Synoptic Analysis of the Nature of Sudan Low PressureSystems (Case Study: December 2001 Storm)

In order to investigate some synoptic aspects of Sudan low systems such asupper and lower level tropospheric circulation pattern and moisture source and theiradvection way, the synoptic pattern on the occurrence time of December 2001 stormover Iran has been noticed. Therefore the maps of surface, 500, 200, 50hpa levelsbefore and after storm were studied. In order to understand the situation of subtropicaljet stream and how it strengthens, maps of zonal wind component and velocitypotential (x) of 200hpa level have investigated. Also, satellite images of cloud form ofstorm have been studied.The results showed that although like previous studies, there was low cycle indexand existence of a ridge in mid-troposphere over the west Mediterranean and a deeptrough over the east Mediterranean had important role on the formation and evolutionof Sudan low; but the main role belonged to the stretching of polar vortex in lowerstratosphere as the main source of vorticity for abnormal circulation and strengtheningof Hadley cell and Subtropical jet stream over Mediterranean and north of Africa.The results indicated that the suitable position of Subtropical jet stream core overMiddle East associated with the settlement and strengthening of a ridge in middle andlower troposphere over the Arabian Sea are necessary for the evolution and developingof Sudan lows.However, the position and speed of subtropical jet stream core over the MiddleEast and it’s axis direction in the upper troposphere control both the mid-troposphericcirculation pattern and the tracks of incoming Sudan lows to Iran.The consequences indicated that settlements and strengthening of ridge/high over theArabian Sea, while making a set positive feedback and subsequently intensify Hadleycell and strengthening subtropical jet stream over Red Sea region, lend to directtransportation of moisture from the Arabian Sea into Sudan lows.Moreover, it was found that the main sources of moisture of Sudan systems aretropical Eastern Africa and southwest sector of Arabian Sea which strengthening whenthey pass over the Read Sea and Persian Gulf.