واکاوی آماری – همدیدی توفان‌های تندری سواحل جنوبی ایران

Thunderstorms are regarded not only as a significant weather event but also as a key element in water and electricity cycles of the atmosphere. Generally, researchers consider the intense weather instability as a result of convection in lower levels of the atmosphere with high levels enough of humidity. Usually statistic instability, the humidity of lower levels of the atmosphere and lifting mechanisms near the ground are the main factors leading to convection. Moreover, the combination of three factors, instability, humidity and convergence in lower levels of the atmosphere plays an important role in increasing the possibility of thunderstorms. Accompanying phenomena like lightning, tornado, hail, winds, heavy precipitations (Changnon, 2001 and 1925) and hazardous atmospheric phenomena like turbulence, freezing, and wind sheering make considerable irrecoverable damages to natural and human environments, therefore recognizing the features of these phenomena have always been attracting the attention of researchers. The present study aims at recognizing statistic of thermodynamic, and synoptic features of thunderstorms of southern coasts of Iran. Referring to the archive of National Meteorological Organization, hourly data of atmospheric phenomena of 10 synoptic stations during a common twenty-year period (1995- 2014) were extracted. The data were processed in temporal scales of year, season and month. The data of upper atmosphere (radio-sound data), available in the website of Wayoming University, were applied to investigate the thermodynamic features of the occurred thunderstorms. The thermodynamic features include KI, SI, TT, LI, CAPE indices and skew- T chart in RAOB software. The days with the occurrence of thunderstorms had 5 mm or more at least in two stations that were selected to find synoptic patterns. As the samples were limited, synoptic patterns were done manually. The required maps were prepared using the data of geopotential height in 1000 - 500 hPa levels. Besides wind components u and v and sea level pressure, extracted from NCEP/ NCAR website, were mapped by GrADS software. Checking yearly frequency of thunderstorm occurrence in the southern coasts of Iran showed that the frequency of occurrence of storms in Booshehr station was more than its frequency in Hormozgan station. Moreover, the thunderstorms of Booshehr have a better chronological orderas it occurs during all common years. However, except for BandarAbbas, there is no chronological order for this phenomena. Therefore, it can be said that the occurrence of thunderstorm in the western coasts of the south of Iran has higher frequency than the central and the eastern regions, making it a potential area in this region for storm formation. The largest number of thunderstorm occurrence in seasonal scale is recorded for fall with 45% and winter with 43% respectively. Following seasonal conditions, the largest number of thunderstorm occurrence in monthly scale is recorded for cold months. In Hormozgan station, November, December, and January have more frequencies, while in Booshehr station January, February and March have more frequencies. Analyzing the applied instability indices showed that there was a slight extreme and great CAPE (more than 2500) in Bandarabas station. Besides the values of convection indices TT and KI for most of the thunderstorms suggested the possibility of convection occurrence. Instability indices LI and SI for the occurred thunderstorms reveals conditions of limited instability. Synoptic analysis shows not only the dominance of the westerly winds extending to the south of Saudi Arabia but also the location of divergent region and positive vorticity advection region in the studied region, making instability conditions raising air. The spread of the westerly winds is either due to formation of blocking system in the atmospheric middle level or their meridional blowing and cold air advection from Europe or the north of Asia to the east of Mediterranean. Statistical findings reveal that the occurrence of thunderstorms of western coasts of the Persian Gulf, have higher potential, and more frequency than the central and eastern regions. In seasonal scale, the largest number of occurrences is recorded for fall and winter respectively, while there is no substantial difference in different hours of day and night in hourly scale. As a matter of fact, they are possible to happen all the times. Synoptic analyses show that there is the dominance of two patterns of blocking systems and westerlies trough in the middle of atmosphere leading to instability and rising air in the studied region. The divergent region and positive vorticity advection region in the studied regions make instability condition and hence rising air. Based on the findings of thermodynamic indices, it can be said that convective activities and local instabilities are rarely responsible for thunderstorm occurrence in the region. Also for the occurrence of severe convective activities and relatively high instability, extreme instability and extreme severe instability is coincident with limited thunderstorm occurrences.