1. A CLIMATOLOGY OF GREEK SUPERCELLS D. Foris (1) and V. Foris (2) (1) Meteorological Applications Center Hellenic Agricultural Insurance Organization Thessaloniki, Greece (2) Physics Department Aristotle University of Thessaloniki Thessaloniki, Greece

2. 14th EMS & 10th ECAC6-10/10/2014, Prague 2 STRUCTURE OF PRESENTATION 1.Introduction 2.Definition 3.Spatial distribution 4.Temporal distribution 5.Kinematic analysis 6.Synoptic environment 7.Thermodynamic environment 8.Radar signatures 9.Conclusions STRUCTURE OF PRESENTATION

3. 14th EMS & 10th ECAC6-10/10/2014, Prague 3 1. INTRODUCTION A thunderstorm is considered severe (according to American NWS) if: –Hail diameter is larger than 25 mm, or –Winds reach at least 93 km/h, or –It exhibits a combination of the above two criteria Although severe thunderstorms are not uncommon in Greece, supercells are rare. During the warm seasons of the last 30 years only 40 such cases were recorded. These were identified during radar watch in the frame of Greek National Hail Suppression Program (GNHSP), which runs since 1984, using 2 radars, installed in Northern and Central Greece (S-band until 2006, C-band thereafter). Most parts of continental Greece fall within the maximum unambiguous range of the above conventional radars. Although supercells start as ordinary cells, they usually develop to meso-β scale, being part of a mesoscale convective system (MCS).

4. 14th EMS & 10th ECAC6-10/10/2014, Prague 4

5. 14th EMS & 10th ECAC6-10/10/2014, Prague 5 2. DEFINITION The conceptual model of supercells was developed by Browning (1962) and refined by Marwitz (1972) and others since then, based mainly on the features they exhibited in the Great Plains of USA. These particular characteristics are: –The presence of a mesocyclone –The symbiotic downdrafts (in forward and rear flank) and tilted rotating updraft –A continuous propagation to the right of mean tropospheric wind –A long distance traveled and a great lifetime –An overshooting top into the stratosphere –A flanking line –A wall cloud –Very large hail –Possible tornadoes –A weak echo region (WER) on the radar RHI –A hook echo on radar PPI –Development in unstable, highly-sheared environment

6. 14th EMS & 10th ECAC6-10/10/2014, Prague 6 PLAN VIEW

7. 14th EMS & 10th ECAC6-10/10/2014, Prague 7 SIDE VIEW

8. 14th EMS & 10th ECAC6-10/10/2014, Prague 8 DEFINITION (continued) The need for a careful adaptation of this model, suitably modified, arises from Greece’s special climatic and topographic characteristics (small terrain dimensions, complex topography with mountain chains alternating with not extended plains, vicinity to the sea). For this reason, a “light” definition is proposed, based on the presence of a hook echo in the horizontal and a (bounded) weak echo region in the vertical, for at least 3 volume scans of the radar (approximately 10 min).

9. 14th EMS & 10th ECAC6-10/10/2014, Prague 9 3. SPATIAL DISTRIBUTION Startup (hotspot) locations are over land in 93% of the cases. Source regions coincide with 5 mountain chains that span the Greek peninsula from NNW to SSE. Therefore, topography plays a key role in the initiation of supercell storms (for the first stages of development).

10. 14th EMS & 10th ECAC6-10/10/2014, Prague 10 3. SPATIAL DISTRIBUTION (continued) Hail occurrence is higher in Thessaly (40%, red) and Central Macedonia (40%, green), while Eastern Macedonia receives a 15% and Western Macedonia only 5%. Hail size from these supercells varies from 18 to 64 mm in diameter, as reported from in situ pictures and reports of agronomists or from a hailpad network installed in Central Macedonia Plain.

11. 14th EMS & 10th ECAC6-10/10/2014, Prague 11 4. TEMPORAL DISTRIBUTION The yearly distributions of the number of supercells and of the 28 days on which these 40 supercells occurred, reveals two facts: 1.That the frequency of occurrence increases dramatically, probably due to climate change that favors extreme instability events, and 2.That in general supercells develop as isolated events, though in some cases supercell outbreaks occur, leading to the formation of multiple storms within the same day, over different locations.

12. 14th EMS & 10th ECAC6-10/10/2014, Prague 12 4. TEMPORAL DISTRIBUTION (continued) The monthly distributions of storms present a maximum in July, while June is the month with most storm days. Spring and autumn show lower frequencies. The June maximum coincides with the maximum of overall convective activity in Northern and Central Greece.

13. 14th EMS & 10th ECAC6-10/10/2014, Prague 13 4. TEMPORAL DISTRIBUTION (continued) The hourly distribution of the initiation time of supercells present maximum frequencies in the interval 10-14 UTC (13-17 LT), which coincides with maximum heating time. This implies that heating, apart from topography, is a triggering mechanism for these storms (as for all types of storms).

14. 14th EMS & 10th ECAC6-10/10/2014, Prague 14 5. KINEMATIC ANALYSIS The average speed of motion ranges from 10 to about 100 km/h.  The maximum (35%) occurs in the class 20-30 km/h.  Only the 20% of the storms moves faster than 50 km/h. The direction of motion is merely eastward.  E, NE, SE directions represent the 92% of the cases.  This is evident from the trajectory map.

