Extraction and Analysis of Synoptic Patterns Resulting in Thunderstorm in Ardabil Plain

Introduction
Thunderstorms pose a significant threat to modern societies and their assets. Despite their local-scale characteristics, severe thunderstorms and associated extreme events like heavy rainfall, hail, gusts, or tornadoes can cause considerable damage to agriculture, buildings, or infrastructure, and facilities. Thunderstorms are highly localized and largely stationary weather systems. They affect a limited area of about 20–50 km2, depending on the size of the cumulus tower. They are associated with shower clouds in which electrical discharges can be seen as lightening and heard as thunder on the ground. They represent an advanced stage in the development of convection in moist air. The importance of the rainfall generated by the thunderstorms lies in the fact that it is largely torrential and of high intensity, and as a result much is lost as runoff which causes flooding. Basically thunderstorms occur more frequently above land areas in the warm season, while they are more frequent in the cold season over oceans. A lot of factors impact their occurrence. Among them the most important are the thermodynamic and kinematic states of the atmosphere, topography, land cover, and its coastal configuration and atmospheric circulation conditions. Ardabil is located in the northwest part of Iran, for this reason it has always been under the influence of the thunderstorms. Due to the geographic location and specific local conditions in this region, every year numerous thunderstorm events happen in this area and cause severe damages to the agriculture, utilities and infrastructure sectors. From this point of view, studying this phenomenon in detail and identifying the synoptic patterns of the ground surface and upper levels in which they are formed in Ardebil are vital and important for the region.
 
Methodology
For this study, initially the related data of Ardabil thunderstorms had been received from the Meteorological Agency of Ardabil. Within the related codes with the thunderstorms, codes from 90 to 99 during the period of 20 years (from 1992 to 2012) were used. After the initial Ardabil thunderstorm’s data analysis, the 88 observational days that thunderstorm occurred in were identified, and out of the 88 days, the 43 days that were compatible with the observational hours of the NOAA data (3: 30, 9: 30, 15, 30, 21, 30) were used for the patterning. This is so that this research does not have any time contradiction with the upper atmosphere data and to be justified with the interpretation and analysis. Then, for the patterning and extraction of patterns in the upper atmosphere and ground surface, the related data to the pressure of the ground surface and geopotential height were obtained from the site which belongs to the National Center for Environmental Prediction (NCEP). Thus, to do this research, the environmental to circulation method was used. In this case, based on the recorded data in the Ardabil station, the occurred thunderstorms were identified and then by using the clustering, extraction and identification of the patterns were performed for the ground surface and upper atmosphere. For the classification and extraction of the ground surface pressure patterns and geo-potential height with the level of 500 hpa, the diverse kinds of hierarchical clustering methods were tested. Finally, based on the results, the clustering method into the Euclidean distance was known as the best method and the results of that were reflected in the following research.
 
 
 
Result and Discussion
According to the results of hierarchical clustering, 4 patterns on the ground surface and in the level of 500 hpa were identified. The extracted patterns could justify beautifully the Ardabil thunderstorm occurrence. The ground surface patterns that lead to the occurrence of thunderstorms are pattern 1: the  formation of low pressure on India, Siberia and high pressure on northern Europe and west China; pattern 2: formation of low pressure on the Ganges Valley, Persian Gulf and North Europe and high pressure on Siberia and west China; pattern 3: formation of low pressure on India and Persian Gulf and high pressure on Siberia and west China; pattern 4:  formation of high-pressure on the Central Asia and front occurring in the northwest part of Iran. The patterns of geo-potential height with the level of 500 hpa that lead to the occurrence of thunderstorms also are pattern 1: formation of the trough on the east Mediterranean and the placement of Ardabil in east part of trough; pattern 2: formation of the omega blocking on the northern parts of the Caspian sea and placement on Ardabil in southwest part of that; pattern 3: the occurrence of the cut-off blocking with low pressure over the central and east part of Turkey and its placement on Ardabil in east part of the trough made by that; and pattern 4: the occurrence of split flow blocking in central Europe and its placement over Ardabil in east part of the trough.
 
Conclusion
According to the results of hierarchical clustering in the ground surface and in upper atmosphere, there are different patterns effective on Ardabil thunderstorm precipitation. In the ground surface, formation of low pressure on the Ganges valley, southwest Persian Gulf, Siberia and northern Europe, also high pressure on the northern Europe, Siberia, west China and the Central Asia played a significant role in the thunder precipitation events. On the other hand, in upper atmosphere, the study area in the east part of trough and the formation of different patterns of blocking (omega, Split flow and cut-off low pressure) provided the condition for occurrence of the thunder precipitation in Ardabil.