Research and Forecasting (WRF) model using two assimilation techniques, 3D-Var in WRF Data Assimilation (WRFDA) system and a nudging scheme for lightning, in order to improve the forecast accuracy and to evaluate the impact of assimilated different datasets. To evaluate the numerical simulations performance, three different verification approaches, object-based, fuzzy and qualitative, are used. The results suggest that the assimilation of lightning data plays a key role in triggering the convective cells, improving both location and timing. Moreover, the numerical weather prediction (NWP)-based nowcasting system is able to produce reliable forecasts at high spatial and temporal resolution. The timing was found to be suitable for helping Air Traffic Management (ATM) operators to compute alternative landing trajectories.
... In fact, Fig. 8 in Punge et al. (2014) and Fig. 6 of Punge et al. (2017) show how, from satellite data, one can derive that NE Italy is a hotspot for hailstorms in Europe, as is the case for lightning flashes (Manzato et al. 2022, Taszarek et al. 2020. This is probably due to FVG's close proximity to the Marano and Grado lagoons and the Adriatic Sea to the south, which are a source of low-level moisture, and the Prealps and Alps to the West, North and East, which provide orographic lift and low-level convergence (Davolio et al. 2016, Miglietta et al. 2016). Manzato (2012) prepared a climatology of the number of hailpads impacted by hail in FVG for 1992 through 2009. ...
Article
Despite that hail is a well known meteorological hazard, it is hard to find long records of hail observed at the ground with high spatial resolution. Most hail climatologies are based on remote–sensing observations or an inhomogeneous network of human observers. In the plain of Friuli Venezia Giulia (NE Italy), a hailpad network of 367 stations has operated since 1988. During the 1988–2016 warm seasons 7,782 hailpads were impacted by hailstones and more than one million dents were observed and automatically analyzed, even though only 63% of them were associated with valid hailstone dents. In this work, this large quantity of direct hail observations is used to build a hail climatology in terms of hailstone size, areal–density and flux of kinetic energy. From the empirical distributions of data collected it is possible to fit statistical distributions to the different hail/hailpads behaviors. In particular, it is also possible to find an approximate estimation of the flux of kinetic energy based only on the largest hail diameter observed on the hailpad. Lastly, temporal and spatial distributions of these characteristics are investigated. Hailstones are larger along a southwestern–to–northeastern alley, that is parallel to the main Prealpine crest, with the very largest sizes being more frequent on the south–western corner. The only hail climate change signal that one can infer from the analysis of these multidecadal trends is that, in more recent years, hailstorms seem to produce fewer and larger hailstones, on average.
... SOP-related studies identified a similar low-level flow characteristic associated with heavy rainfall over northeastern Italy. In both the analyzed events that occurred during IOP2b (Manzato et al., 2015;Miglietta et al., 2016) and IOP18 , the blocking of southerly low-level marine inflow in the form of a northeasterly barrier wind in front of the Alps produced strong and localized convergence, favoring convection triggering (Fig. 4e). Through additional modeling investigations of similar events in the past, this was recognized as a typical mechanism for deep convection (even supercell) development over the area. ...
Article
Full-text available
Heavy precipitation (HP) constitutes a major meteorological threat in the western Mediterranean (WMed). Every year, recurrent events affect the area with fatal consequences for infrastructure and personal losses. Despite this being a well-known issue widely investigated in the past, open questions still remain. Particularly, the understanding of the underlying mechanisms and the modeling representation of the events must be improved. One of the major goals of the Hydrological Cycle in the Mediterranean Experiment (HyMeX; 2010–2020) has been to advance knowledge on this topic. In this article, we present an overview of the most recent lessons learned from HyMeX towards an improved understanding of the mechanisms leading to HP in the WMed. The unique network of instruments deployed as well as the use of finer model resolutions and coupled models provided an unprecedented opportunity to validate numerical model simulations, develop improved parameterizations, and design high-resolution ensemble modeling approaches and sophisticated assimilation techniques across scales. All in all, HyMeX, and particularly the science team heavy precipitation, favored the evidencing of theoretical results, the enrichment of our knowledge on the genesis and evolution of convection in a complex topography environment, and the improvement of precipitation forecasts. Illustratively, the intervention of cyclones and warm conveyor belts in the occurrence of heavy precipitation has been pointed out, and the crucial role of the spatiotemporal distribution of atmospheric water vapor for the understanding and accurate forecast of the timing and location of deep convection has been evidenced, as has the complex interaction among processes across scales. The importance of soil and ocean conditions and the interactions among systems were highlighted, and such systems were specifically developed in the framework of HyMeX to improve the realism of weather forecasts. Furthermore, the benefits of cross-disciplinary efforts within HyMeX have been a key asset in bringing our knowledge about heavy precipitation in the Mediterranean region a step forward.
