Download Extremely strong convection in Lithuania on 8th of

EXTREMELY STRONG CONVECTION IN LITHUANIA
ON 8th OF AUGUST 2010
Viktorija Grusaite
Lithuanian Hydrometeorological Service, Rudnios 6, LT-09300 Vilnius, Lithuania
Abstract
Squalls, heavy rain and thunderstorms (mostly in the central part of the country) were reported at the
night of 8th of August. The strongest wind gusts reached even 30 m/s (Kaunas airport) and exceeded
the value of 16 m/s and higher in the major area of the country. During the day time the rainfall (~41
mm/1 h and ~69 mm/ 8 h) was registered in the western part of Lithuania. Moreover, there were
thunderstorms with hail of diameter 2 cm. Synoptic situation was favourable for this event to occur.
The warm front was approaching the north part of Lithuania and cold front with waves was parallel to
the southeasterly flow. Thus, Lithuania was in warm sector – within very unstable subtropical airmass.
At the midnight, the convergence line emerged. Warm air trough at the 850 hPa level approached the
western part of Lithuania. The advection of cold air was getting stronger and as a result atmospheric
instability was intensifying as well. This cold front started to move towards Lithuania. Furthermore, the
cold air advection at the higher levels was contributing the development of convection at the
convergence line. Cb clouds tops achieved the height of the 15-17 km. According to the sounding
data, cloud tops broke the tropopause level and demonstrated how intensive the convection was.
There were three main factors operating together: very strong ascent of warm and moist air, strong
convergence at the surface level and cold air advection at higher levels. Numerical models (7 th of
August 00UTC) provided information 24-30 hours in advance that Lithuania will be in a warm and
moist airmass. There was no indications about the convergence line. Moreover, the models lagged
behind of approximately 12 hours. Hence, strong convection occurred at night instead of the day. Due
to these extreme weather phenomena, significant damages took place. According to the government
announcement the squall had made damage to the forests of about 7 million Euro, even four people
were lost. Sounding data, various MSG images and cross sections of atmospheric parameters were
analysed during the presentation.
SYNOPTIC SCALE OVERVIEW
There is known that thunderstorms to form usually need:
• warm, moist air at low levels;
• cool dry air at upper levels;
• synoptic scale disturbance;
• upper level divergence.
The synoptic situation was very favourable for the extremely strong convection that occurred 8th of
August 2010. The blocking anticyclone was situated over North-East part of Europe for a quite long
time. Very hot and moist tropical air was transported constantly into Lithuania. Air temperature
reached new summer records. The 7 th day of August was reported as the hottest day and temperature
had reached 30-34°C.
As the blocking anticyclone started move slowly to the east, the colder air started to spread towards
Lithuania. Though, the east part of the trough was still blocked by the anticyclone located NE part of
Europe. A shallow low formed with central pressure of 1011 hPa over the Baltic Sea (fig. 1). Over the
anticyclone center (over Russia) there were temperatures of 35-38°C, 18-24°C at the 850 hPa surface
and the 0°C isotherm was located at 4 000 m height. The inflow of cold air was on the west side of the
trough. The temperature at the surface was around 15-20°C and 7-10°C at the 850 hPa level.
Furthermore, the air was moist as well. Over that area the cold front formed which later influenced the
weather in the eastern part of Europe.
Figure 1: Synoptic situation on the 08.08.2010 at 0000 UTC.
THE EVOLUTION OF EXTREMELY STRONG CONVECTION AND THE USE OF SATELLITE
IMAGERY
The 2100 UTC (07.08.2010) surface analysis reveals the approaching cold front with waves towards
South-west part of Lithuania (not shown). Ahead of this cold front the moist air started to accumulate,
the dew point depression (T-Td) exceeded 1-3 °C and 0 °C in the south part of Lithuania. The
instability line started to form. At 2200 UTC the first thunder was registered. According to the
0000 UTC surface analysis the convergence line moved to the northeastern parts (fig. 2). After 13
minutes the supercell reaches the center of Lithuania with the records of thunders and strong winds
over 59 knots (Kaunas airport), or around 30 m/s.
Figure 2: Synoptic situation on the 08.08.2010 at 0000 UTC over Lithuania.
The extremely strong convection with strengthening winds was formed under the tongue of warm air at
the 850 hPa layer and moist accumulated air at the same level. Favourable condition for development
of strong convection is high instability remaining for some time in the threat area. This is easy to depict
by looking at the sounding data from Kaunas meteorological station at 0000 UTC of 7 th of August (fig.
3). One of the main clue about potential squall formation was maximum wind – low level jet - at the
boundary layer (ground surface – 3000 m height). In this case strongest wind was registered 17,5 m/s
at 850 hPa, 20 m/s at 700 hPa, and 17,5 m/s at 500 hPa and at 300 hPa level as well. For this reason
extreme weather forms in frontal zone on periphery of anticyclone (fig. 2).
