عنوان مقاله [English]
نویسندگان [English]چکیده [English]
In order to investigate some synoptic aspects of Sudan low systems such as
upper and lower level tropospheric circulation pattern and moisture source and their
advection way, the synoptic pattern on the occurrence time of December 2001 storm
over Iran has been noticed. Therefore the maps of surface, 500, 200, 50hpa levels
before and after storm were studied. In order to understand the situation of subtropical
jet stream and how it strengthens, maps of zonal wind component and velocity
potential (x) of 200hpa level have investigated. Also, satellite images of cloud form of
storm have been studied.
The results showed that although like previous studies, there was low cycle index
and existence of a ridge in mid-troposphere over the west Mediterranean and a deep
trough over the east Mediterranean had important role on the formation and evolution
of Sudan low; but the main role belonged to the stretching of polar vortex in lower
stratosphere as the main source of vorticity for abnormal circulation and strengthening
of Hadley cell and Subtropical jet stream over Mediterranean and north of Africa.
The results indicated that the suitable position of Subtropical jet stream core over
Middle East associated with the settlement and strengthening of a ridge in middle and
lower troposphere over the Arabian Sea are necessary for the evolution and developing
of Sudan lows.
However, the position and speed of subtropical jet stream core over the Middle
East and it’s axis direction in the upper troposphere control both the mid-tropospheric
circulation pattern and the tracks of incoming Sudan lows to Iran.
The consequences indicated that settlements and strengthening of ridge/high over the
Arabian Sea, while making a set positive feedback and subsequently intensify Hadley
cell and strengthening subtropical jet stream over Red Sea region, lend to direct
transportation of moisture from the Arabian Sea into Sudan lows.
Moreover, it was found that the main sources of moisture of Sudan systems are
tropical Eastern Africa and southwest sector of Arabian Sea which strengthening when
they pass over the Read Sea and Persian Gulf.