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A CLIMATOLOGY OF GREEK SUPERCELLS

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INTRODUCTION A thunderstorm is considered severe (according to American NWS) ... (for the first stages of development). 3. SPATIAL DISTRIBUTION (continued) ... – PowerPoint PPT presentation

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Title: A CLIMATOLOGY OF GREEK SUPERCELLS


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
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

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

4
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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
PLAN VIEW
7
SIDE VIEW
8
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).

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
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
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.

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
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
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
5. KINEMATIC ANALYSIS (continued)
  • Trajectories of supercell storms

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

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
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
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
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.
24
8. RADAR SIGNATURES (continued)
  • Hook in the horizontal, (Bounded) Weak Echo
    Region in the vertical

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 60-170o (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.

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
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.
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