... Areas of Europe so far exempted by these phenomena could be hit more frequently by such events thus causing damages and devastation to densely populated Mediterranean regions that are that could be neither accustomed nor prepared to face similar hazards. In addition, some studies have highlighted differences of Mediterranean supercells with events occurring in Great Plains (Taszarek et al., 2020;Miglietta et al., 2016). ...
Article
Ongoing climate change is considered to be responsible for the intensification and increased frequency of extreme weather events. The Mediterranean basin is not exempted by such modification processes as testified by the recent increase in the number of severe convective storm and mesocyclones. In recent years this kind of events caused many victims and damages in the such area, due to its geomorphological configuration and to the massive urban development of the last decades. Due to their local-scale nature Mediterranean extreme events can hardly be forecast with the use of numerical weather prediction systems and cannot adequately observed by using satellite platforms. These methods are not as straightforward as those based on radar observations because their spatial-temporal resolution is not necessarily adequate to resolve exceptional atmospheric phenomena as a weather radar can indeed offer. This article uses weather radar observations of an exceptional Mediterranean hail-bearing supercell that hit the central-eastern coast of the Adriatic Sea on 10 July 2019 causing flash flood and giant hail. Two operational dual-polarization Doppler C-band radars, managed by the Civil Protection Department of Italy, were able to observe, in an operational framework, the genesis and the evolution of the supercell, allowing for a detailed analysis of the event. Exploiting the unique time-resolved high-resolution three-dimensional available measurements, the combined use of a wind-field retrieval scheme and the interpretation of the dual-polarization radar observed features reveals some unique microphysical processes, rarely documented through radar observations at the Mediterranean latitudes. Results of this work improve the understanding of mechanisms and processes of giant hail formation and it is crucial to improve the nowcasting and forecasting skill for the early detection of hazardous meteorological events.
... The project "Comparison of Tornadic Supercells and their environmental conditions in Japan and Italy", funded by the cooperation program between the Japan Society for the Promotion of Science (JSPS) and the Italian National Research Council (CNR), promoted further research on tornadoes in Italy. Starting from the analysis of a tornado-spawning supercell in northeastern Italy [26], which showed peculiar mesoscale environmental conditions compared to those typical, for example, of the US Great Plains, the project had the objective of identifying similarities and differences in the environments conducive to tornadogenesis in Italy with respect to Japan and the US. A waterspout over the Liguria region was deeply investigated during the project [27], showing that waterspouts of modest intensity can be identified using radar reflectivity and Doppler radar velocity, even when the vortex is a few tens of km away from the radar site. ...
Article
Full-text available
Characteristics of extratropical cyclones that cause tornadoes in Italy are investigated. Tornadoes between 2007 and 2016 are analyzed, and statistical analysis of the associated cyclone structures and environments is performed using the JRA-55 reanalysis. Tornadoes are distributed sporadically around the cyclone location within a window of 10° × 10°. The difference in the cyclone tracks partially explains the seasonal variability in the distribution of tornadoes. The highest number of tornadoes occur south of the cyclone centers, mainly in the warm sector, while a few are observed along the cold front. Composite mesoscale parameters are examined to identify the environmental conditions associated with tornadoes in different seasons. Potential instability is favorable to tornado development in autumn. The highest convective available potential energy (CAPE) in this season is associated with relatively high-temperature and humidity at low-levels, mainly due to the strong evaporation over the warm Mediterranean Sea. Upper-level potential vorticity (PV) anomalies and the associated cold air reduce the static stability above the cyclone center, mainly in spring and winter. On average, the values of CAPE are lower than for US tornadoes and comparable with those occurring in Japan, while storm relative helicity (SREH) is comparable with US tornadoes and higher than Japanese tornadoes, indicating that the environmental conditions for Italian tornadoes have peculiar characteristics. Overall, the conditions emerging in this study are close to the high-shear, low-CAPE environments typical of cool-season tornadoes in the Southeastern US.
