A CLIMATOLOGY OF GREEK SUPERCELLS

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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

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STRUCTURE OF PRESENTATION
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

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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-ß scale, being part of a
  mesoscale convective system (MCS).

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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

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PLAN VIEW
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SIDE VIEW
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DEFINITION (continued)

• The need for a careful adaptation of this model,
  suitably modified, arises from Greeces 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).

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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).

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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.

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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
• That the frequency of occurrence increases
  dramatically, probably due to climate change that
  favors extreme instability events, and
• 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.

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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.
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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).
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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.

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5. KINEMATIC ANALYSIS (continued)

• Trajectories of supercell storms

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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.

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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-track Long-track
Short-lived 15 (37,5) 5 (12,5)
Long-lived 8 (20) 12 (30)
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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.

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6. SYNOPTIC ENVIRONMENT (continued)
Example of supercell development area in the left
exit region of the jet streak.
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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

Index Range (25-75 percentile) Median (threshold) US criteria
K 28 to 33 31 gt 40
TT 46 to 52 48 gt 51
LI 1 to -4 -2 lt -4
SW 2 to 0 1 lt -4
TEI 7.5 to 18.5 12 gt 9
SWEAT 148 to 284 227 gt 300
BRN 24 to 228 60 10 to 45
shear 1.9 to 4.7 3.2 gt 5
dir_shear 79 to 171 135 gt 70
cap 3.8 to 5.8 4.6 gt 2
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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.

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7. THERMODYNAMIC ENVIRONMENT (continued)

• Hodographs in most cases comply with veering with
  height, leading to right-movers, presenting the
  characteristic turning in low-levels.

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8. RADAR SIGNATURES
Min Max 25-75 percentile Median
Zmax (dBZ) 59 74 65 67 66
Hmax (km) 9.3 16.5 12.0 14.3 13.0
H45 (km) 6.8 14.5 11.2 12.5 11.7
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.
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8. RADAR SIGNATURES (continued)

• Hook in the horizontal, (Bounded) Weak Echo
  Region in the vertical

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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 60-170o (average 131o), while in
  12 of the cases the outflow was to the left of
  the inflow by 63-123o (average 89o)
• For Great Plains storms this angle is 90o.
• In 73 of the cases supercells were right movers,
  deviating 5-77o (average 29o) to the right of
  mean tropospheric wind, while 27 were
  left-movers, deviating 10-48o (average 24o) to
  its left.
• For Great Plains storms this deviation is
  60o to the right.

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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

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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 reflectivitys threshold seems to be 66
  dBZ and tops 13 km
• Hook echo and weak-echo region are always present
• The angle between inflow and outflow is 130o on
  average
• They deviate about 30o to the right of mean
  tropospheric wind
• Future work includes more elaborate examination
  using vorticity and divergence maps, satellite
  images, more specialized indices, etc.