On the Risk Assessment of Severe Convective Storms and Some Weather Hazards over Bulgaria ( ) - Meteorological Approach

Transcription

1 On the Risk Assessment of Severe Convective Storms and Some Weather Hazards over Bulgaria ( ) - Meteorological Approach Lilia Bocheva, Petio Simeonov, Ilian Gospodinov National Institute of Meteorology and Hydrology Sofia, BULGARIA Abstract Severe convective storms (SCS) produce dangerous weather phenomena during the warm half of the year like heavy and very intense rainfall, thunderstorms, hail-fall. They are often associated with strong to violent wind gusts and sometimes even with such dangerous events like squall and tornado. From meteorological point of view these events are defined as small-scale severe weather phenomena (SCSWP). The present investigation is based on a proper selection of SCS and the associated with them severe and hazardous meteorological events like heavy ( mm/24h) and torrential (totals 60 mm/24h in one station are considered) precipitation, wind (speed 20 m/s), hail and thunderstorms in six different parts of the Bulgarian territory. Their monthly and seasonal distribution is obtained. The frequency of days with torrential rainfall (Q 60 mm/24h), extended thunders and hailfall, and wind have been analyzed separately. Statistically significant increase (about 30-50%) of days with torrential 24-hours precipitation is revealed during the period of investigation ( ) in the central and east parts of the country, while in South-West Bulgaria these dangerous events decrease with about 20-35%. The increase in frequency of stormy days in the autumn months September and October is observed in almost all parts of the country. Keywords: extreme precipitation, thunderstorms, hail, Bulgaria Introduction Severe convective storms (SCS) produce such dangerous weather phenomena during the warm half of the year like heavy and very intense rainfall, thunderstorms, hail-fall, which are often associated with strong to violent wind gusts and sometimes even with such dangerous events like squall and tornado. From meteorological point of view these events are defined as small-scale severe weather phenomena (SCSWP Brazdil, 2002). Although these SCSWP are sparse in space and time they have detrimental influence on the economy and the society especially in the small countries. Often they cause significant property and infrastructure damage as well as loss of life. According to the WMO experts about 90% of natural disasters are due to weather, climate and water (Fig.1 by WMO, 2006). а) b) Figure 1. Losses caused by natural disasters according to the type of hazardous events related to weather, climate and water (a) and juxtaposing of damages in the developed and the least developed countries (b). BALWOIS Ohrid, Republic of Macedonia - 25, 29 May

2 During the last almost 10 years, series of hazardous events, more often associated with severe convective storms and heavy rainfall, have affected Bulgaria. They have caused local floods, significant property damage and also loss of life and thus have had a considerable impact on the Bulgarian economy. The aim of the study presented here is to look at the annual and monthly distribution of SCS associated with torrential precipitation events, thunder- and hail-storms over the territory of Bulgaria which is divided on 6 different parts. The distribution of precipitation across the territory of Bulgaria and their seasonality are mainly caused by circulation conditions and topographic particularities. The zonal extension of the Stara Planina mountain chain and the Rila-Rhodope mountainous massif present a natural barrier to the invasion of cold air masses towards the southern part of the country. These mountains are also a barrier to warm air masses which have to overflow them. Actually, the country is naturally divided into North and South Bulgaria by Stara Planina, which affects the precipitation and temperature regime of either side. However, a significant difference in the precipitation regime has recently been observed between the western and eastern parts of the country (Bocheva et al., 2008; Bocheva et al., 2009). If we look at the variance of the thunderstorm distribution, we can say that there is no statistically significant recent increase in the thunderstorm activity Bulgaria (Simeonov et al., 2006). But it is interesting to look at the situation in the different parts of the country. Method of investigation The study is based on data of heavy and torrential precipitation events from the meteorological database of the National Institute of Meteorology and Hydrology (NIMH) of Bulgaria for the period We consider all 375 synoptic, climatological, and rain-gauge stations (Fig.2) in which regular observations were completed during the whole period or part of it. In all stations the daily precipitation total is measured in 7.30a.m. local time with classic ground-level precipitation gauges. The automatic stations data is not included in this study. ROMANIA SERBIA BLACK SEA MACEDONIA TURKEY GREECE Figure 2. The NIMH weather stations network: synoptic (squares), climatological (triangles) and raingauge (circles) stations. We consider all days with thunderstorm occurrence in all synoptic and climatological stations within the studied period. We define a thundery day as follows: a day when at least one thunderstorm occurred between 00:00 and 24:00 h local time; even distant thunderstorms are taken into account; if a thunderstorm occurred at the turn of 2 days, it is included in both days (but these cases occur only rarely). BALWOIS Ohrid, Republic of Macedonia - 25, 29 May

