Lyubov Trifonova, Anelia Gocheva, Tania Marinova, Lilia Bocheva

1. BALWOIS-200623-26 May, 2006 Ohrid, Republic of Macedonia Climatological Analysis of the Synoptic Situations Causing Dry Wind and Droughty Spells in Bulgaria Lyubov Trifonova, Anelia Gocheva, Tania Marinova, Lilia Bocheva National Institute of Meteorology & Hydrology Sofia, BULGARIA

2. Dry wind and Droughty spells(air temperature - humidity combination) - high air temperature, - low humidity - moderate wind - at the ground level Criterion for dry wind and droughty spells: • based on the criterion of M.Kulik • air temperature  25 ºC, • relative humidity  30 % • wind velocity  5 m/s (for dry wind) • at least during 3 consecutive days

3. Dry wind and droughty spells (used information) • Analyzed 249 stations in Bulgaria (1961-2000) • Eliminated stations: • Costal and mountain station • Stations with: • closure or crucial shifts; • frequent interruptions; • short available data periods; • partial but frequent lacks of data during the warm half-year • Number of stations are reduced to 150 stations (with high quality data series)

4. Dry wind and Droughty spells in Bulgaria • > 2500 cases of droughty days are identified • All cases are grouped to periods with: • 3 or more days • at least 10 stations registered drought or 3 stations registered dry wind • 74 cases of droughty spells and 28 dry wind spells are defined during 40 years period

5. Distribution of droughty spells (1961-2000) Droughty spells 1961-1970 - 26% 1971-1980 - 5 % 1981-1990 - 27% 1991-2000 - 41% Dry wind spells 1961-1970 - 21% 1971-1980 - 18 % 1981-1990 - 29% 1991-2000 - 32% Drought results are extreme for 1993 and 2000 yr.

6. Distribution of dry wind and droughty spells (1961-2000) Droughty spells July-September - about 80% July – 21% August – 37% September – 19% Dry wind spells July and August – about 70% July – 32% August 39%

7. Analysis of the synoptic situationscausing dry wind and droughty spells in Bulgaria • The NIMH’s historical archive of synoptic maps and NCEP/NCAR Reanalysis data files are used in the investigation • The considered synoptic situations are combined into 4 groups based on: • view of the isobars on the geopotential heights on the surface and upper-air • air temperature on 850 hPa; • formation and evolution of high- or low pressure centers and frontal systems, extending of ridges and troughs.

8. Synoptic situation 1: SW advection Geopotential height [dm] on 500 hPa Geopotential height [dm] on 1000 hPa • Deep cyclone over Northern Europe • Low gradient baric fieldover Bulgaria • Droughty spells prolong 3 - max 4 days • Dry wind phenomenon occurs mainly in the Western and Central part of Bulgaria • Extreme SW advection causes Foehn in many region of downwards of the mountains in the Western Bulgaria • Sand storms can be caused • Droughty spell is interrupted after the passing of the frontal system • Deep trough over Western Europe • Ridge over Atlantic • Advection of warm air from SW • Air temperature on 850 hPa increases up to 12 -14ºC in April, > 16ºC in May, Sept. and Oct. > 18-20ºC during summer months

9. Synoptic situation 2: NW advection Geopotential height [dm] on 500 hPa Geopotential height [dm] on 1000 hPa • Two deep troughs over the Atlantic and Eastern Europe • Ridge over Central Europe up to Scandinavia • NW advection on the eastern slope of the ridge • Air temperature on 850 hPa increases up to 12 -14ºC in June and October, up to 16 -18ºC in July - September • Ridge of high-pressure spread over Western and Central Europe and move to the East • Transformed Atlantic air mass is moving into the regions of Bulgaria • After 1-2 days of positive radiation balance the air temperature and humidity reach drought conditions • NW advection cause drought mainly for 4 days • In the periphery of ridge dry wind phenomena are registered, sand storms are possible

10. Synoptic situation 3: Radiation overheating Geopotential height [dm] on 500 hPa Geopotential height [dm] on 1000 hPa • Deep low-pressure field is over Northern Europe • Low gradient field usually from anticyclone type settles down over regions a few days before the registered drought • Positive radiation balance causes prolong radiation overheating and droughty spells for 5 - 6 days • Droughty spells are accompanied by dry wind mainly at the end of period when this synoptic situation transforms to a situation with SW advection and prolong up to 8 -10 days and even more • Active frontal zone removes to the north • Isobars have mainly zonal direction • A low baric gradient is over Southern Europe and Bulgaria with a weak ridge from south • Air temperature on 850 hPa increases up to 18-20 ºC over Southern Europe

11. Synoptic situation 4: NE advection Geopotential height [dm] on 500 hPa Geopotential height [dm] on 1000 hPa • Deep trough with closed contour in the upper air is over Western Europe and East Atlantic • Ridge of high-pressure also with closed contour is over Eastern Europe and Russia • Air temperature on 850 hPa increases up to 12 ºC over South-East Europe and South Russia • Stationary vast anticyclone spread over Eastern Europe and Russiaand the air transforms • Low gradient cyclone field lies over Western Europe and Africa • Such situation occurs very seldom

12. Distribution of the synoptic situationscausing dry wind and droughty spells in Bulgaria Drought Dry Wind SW 57% 68% NW 26% 25% RO 16% 7% NE 1% 0% • SW advection causes drought mainly for 3-4 days from April till October, but more often from July till September especially in August. • NW advection causes drought mainly for 4 days from June till October, but more often in July • Radiation overheating causes the droughty spells for 5-6 days mainly in August and very seldom in September and others summer months • NE advection causes droughts very seldomunder the condition that it is registered at least during 3 consecutive days and at 10 stations • Often the synoptic situations are combined and droughty spells prolong more than 10 days

13. Conclusions • More then 2500 cases of droughty days (on the base of THC analysis) in the representative stations are identified from the NIMH’s database for the period 1961-2000. • 74 droughty spells and 28 dry wind spells are registered during this 40 years’ period under precondition that droughts are registered at least during 3 consecutive days and at 10 stations (at 3 stations for dry wind) • 80% of droughty spells are observed for the period July-September and about 70 % of dry wind spells happen in July and August • 4 typical synoptic situations that caused those droughty and dry wind spells over Bulgaria are defined after analysis of the weather conditions • Droughty and dry wind spells are caused by synoptic situation with SW, NW advection, during weather with radiation overheating and very seldom with NE advection. Shares of the situations are respectively 57 %, 26 %, 16 % and 1 % for droughty spells and 68 %, 25 %, 7 % and 0 % for dry wind periods. Often the periods are combined and droughty spells prolong more than 10 days. During SW and NW advections sand storms are possible • During the last decade of the 20-th century the number of droughty spells when such periods are observed at least at 10 stations are increased – 41 % of all cases comparable with others decades 26 %, 6 % and 27 % respectively. The same is registered for dry wind spells when the phenomenon is observed at least at 3 stations – 32 % of all cases occur during the last decade and 21%, 18 % and 29 % respectively for the others decades. The results of the investigation confirm changes in weather conditions at the end of last century and increasing of the climate extremes

14. Thank you for your attention!