Figure 3: Sounding data on the 07.08.2010 at 0000 UTC.
Figure 4: Sounding data on the 08.08.2010 at 0000 UTC.
Thunderstorms and squalls became more intense where low pressure system developed so-called
mesocyclone. On the left side of convergence line a small-scale high pressure system formed (fig. 2).
Extremely strong convection developed in the warm air ahead of cold front where the surface air was
strongly heated during daytime. According to the general experience, 90% of convective wind
strengthening over 20 m/s occurs when the upper level ridge of warm air located in the East is pushed
away by the cold trough from the west.
The best way for the nowcasting was the use of satellite images. It should be noted that the weather
radar officially was launched in August 2011 in Lithuania. Therefore, it was a good option to monitor
the movement and the development of the supercell by having various satellite images and vertical
cross sections. The look at the MSG water vapor 6.2 channel image with the set geopotencial height
at 500 hPa shows the approaching supercell with trajectory parallel to isolines at 500 hPa level (fig. 5).
The Global Instability Index (GII) product describes the instability of the clear atmosphere that
indicates extremely unstable atmosphere around the approaching supercell (fig. 6).
Figure 5: 07 August 2010 21:00 UTC – MSG WV 6.2;
black: MSLP; green: 500 hPa;
Figure 6: 07 August 2010 21:00 UTC – MSG IR 10,8; Global
Instability Index (GII).
High CAPE values are also good indications for predicting strong convection (fig. 7).
Figure 7: 07 August 2010 21:00 UTC – MSG IR 10.8; CAPE.
Moreover, the vertical cross sections can provide useful information about the atmosphere stability,
temperature and vorticity advection, relative humidity, potential vorticity, convergence/divergence,
omega, cloud cover and wind. Figure 8 shows the height where is located maximum wind with
temperature advection at 7 August 2010 2100 UTC.
Figure 8: 07 August 2010 21:00 UTC – beige: wind; red: warm advection; dashed blue: cold advection.
At the time when supercell moved into central Lithuania the satellite images showed bright colors of
the cloud top indicating the strong ascent (fig. 9). At the MSG Dust RGB image over the center of
supercell there is brighter color circle (orange color) that might indicate the overshooting top (fig. 10).
Figure 9: 08 August 2010 00:00 UTC – MSG
Airmass RGB; green: Equiv. thickness; red: WA
700 hPa; blue: CA 700 hPa.
Figure 10: 08 August 2010 00:00 UTC – MSG Dust RGB.
The sounding was made some tens of kilometers away from the epicenter of the supercell at 0000
UTC. The data from this sounding revealed CAPE value of 411 J/kg. But according to the figure 9,
CAPE was much higher around 1000 J/kg and wind shear of 20 m/s. Figure 12 represents the
trajectory of the supercell over few hours. Unfortunately, the track of the supercell was mostly
between meteorological stations and there are not descriptive data about how strong the supercell
was. But there are evidences of the disproportionate amount of damage and 4 casualties.
Figure 11: 08 August 2010 00:00 UTC – MSG IR 10.8;
ECMWF Windshear 0-6 km; ECMWF MLCAPE (J/kg).
Figure 12: MSG IR 10.8; the trajectory of the supercell.
CONCLUSION AND RECOMMENDATIONS
Nowadays the extreme weather phenomena are getting more common and severe. The last record of
the extremely strong convection with huge damage amount was in 27 July 1951. In 2011 there were
registered 13 strong convective storms with 2 tornado events. It is urgent to know features of potential
extreme phenomenon such as a supercell over specific or neighboring areas. The case studies helps
to collect meteorological information and indicate or predict extreme weather.
Recommendations for the forecasting potential convective wind strengthening:
1. Determine intensity of potential convective development:
• unstable and warm air transfer into the threat area, unstable stratification above -23 °C
isotherm;
• favourable conditions for strong convection are:
o ridge of warm and moist air at 850 hPa level;
o trough of cold air at 500 hPa.
2. Sounding data:
• set the stratification curve;
• determine moist layer (dew point depression < 12 °C in a layer from surface to 3000 m;
except the ground level);
• cloud top height – overshooting tropopause.
3. Detect convergent flow areas.
4. Wind field:
• wind shear in direction and speed;
• mesoscale jets where the speed is higher than 10 m/s up to 700 hPa level.
References:
Satellite images – www.satreponline.org
Satellite image with CAPE and WS - M. Barczyk, M. Ozka (Institute for Meteorology and Water
Management)
Image of the trajectory – K. Kornel (Hungarian meteorological service)