3 The hailstorms are registered only if they occur in the vicinity of a given weather station and their frequency seems to be under-evaluated in recent times. The hail occurrence is taken also only from synoptic and climatological stations where this type of phenomena has consistently been recorded. The selected convective storm days are separated into five classes for each of the six parts of Bulgaria: A - days with Q 30 mm and thunders (Ts), with or without hailstorm (Hs); B - days with Q 30 mm and thunders (Ts), with hailstorm (Hs); C - days with Q 30 mm and thunders (Ts), with or without hailstorm (Hs), and with wind (w 20 ms -1 ); C2 - days with Q 30 mm, Ts in all stations, hail in all station, w 20 ms -1 (with or without tornado), severe events. D as C2 but days with Q 60 mm. A brief overview of tornado occurrence is done too. Tornado occurs very rarely in Bulgaria, but it causes great local damage in the affected area. The highest wind speed of tornado in our country is about 100 m/s (Latinov, 2006), which exceeds 3 times the criteria for hazardous meteorological events (HME) for strong winds (Table 1). Table 1 Criteria for some severe and hazardous meteorological events (SME and HME) Type of SME and HME Strong wind (phoen or squall ) Criterion for SME intensity Mean wind speed m/s for 2 min. Criterion for HME intensity Mean wind speed 30 m/s for 2 min. Strong wind gusts Wind speed m/s Wind speed 35 m/s Rain Intensity l/m 2 for 6 hours Intensity 30 l/m 2 for 6 hours Hail Diameter 6 19 mm Diameter 20 mm Thunderstorm with or without precipitation Every case All over the country The territory of Bulgaria is small, but it has a very diverse relief. The local and regional precipitation regime is highly influenced by latitude, altitude, topography, the proximity of sea bodies and the prevalent atmospheric circulation. North and South Bulgaria are naturally separated by a mountain barrier namely the mountain of Stara Planina. Western Bulgaria is dominated by mountain bodies of various height, shape, size and orientation. Eastern Bulgaria is rather flat and borders the Black Sea. The sea body has a decisive influence on the precipitation regime there. North Bulgaria is also rather flat while the very south of the country is mountainous. We divide Bulgaria into 6 regions on administrative principle: North-West (NW), North-Central (NC), North-East (NE), South-East (SE), South-Central (SC), and South-West (SW) Bulgaria (BG) (See Fig.2). They also match to some extend the different climate zones of the country. Table 2 summarizes the geographical features of the six regions. Table 2 Geographical features of the regions of Bulgaria. Geography of Orography Relation to sea bodies regions NW Bulgaria Flat, mountain ridge at southwest border Far from sea NC Bulgaria Flat, mountain ridge at south border Far from sea NE Bulgaria Flat, mountain ridge at southwest corner Borders the Black Sea SE Bulgaria Flat, low mountain at south border Borders the Black Sea, close to Aegean Sea SC Bulgaria Flat in the middle, mountainous massive in the Closest to Aegean Sea south, mountainous ridge in the north SW Bulgaria Very mountainous Close to Aegean Sea BALWOIS Ohrid, Republic of Macedonia - 25, 29 May

4 Brief climatic statistic Significant increase of days with torrential (Q 60 mm/24h in one station are considered) precipitation is revealed during the period in NC Bulgaria (about 45%) and NE parts of the country (about 31%), while in West Bulgaria (especially in SW Bulgaria) the number of these dangerous events decreases with about 20-33%. In the same time, an increase in frequency of heavy precipitation days (Q 30 mm/24h) in September is observed in all parts of the country (see Bocheva et al., 2009). Commonly, the SCS are not only attended by heavy rain events, but also by hail and thunderstorm activity, which on their own cause material damage and life loss. Their annual distributions in different regions in Bulgaria are presented in Fig.3 and 4. The number of registered thunderstorm days increases during the second half of observed period almost in all regions, but this growth is statistically significant after year 2000 only in NE Bulgaria (about 15%). In the most endangered of Ts activity part of the country - SC Bulgaria there is no significant variation in the annual number of observed Ts days during the studied period. The increasing of Ts recently is mostly due to the increasing in number of days with such event 2-5 times in January and partly in February, as well as growth with about 50 % in September all over the country. 140 NW NC NE SE SC SW Number of thunderstorm days Year Figure 3. Annual distribution of days with Ts. 50 NW NC NE SE SC SW Number of hail days Year Figure 4. Annual distribution of days with Hs BALWOIS Ohrid, Republic of Macedonia - 25, 29 May

