Combinatory Mediterranean-Sudanese systems role in the occurrence of heavy rainfalls (case study: south west of Iran)

Abstract

Sudanese systems are low-pressure thermal s ystems which originate from the Red Sea and their low-pressure isoheights are detectable around south of the Red Sea, Sudan and Ethiopia. These systems when combined with Mediterranean troughs in cold seasons, and absorbing more humidity from the Red Sea, reach the south west of Iran and result in heavy rainfalls in western, southwestern and southern parts of Iran. In all these synoptic–dynamic systems, low-pressure isobars or low isoheights are considerable. 11 severe torrents which happened in southwest of Iran (1998–2008) are selected and using Japanese ReAnalysis data (JRA25), anomalies of dynamic and thermodynamic indices 2 days prior to the start of heavy rainfalls and during rainy days have been analyzed. The selected indices in this research are relative vorticity, convergence and divergence, vorticity advection, specific humidity and moisture advection. Pressure levels of 500 and 700 were analyzed to detect the location of troughs, vorticity and vertical wind velocity. Also level of 1000 hPa was investigated to determine the atmospheric convergence in near surface levels. It should be mentioned that to determine the advection of moisture, vertical profile of moisture advection (1000–250 hPa levels) was analyzed. According to the results of this research and analyzing 6-hourly recorded rainfall amounts, it is understood that in the reference events, before the start of the torrent, geopotential height values in the under-studied region decrease and at the time of maximum rainfalls, the geopotential height reaches to its lowest values and by the end of the torrent, the values start increasing; whereas parameters related to convergence and vertical movements, such as relative vorticity, moisture advection, convergence of moisture and specific humidity amounts correspond to the same trend of rainfall from the beginning to the end.