15. 14th EMS & 10th ECAC6-10/10/2014, Prague 15 5. KINEMATIC ANALYSIS (continued) Trajectories of supercell storms

16. 14th EMS & 10th ECAC6-10/10/2014, Prague 16 5. KINEMATIC ANALYSIS (continued) The duration (lifetime) of these storms ranges from 1,5 to 9 h, while the distance traveled from 25 to 350 km.

17. 14th EMS & 10th ECAC6-10/10/2014, Prague 17 5. KINEMATIC ANALYSIS (continued) It seems reasonable to divide the storms into 4 regimes: –In time: short-lived and long-lived –In space: short-track and long-track Short-trackLong-track Short-lived15 (37,5%) 5 (12,5%) Long-lived 8 (20%)12 (30%)

18. 14th EMS & 10th ECAC6-10/10/2014, Prague 18 6. SYNOPTIC ENVIRONMENT The 28 days are categorized according to the prevailing synoptic types. By far the most favorable weather types are SW-flow (10 cases) and SWT-shortwave trough passage (7 cases). Less frequent are NW-flow and zonal flow (3 cases each), cut-off low and ridge (2 cases each) and closed low (1 case). The presence of a jet streak in the upper troposphere is known to be a factor that favors supercell development. From the 26 available soundings, a jet streak was present in 19 cases (73%), ranging from 60 to 100 kt. The position of the jet is crucial: its left exit region is the most favorable for supercell formation, as this is the region of Positive Vorticity Advection (PVA) and of divergence aloft.

19. 14th EMS & 10th ECAC6-10/10/2014, Prague 19 6. SYNOPTIC ENVIRONMENT (continued) Example of supercell development area in the left exit region of the jet streak.

20. 14th EMS & 10th ECAC6-10/10/2014, Prague 20 7. THERMODYNAMIC ENVIRONMENT Choice of a representative sounding in space and time proximity Too small sample (only 26 available soundings), though indicative Examination of stability indices and severe thunderstorm indices IndexRange (25%-75% percentile)Median (threshold)US criteria K28 to 3331> 40 TT46 to 5248> 51 LI1 to -4-2< -4 SW2 to 01< -4 TEI7.5 to 18.512> 9 SWEAT148 to 284227> 300 BRN24 to 2286010 to 45 +shear1.9 to 4.73.2> 5 dir_shear79 to 171135> 70 cap3.8 to 5.84.6> 2

21. 14th EMS & 10th ECAC6-10/10/2014, Prague 21 7. THERMODYNAMIC ENVIRONMENT (continued) The amount of buoyancy and shear in the environment helps determine storm type. The scatterplot of CAPE-SRH enlightens the interplay between them. It seems that environments favoring supercell formation are more influenced by CAPE than by helicity.

22. 14th EMS & 10th ECAC6-10/10/2014, Prague 22 7. THERMODYNAMIC ENVIRONMENT (continued) Hodographs in most cases comply with veering with height, leading to right- movers, presenting the characteristic turning in low-levels.

23. 14th EMS & 10th ECAC6-10/10/2014, Prague 23 8. RADAR SIGNATURES MinMax25-75% percentileMedian Z max (dBZ)597465 – 6766 H max (km)9.316.512.0 – 14.313.0 H 45 (km)6.814.511.2 – 12.511.7 Hmax and H45 are highly dependent on season. Minima appear in early spring, while maxima appear in July and August, when the depth of the troposphere is maximum.

24. 14th EMS & 10th ECAC6-10/10/2014, Prague 24 8. RADAR SIGNATURES (continued) Hook in the horizontal, (Bounded) Weak Echo Region in the vertical

25. 14th EMS & 10th ECAC6-10/10/2014, Prague 25 8. RADAR SIGNATURES (continued) Cell model developed with 2 couplets: inflow-outflow and mean wind-storm motion.  In 88% of the cases the outflow was to the right of the inflow by 60-170 o (average 131 o ), while in 12% of the cases the outflow was to the left of the inflow by 63-123 o (average 89 o ) For Great Plains storms this angle is 90 o.  In 73% of the cases supercells were right movers, deviating 5-77 o (average 29 o ) to the right of mean tropospheric wind, while 27% were left-movers, deviating 10-48 o (average 24 o ) to its left. For Great Plains storms this deviation is 60 o to the right.

26. 14th EMS & 10th ECAC6-10/10/2014, Prague 26 9. CONCLUSIONS Although the sample of Greek supercells is small, some preliminary climatological results may be drawn, given their common radar features of hook echo and weak-echo region (echo-free vault): –Mountain chains are favorable locations for supercell initiation –Hail occurrence maxima are observed in Central Macedonia and Thessaly –The number of such storms and storm days is continuously increasing –Supercells occur either isolated or in families (outbreaks) –June and July present the highest frequency –Daily maximum heating triggers their initiation –They travel mainly due east at an average speed of 20-30 km/h –They can be short- or long-lived and of short- or long-track –They usually occur under SW-flow or SWT passage –The presence of a jet streak favors their development, especially in its left exit region

27. 14th EMS & 10th ECAC6-10/10/2014, Prague 27 9. CONCLUSIONS (continued) –Stability and severe thunderstorm indices show in general values lower than international standards –A large CAPE contributes more than helicity to their formation –Hodographs are typical of right-movers –Maximum reflectivity’s threshold seems to be 66 dBZ and top’s 13 km –Hook echo and weak-echo region are always present –The angle between inflow and outflow is 130 o on average –They deviate about 30 o to the right of mean tropospheric wind Future work includes more elaborate examination using vorticity and divergence maps, satellite images, more specialized indices, etc.