5 The annual distribution of hail-fall days is presented on Fig. 4. Obviously, the most endangered regions are SC and SW Bulgaria with about 27 hail-fall days per year. In contrary, in NW Bulgaria the mean annual number of Hs days is about 8 during the period However, the number of meteorological station is different in each region and hail-fall is very difficult observe. They are registered only when they occur in the vicinity of a weather station. Our comparison between different regions is made on the basis of calculated mean number of Hs days in one station for each region (Fig. 5). So, in the western part of the country and in SE Bulgaria the number of Hs days decreases with about 10 % after 2000, while in NC and NE Bulgaria it raises with about %. The tendency in the monthly distribution of Hs days in each region is the same as the one in the monthly distribution of Ts days, presented above. There is not typical increase of hail-fall events observed in winter and also growth with about % in September in almost all regions in the country. Mean number of days with hail in one station NW NC NE SE SC SW Region Figure 5. Mean number of days with Hs in one station. For 18-years period of investigation 948 SCS days are studied for the whole country. These SCS days are divided into 5 classes (A, B, C, C2 and D) for each of the six parts of Bulgaria according to the criteria described in the first section. In Bulgaria, the maximum occurrence of stormy days was in 2005 (85 days), where the number of severe storm days (class C2) is 13 and heavy rain days (class D) is 3. In the summer of that year there were floods which caused damage and victims. Also an increase of the number of days from categories C2 (about 30%) and D (about 500%) in the recent 8 years compared to the previous ones is observed. The annual distribution of SCS days for NE and SW Bulgaria are presented on Figure 6. In difference to the changes in the distribution of torrential precipitation found by Bocheva et al., 2009, the most significant recent increase in SCS days is observed in SW Bulgaria and the mountainous part of SC Bulgaria. In SW Bulgaria (Fig. 6b) the maximum annual number of SCS days was in 2007, but the biggest number of heavy rain stormy days (class D) was in For all regions statistically significant increase in the number of SCS days from class D (between 300 and 500%) has been observed since The increase of days from categories C2 is different for the different regions. It is statistically insignificant for SW Bulgaria and statistically significant for NC and NE Bulgaria (about 45%). According to the monthly distribution of SCS days, recently their number increases in August, September and especially in January in almost all six regions. Tornado and water spout In very rare cases the SCS produce tornado. In our country they typically occur over rough mountains terrains or over the Black Sea. It is very difficult to predict this dangerous phenomenon without specialized equipment. There are only few documented tornado events in Bulgaria. The meteorological statistics suffers from lack of data and there are only few scientific analyses done. See, for example, Simeonov and Georgiev, 2001; Simeonov and Georgiev, 2003; or Latinov, Sometimes a downburst or squall is taken for a spout. In order to distinguish the spout (tornado) and BALWOIS Ohrid, Republic of Macedonia - 25, 29 May

6 to determine its intensity according to Fudjita scale for instance, it is important to know not only the physical and time-spatial characteristics of such type of events, but also the kind and the pattern of damage. 30 A B C C2 D Storm days in NE Bulgaria Year a) NE Bulgaria 30 A B C C1 D Storm days in SW Bulgaria Year b) SW Bulgaria Figure 6. Annual distribution of 5 classes of SCS days Examples from the near past are given below: Tornado occurred on 5 June 1989 at about h local times over the village Bohot, region of Pleven. In the course of 5 minutes the spout covered the distance of 3 4 km and destroyed two-storied, solid-built houses. 144 houses collapsed completely and other 186 were partially destroyed. The strip of damage was about 200 m wide, and the strip of the house parts which were lifted by the trunk of the spout was about 50 m. Tornado, which occurred on 19 May 1999 in the region of Kurdzhali (Zhaltusha village), caused damages for USD. On 22 May 2001 tornado caused significant damage (flora and houses) in the Vitosha mountain (Jeleznitsa village) for USD. The tornadoes however have become more frequent last years in Bulgaria and some of them were studied by applying classic meteorological data radar imagery and filed inquiry (see Simeonov et al., 2009). Some characteristics of the documented tornadoes are given in Table 3. The losses of BALWOIS Ohrid, Republic of Macedonia - 25, 29 May