Article
Full-text available
A new lightning–flash and convective initiation climatology is developed over the Alpine area, one of the hotspots for lightning activity in Europe. The climatology uses cloud–to–ground (CG) data from the European Cooperation for LIghtning Detection (EUCLID) network, occurring from 2005 to 2019. The CG lightning data are gridded at a resolution of approximately 2 km and 10 min. A new and simple method of identifying convective initiation (CI) events applies a spatiotemporal mask to the CG data to determine CI timing and location. Although the method depends on a few empirical thresholds, sensitivity tests show the results to be robust. The maximum activity for both CG flashes and CI events is observed from mid–May to mid–September, with a peak at the end of July; the peak in the diurnal cycle occurs in the afternoon. CI is mainly concentrated over and around the Alps, particularly in northern and northeastern Italy. Since many thunderstorms follow the prevailing mid–latitude westerly flow, a peak of CG flashes extends from the mountains into the plains and coastal areas of northeastern Italy and Slovenia. CG flashes and CI events over the sea/coast occur less frequently than in plains and mountains, have a weaker diurnal cycle, and have a seasonal maximum in autumn instead of summer.
Article
Hailstorms are relatively frequent in Friuli Venezia Giulia region, northeastern Italy, and, for that reason, a network of manual hailpads has been set up there since the late 1980s. On July 4, 2007, a record number of hailpads in the network was impacted by hail (115 out of about 360 total stations). To the best of the authors' knowledge, no other single case-study of hailstorm has information coming from such a large quantity of hailpads impacted by hail. These hailpads are analysed using an automatic software, able to interpolate each hail dent with an ellipse. From these data, the distribution of hailstone diameters, areal hailstone density on the pads and of the total kinetic energy flux are computed and analysed in the present study. A parallel analysis of radar maximum reflectivity and cloud-to-ground lightning shows that the evolution of the hailstorm can be divided into three different phases, the first phase being the most severe in terms of lightning and hail. Finally, numerical simulations performed with a nonhydrostatic mesoscale model, implementing a prognostic module for the explicit simulation of hail (WRF-HAILCAST), show that the simulation system provides useful information on the severity of the event in terms of maximum hailstone diameter and reflectivity shape.
Article
Full-text available
During the first HyMeX Special Observation Period (SOP1) field campaign, the target site of North Eastern Italy (NEI) experienced a large amount of precipitation, locally exceeding the climatological values and distributed among several heavy rainfall episodes. In particular, two events that occurred during the last period of the campaign drew our attention. These events had common large-scale patterns and a similar mesoscale setting, characterised by southerly low-level flow interacting with the Alpine orography, but the precipitation distribution was very different. During IOP18 (31 October – 1 November 2012), convective systems were responsible for intense rainfall mainly located over a flat area of the eastern Po Valley, well upstream of the orography. Conversely, during IOP19 (4 – 5 November 2012), heavy precipitation affected only the Alpine area. In addition to IOP18 and IOP19, the present study analyses other heavy-precipitation episodes that display similar characteristics and which occurred over NEI during the autumn of recent years. A high-resolution (2-km grid spacing) non-hydrostatic NWP model and available observations are used for this purpose.
Article
Full-text available
The possibility offered by the internet to share pictures of tornadoes and the storm-report archiving in the European Storm Weather Database, have made it apparent that their occurrence over Europe has been underestimated. Together with weak waterspouts and tornadoes, large and intense vortices are occasionally observed. Among these, an EF3 multi-vortex tornado, with a path width of some hundreds of meters affected southeastern Italy on 28 November 2012, causing one casualty and estimated damage of 60 M to the largest steel plant in Europe. A tide gauge positioned near the location of tornado landfall and a vertical atmospheric profile available a few hours later near the affected region represent unique sources of information for these events in the Mediterranean. During its transit across the port of Taranto, a waterspout, which was to become the tornado, was observed to have induced a sea-level rise of about 30 cm. The supercell responsible for the tornado developed from convective cells triggered by orographic uplift over the Apennines. The 0 – 1 km wind shear was exceptional in comparison with other Italian tornadoes, and was remarkable in comparison with US events as well. Other indices for severe convection diagnosis also showed extremely high values. The occasional occurrence of events with similar or stronger intensities over Italy emphasizes the need for the Distributed National Weather Service, which will integrate Italian meteorological institutions under one agency and is currently under development, to devise a warning system dedicated to the monitoring and prediction of severe convective events.
