Investigation Subtropical High Pressure Spatial Variations in Summer Rainfalls of the Southern Half of Iran

In order to identify the spatial relation between the subtropical high pressure system with summer rainfall in the southern half of Iran, the data about the daily rainfall of synoptic stations of the southern half of Iran were examined from 1987 to 2005 thus six widespread raining periods were selected in this regard. After exploring the ground level pressure map and the geopotential height maps at levels of 300, 500 and 700 hpa throughout the selected rainy days, two different patterns were detected. In the first pattern, the moisture of the Indian ocean and the contiguous seas transfers to the low level of troposphere as a result of monsonal cyclonic movement system. In the median levels, the western trough has expanded towards the southern half of Iran and resulted in the recoil of the subtropical high pressure toward the lower latitude. In addition, the subtropical high pressure axis places in the western-eastern manner. Throughout the rainfalls in this model, the maximum decrease in geopotential height occurs at 300 and 500 hpa respectively and continues into the median levels of the convection layer due to dwindling subtropical system. In the second pattern, the monsoon system has directed toward our area of investigation and severe rainfalls took place as the monsoon systems approached. In this model, the western streams trough have expanded toward the Mediterranean basin and results in the recoil of the subtropical high pressure flow toward the northern part of Africa, out of which a high pressure core is separated and located on Iran. In this situation, the high pressure system transfers to the upper levels also,the condition for mansoon wet stream elevation and convectional rainfalls has been provided. In this model, the maximum decrease in the geopotential height has occurred at 700hpa and the thickness of the convection layer is lower than the first model which would reach to 700 hpa level.