National Institute of Meteorology & Hydrology, Sofia, BULGARIA

1. National Institute of Meteorology & Hydrology, Sofia, BULGARIA On Frequency Distribution and Intensity of Severe Convective Storms over Bulgaria Lilia Bocheva, Petio Simeonov, Tania Marinova BALWOIS 2006, Ohrid, Republic of Macedonia, 23 – 26 May 2006

2. Convective events produce very intense rainfall that often leads to river floods and flash floods causing significant losses of life and property damages, landslide activation, and other social and economic problems. The upward tendency of damages caused by natural disasters, great parts of which are severeconvective storms (SCS) from the Small-scale Weather Phenomena (SCSWP), supports the idea that extreme events, associated with the effects of climate change, occur with greater frequency (Easterling et al., 2000). • The series of hazardous events which affected Balkans and in particular Bulgaria last summer (from May to September 2005) were connected with SCS and really heavy rainfall events which produced floods and damages almost all over the country. These episodes show that additional information and investigations of such type of phenomena are necessary in order to be able to predict them more precisely. • The aim of this investigation is to study monthly and seasonal distribution of some severe and hazardous events like intense and great 24-hour precipitation, hail and thunderstorms produced by SCS during the warm part of the year (April–September) for the period 1961 – 2005.

3. Method of investigation :►Thunderstorm and hail occurrence, as well as precipitation from 67 climatological and 26 precipitation stations with altitude below 1000 m for the period 1961 – 2005;►Potential dangerous events- heavy rain (totals ≥ 30 mm/24 h), thunderstorm and hail in at least 4 of 27 districts in Bulgaria (or almost 15% from the area of the country affected)

4. Method of investigation ►The data used in analysis include the monthly number of the days with thunderstorms and hail, monthly precipitation amounts and daily distribution of heavy rain cases and hail-fall for every station and year; ►The frequency of thunderstorm and hail occurrence as well as monthly precipitation amounts for two periods (1961–1990, 1991–2005) were determined by specially developed Transact-SQL store procedures in Meteorological Database Management Division (MDBMD) of NIMH; ►All cases of thunderstorm occurrence at climatological stations (precipitation stations do not report about thunderstorms) in the studied period were investigated; ►The hailstorms are recorded only when they occur in limited site where the weather station is located and so their frequency seems to be under-evaluated recently; ►The percent deviation of monthly precipitation amounts from the mean values (1961– 1990) was calculated for each station and summarized for all country; ►For all stations the inter-monthly (number of cases in each decade of the month) and inter-seasonal distributions of mass heavy rain and hail events, detected at least in four districts in Bulgaria, are computed .

5. Annual distribution of heavy rain events (1961 - 2005) • No significant trends in distribution of average precipitation totals during the warm half of the year were found (without 2005 rainfall sums shows decreasing trend) • Mean seasonal number of heavy rain days (in more than 4 districts) for two periods: • 1961-1990 – 7.6 days • 1991-2005 – 12.2 days Increase of potential dangerous heavy rain events with about 60% is observed during the last 15 years

6. Annual distribution of heavy rain events (1961 - 2005) Recently in Bulgaria, like in the other Mediterranean countries (Alpert et al., 2002), a disproportional increase in the frequency of 1-day heavy precipitation is observed. In order to find the reasons leading to this type of extreme events, a statistical estimation, as well as detailed investigation and classification of synoptic situations connected with them, has to be carried out and this will be the next step in our work.

7. Annual distribution of mean seasonal thunderstorm days (1961 - 2005) Commonly the SCS are not only attended by heavy rain events, but also by hail and thunderstorm activity, which on their own account cause material damages and life loses. The number of registered thunderstorm days increases during the observed period, but the growth of their trend is slower in comparison with those, obtained for heavy rain events. The mean seasonal number of days with thunderstorm occurrence during the second period (1991–2005) is 24.4 days which in comparison with the first period (1961–1990) – 21.5 days, represents an increase with about 14%, which is 4 time less then the observed increase of heavy rain events.

8. Annual distribution of seasonal number of hail-fall days reported in more than 4 districts (1961 - 2005) Annual distribution of mean seasonal (April–September) number of days with hail- fall, has a decreasing linear trend and the maximum value is in the year 1973. There are two factors responsible for the obtained results: cloud seeding for hail suppression which began in 1972 with protected area of about 15 600 км² and insufficient density of climatological stations where this type of phenomenon is observed. Thus, for better investigation of hail detection events we need more information not only from ground meteorological observations but also radar and upper-air sounding data.

9. Average number of days with thunderstorm and hail on the basis of data from all stations Monthly changes in registration of hail- fall and thunderstorm events were obtained through comparison of two periods: 1961–1990and 1991–2005 • More considerable increase in average number of thunderstorms is observed in July (with 16%), August (with 19%) and September (with 50%), which has not been typical for such type of SCS before. • An increasing trend in average number of days with hail in September (50% growth) has been observed recently.

10. Inter–monthly distribution of heavy rain and hail-fall days in more than 4 districts in Bulgaria Heavy rain days: • 1961–1990 - the greatest number of heavy rain days is observed in II decade of June and I of July • 1991–2005 - more frequently occurred in II and III decades of July and I decade of August and their extension is more than 150%.Untypical increasing in frequency of heavy rain episodes in all decades of September (average with 50–100%) is observed. Hail-fall days: • 1991–2005 – increase in III decades of July (with 58%) and August (with 200%), and a registration of this phenomenon for the first time in I and III decades of September.

11. Conclusions • In the present analysis, based on data from 93 meteorological stations in Bulgaria for the period 1961 – 2005 during the warm half of the year (April–September), the extreme daily rainfall increase is observed in spite of the fact that total rainfall generally decreases. • Recently SCS and a number of extreme events, produced by them, like thunderstorms, hail and heavy 1-day precipitation, more frequently occurred in August and September, than during the period 1961–1990. • The increase of heavy precipitation days is 4 times more then the variations of the other observed potential dangerous phenomena – thunderstorms and hail-fall. • The results of the investigations confirm increasing of the climate extremes at the end of last century and at the beginning of 21th century and require detailed analyses of changes in weather conditions.

12. Thank You for Attention! Thank you for your attention!