Book
Mesoscale Meteorology in Mid-Latitudes presents the dynamics of mesoscale meteorological phenomena in a highly accessible, student-friendly manner. The book's clear mathematical treatments are complemented by high-quality photographs and illustrations. Comprehensive coverage of subjects including boundary layer mesoscale phenomena, orographic phenomena and deep convection is brought together with the latest developments in the field to provide an invaluable resource for mesoscale meteorology students. Mesoscale Meteorology in Mid-Latitudes functions as a comprehensive, easy-to-use undergraduate textbook while also providing a useful reference for graduate students, research scientists and weather industry professionals. Illustrated in full colour throughout. Covers the latest developments and research in the field. Comprehensive coverage of deep convection and its initiation. Uses real life examples of phenomena taken from broad geographical areas to demonstrate the practical aspects of the science.
Chapter
General characteristicsSquall line structureSquall line maintenanceRear inflow and bow echoesMesoscale convective complexes
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This expository paper documents an experimental, real-time, 10-member, 3-km, convection-allowing ensemble prediction system (EPS) developed at the National Center for Atmospheric Research (NCAR) in spring 2015. The EPS is particularly unique in that continuously cycling, limited-area, mesoscale en- semble Kalman filter analyses provide diverse initial conditions. In addition to describing the EPS con- figurations, initial forecast assessments are presented that suggest the EPS can provide valuable severe weather guidance and skillful predictions of precipitation. The EPS output is available to operational forecasters,many of whomhave incorporated the products into their toolboxes.Given such rapid embrace of an experimental system by the operational community, acceleration of convection-allowing EPS development is encouraged.
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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.
Article
Friuli Venezia Giulia (FVG) is a region in Italy with very complex orography, having an annual rainfall amount that varies from about 900 mm on the coast to more than 3200 mm in the Julian Prealps. A network of 104 raingauges placed around the FVG territory was used to extract the absolute maximum rain accumulated every 6 h, during the period 16 February 2006 to 15 February 2015 (9 years). Interannual, annual, weekly and daily cycles of three classes of rain intensities are analyzed, finding that significant rainfalls (MaxRain > 5 mm) are more frequent in the May to mid-August period, while the heaviest rainfalls (> 40 mm) are more probable between May and the beginning of December, with a peak at the very beginning of November. ECMWF 6-h forecasts at 18 gridpoints (spaced at 0.25°) above the FVG region are studied for the same period, to find the maximum 6-h rain forecasted by the ECMWF model from + 6 to + 48 h and correlate it with the observed maximum rain of all the 104 raingauges. It is found that the correlation coefficient R is higher at 0000–0600 UTC and minimum at 1800–0000 UTC, while the BIAS is always negative (underestimation), varying between − 3.5 and − 6.9 mm. Looking at more homogeneous subareas, ECMWF has a much worse BIAS and RMSE for the Prealps zone, while its correlation coefficient is lower for the coastal and plains zones. For comparison, a similar exercise is repeated using a LAM model (ALADIN-ARSO), finding better BIAS and RMSE, but a lower skill for the mean correlation coefficient. Hence, a linear statistical method (multiregression with exhaustive input selection) for forecasting the maximum 6-h rain using as candidate predictors the direct model output (absolute values, anomalies, standardized values, plus mean, max and SD in time and space) is developed independently for four different sub-regions and two periods of the year starting from the ECMWF forecast. It is found that the strong BIAS in the Prealpine area can easily be removed, substantially improving the forecast, in particular during the October–April period, while the plains and coastal area, in particular during May–September, have the lowest predictability.