Document Type : Research Paper
Authors
shahid
Abstract
As a result of Iran’s geographical position towards atmospheric general circulation, most of its area has an arid or semi-arid climate. In this research, the author has calculated precipitation start using the daily precipitation information obtained from 30 stations located in 8 southern and south-western provinces of Iran during a 36 year statistical period and of course, based on selected criteria. In ten years, precipitations have started sooner than normally expected. Entire up to 700 HPa levels and sea level pressure maps belonging to 1st of September till the day of precipitation start have been illustrated with Grads software using atmospheric data obtained from Ncep/ncar site. Results of several objective observations of maps manifest that early precipitations in south and south-west sides of Iran are generally occurred in three main patterns. Pattern A) as African sub-tropical high pressure passes above central Mediterranean in a northward move and its eastern trough integrates with migratory anticyclone (deployed on the north side of Caspian Sea), it provides the necessary context for cold advection to move towards north of African high pressure. Afterwards, the migratory anticyclone trough integrates with anticyclone ridge of Arabian high pressure and therefore a very suitable situation is provided for advection of moisture to move inside the Sudan low system. In pattern B) almost thirty days before start of precipitations, the Azour anticyclone adopts a northward movement.
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