A Study of Summer Heavy Rainfalls in Golestan Province (2001-2007)

IntroductionOccurrences of heavy rainfalls depend on atmospheric circulations, geographical position and topography which lead to flash floods. They are considered as a serious threat for human beings and their properties. A major challenge associated with flash floods is the quantitative character of summer rainfall forecast. During the summer mainly in August (2001-2007), some synoptic patterns have produced wide spread heavy rain in Golestan area. Deep, moist convection normally occurs during warm season when high moisture content is possible and buoyant instability promotes strong upward vertical motions, then severe precipitations occur most frequently in the mountainous areas of Golestan province. Historically, lack of data in mountainous area makes the investigation difficult. The impact (and cost) of sever precipitations and flash flood occurrences in Golestan province has increased. Therefore, it is necessary to understand conditions that lead to heavy showers over Golestan area. In this study weather patterns leading to heavy rain falls within 7 years 2001-2007), in Golestan province are investigated. For this purpose, synoptic scale patterns have been investigated by using meteorological data and maps.Materials and methodsGolestan province is located between 36° 44΄ N (north latitude) to 38° 05΄ N and 53° 51΄ E (East longitude) to 56° 14΄ E. Summer extreme precipitation events was sometimes considerable during the recent years which produced flash floods in Golestan area, and financial losses, and fatalities had a lot to follow. The purpose of this study is to determine the pressure patterns in summer seasons that produced heavy rain falls in Golestan province. Actual weather charts at standard pressure levels and total precipitation of meteorological stations from Iran meteorological organization and also gridded sea level pressure (SLP) and 500-hpa geo-potential height data from the National Center for Environmental (NCEP) were used to the synoptic analysis. These data analyzed at 0000 and 1200UTC were prepared for the summer flash floods cases from four days before flash flood occurrence in the period of 2001-2007 over Golestan (Table 1). But here, given the limited volume of the paper, only a few maps are presented. Also, total daily precipitation of meteorological stations has investigated for theevents of Golestan floods. Finally the characteristics of effective patterns atmospheric to produce summer severe precipitations in the area are offered.Results and DiscussionThe heavy rain falls and flash floods are not typical of usual summertime regime in Golestan. The summertime heavy rain falls and storms are examined synoptically by compositing sea level pressure, 850mb and 500mb height data. All types of events are controlled by the lower and upper-level flows. This typically involves, for flash floods, the intrusion into Caspian sea of a surface high pressure ridge in the northerlies with attendant cold air advection and passing the inverted tough in 500mb height from north to southeast over Caspian sea and additionally, a few days before the occurrence, enhanced upper ridge and warm air of the subtropical anticyclones with associated heat low pressure over the region and southern Caspian sea. In conjunction with intense surface heating and topographic effects, these produce widespread atmospheric destabilization over the mountain area in Golestan. In the Golestan region, cutoff lows over Oral Lake are also to be important triggers for summer heavy rainfall primary due to instability created by their cold temperatures at mid-troposphere level.ConclusionThe results indicate that the synoptic-scale forcing such as short waves (inverted and shallow troughs) in mid troposphere, cold advection and penetration of high-pressure systems on Caspian Sea play an effective role in the summer heavy rainfall episodes in Golestan province. Within the days before the heavy rainfall period,temperature increase in low level troposphere (850hPa) in the region leads to significant evaporation in the environment and prepares considerable capacity for acceptance of humidity. Then, pass of middle shallow and inverted troughs and high-pressure penetration afterwards and existence of Alborz mountains, leads to rising of warm and humid air in low level and availability of cold air in mid atmosphere that provides suitable conditions to form potential thunder storms and cumulonimbus clouds which can cause heavy showers. Therefore, penetration of the high pressures result in establishment of the northern cold and dry flows over the Caspian Sea and transition of humid to northern Alborz slopes (southern Caspian sea), coincide with the advection of cold air in low levels of atmosphere most importantly in 850hP and crossing of inverted troughs, are the main factors of causing torrential rains in the region.