7 property, wood and crops damage caused in those cases has been estimated to be between 80 and 640 thousand EURO Table 3 Some precipitation characteristics of six tornado events in five severe storm days based on radar and rain-gauge records. No Date Local storm duration Maximum radar integrated precipitable area Maximum radar rainfall untensity Stations Daily point rainfall Maximal hailstone size minutes km 2 mmh -1 mm cm 1 2 June May April July A 2 June B 2 June In all cases associated with tornado development over Bulgaria we can observe on the synoptic charts: Slowly moving cold atmospheric fronts, which separate tropical and polar air masses and in many cases the arctic front is very close to the polar front; Strong temperature gradient; Powerful high-level frontal zone, connected with jet stream; Shallow and quickly moving cyclonic vortexes over the territory of the country; Bipartite advection of cold air from East-Northeast and strong warm flow in high levels from West-Southwest; Conditions for thermal advection overheating is the biggest. Conclussions Statistically significant recent increase in the number of the days with Ts, Hs and SCS is observed in NE Bulgaria. In the same time the most significant increase of SCS days is observed in SW Bulgaria and the mountainous part of SC Bulgaria. During the last decade the frequency of HME and the extreme events connected with them as thunderstorms, hail, and heavy rainfall, are registered more frequently at the end of summer (August and September) and in January. These first results can be the basis of further investigation of the influence of the climate change on the increase of the frequency and the intensity of meso-scale storms. The obtained results are helpful for the development of methods for their middle and longrange forecasting, as well as for the assessment of the most probable damages. In order to investigate more precisely the risk of severe hydro-meteorological events, it is necessary to build modern national database including the reported damage caused by the different hazardous events for the sectors of the economy. It can be done by restoring and unifying the available archive data in the insurance companies, the national civil-protection service, ministries, the national statistical institute, and others. References Brazdil, R., 2002: Small-scale severe weather phenomena. In: IPCC Workshop Report on changes in Extreme Weather and Climate Events, Beijing, China, BALWOIS Ohrid, Republic of Macedonia - 25, 29 May

8 Bocheva, L., P. Simeonov and T. Marinova, 2006: On frequency distribution and intensity of severe convective storms over Bulgaria. Proceedings of the Conference on Water Observation and Information System for Decision Support (BALWOIS), May 2006, Ohrid, Macedonia, (CD version) Bocheva L., P. Simeonov, I. Gospodinov, T. Marinova, 2008: Torrential Precipitation Events in Bulgaria: A Comparative Analysis for East Bulgaria. BALWOIS 2008, May 2008, Ohrid, Republic Macedonia (CD version). Bocheva L., I. Gospodinov, P. Simeonov, T. Marinova, 2009: On change in extreme daily precipitation characteristics in Bulgaria ( ). 5th European Conference on Severe Storms, October 2009, Landshut, GERMANY Latinov, L., 2006: Synoptic Conditions for Snow Storms, Black Ice, Tornado and Dust Devils in Bulgaria, Sofia, 191 (in Bulgarian, abstract in English) Simeonov, P., L. Bocheva, T. Marinova, 2006: Risk assessment of severe meteorological events on convective storms during the warm half of the year. Proceedings of the First National Research Conference on Emergency Management and Protection of the Population, Sofia, (In Bulgarian) Simeonov, P. and Ch.Georgiev, 2001: A case study of tornado-producing storm south of Rhodopes mountain in the Eastern Mediterranean. Atmospheric Research, Vol.57, Simeonov, P. and Ch.Georgiev, 2003: Severe wind/hail storms over Bulgaria in period synoptic- and meso-scale factors for generation. Atmospheric Research, Vol.67 68, Simeonov, P., I. Gospodinov, R. Petrov, L. Bocheva, 2009: Recent severe rain/hail storms and several tornado events in Bulgaria ( ). 5th European Conference on Severe Storms, October 2009, Landshut, GERMANY WMO Brochure No. 993: Preventing and mitigating natural disasters, 2006, Geneva, 34 pp. BALWOIS Ohrid, Republic of Macedonia - 25, 29 May