National Institute of Meteorology and Hydrology - Bulgaria
Meteosat water vapour (WV) imagery and data from surface observations are used to study the severe storm on 15 May 1999 over the mountain area of the most southern part of Bulgaria, close to the Mediterranean coast. The case was associated with a local tornado and hail, and caused damages of property valued above 250 000 US dollars. Hourly High Resolution Image (HRI) data in 6.3 µm channel of Meteosat are used to identify the continuously decreasing of mid- and upper level humidity during a period of 9 hours prior to the tornadic event. Three-hourly data from 5 synoptic stations (altitude range: 140-1920 m) from the same period showed increasing of surface air temperature and vapour pressure around the area of subsequent development of the severe storm. In this way, it was revealed that convection developed rapidly within a region of mid-upper level dryness, after the dry air had overtaken a low-level layer, in which diurnal heating and advection of warm and moist Mediterranean air were significant.
The storm over the Eastern Mediterranean on 15 May 1999 occurred after appearance of a pronounced ‘C’-shaped dark zone in the WV imagery located just to the south-west and near the upstream edge of the convective cloud cell. Another author previously reported that a high percentage of storms that exhibit such a persistent WV dark feature has been accompanied by severe weather (large hail, strong wind and tornados) over the U.S.A.
In this study, a new quantity referred to as Potential Instability WV Index (PIWVI) is proposed as a measure of the potential for developing extremely severe convection. The PIWVI is calculated using a combination of two different kinds of data:
• Synoptic station observations of surface moisture.
• HRI Meteosat WV data averaged in an area of 7×7 pixels around the synoptic stations.
Nine hours prior to the tornadic event, a continuously increasing of PIWVI was observed only at the station, located just to the west of the tornado release point where the pronounced ‘C’-shaped dark zone appeared in the imagery. At the other stations, the PIWVI was much less and not continuously increasing with time, and although convective clouds developed close to some of that stations, they were not associated with such dramatic severe weather events. The results suggest that using hourly WV imagery jointly with three-hourly data from surface synoptic observations might serve in providing early warnings and detection of convection. Such an approach can be useful as analytical tool for monitoring pre-convection situations and convective activity in cases of lack of proximity soundings for severe storm events.