چكيده لاتين :
Extended Abstract
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, cause considerable damage to agriculture, buildings, or infrastructure facilities. Thunderstorms are highly localized and largely stationary weather systems affecting 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 lightning and heard as thunder on the ground, and they represent an advanced stage in the development of convection in moist air. The importance of rainfall generated by 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 on 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 issues. Ardabil is located in the northwest of the country, 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 thunderstorms happen in this area and cause severe damages in the agriculture, utilities and in restructure 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 lead to the occurrence of thunderstorms in Ardebil are vital and important for the region.
Material and Method
For this study, firstly 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 analyzing, the 88 observational days that thunderstorm occurred in it were identified, and out of the 88 days, 43 days, that were compatible with the observational hours of4 for the NOAA data (3: 30, 9: 30, 15, 30, 21, 30) were used, for the patterning, so that the done research not to 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 geo potential height were obtained from the site which belongs to the National Center for Environmental Prediction (NCEP). So as to do this research the environmental to circulation method was used. In this case, first 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 for the ground surface and upper atmosphere launched, that leading to the occurrence of this phenomenon. 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 tested, and 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 Discuss
According to the results of hierarchical clustering, 4 patterns on the ground surface and in the level of 500 hpa were identified that the extracted patterns could justify beautifully the Ardabil thunderstorm occurrence. The ground surface patterns that lead to the occurrence of thunderstorms include pattern 1: the formation of low pressure on India, Siberia and high pressure on northern Europe and west of China, pattern 2: the formation of low pressure on the Ganges Valley, Persian Gulf and North Europe and high pressure on Siberia and west of China, pattern 3: the formation of low pressure on India and Persian Gulf and high pressure on Siberia and west of China, pattern 4: the formation of high-pressure on the central Asia and front occurring in the northwest of Iran. Also the patterns of geo-potential height with the level of500 hpa that lead to the occurrence of thunderstorms include pattern 1: the formation of the trough on the east Mediterranean and the placement of Ardabil in east of trough, pattern 2: the formation of the omega blocking on the northern parts of the Caspian sea and placement of Ardabil in southwest of that, pattern 3: the occurring of the cut-off blocking with low pressure on the central and east of the Turkey and placement of the Ardabil in east of the trough made by that, and pattern 4: the occurring split flow blocking in central Europe and placement of the Ardabil in east of trough.
Conclusion
Different patterns according to the results of hierarchical clustering in the ground surface and in upper atmosphere are impressive on Ardabil thunderstorm precipitation. In the ground surface, the formation of low pressure on the Ganges valley, southwest of Persian Gulf, Siberia and northern Europe, also high pressure on the northern Europe, Siberia, west of China and the Central Asia played a significant role in the thunder precipitation occurrence. On the other hand in upper atmosphere, the study area in the east of trough and the formation of different patterns of blocking (omega, Split flow and cut-off low pressure) provided the condition so the thunder precipitation in Ardabil to be occurred.
Key word:
Ardabil Plain, Hierarchical Clustering, Synoptic patterns, Thunderstorm.
