عنوان مقاله [English]
نویسندگان [English]چکیده [English]
In this research, the frequency of the jet streams associated with extreme and pervasive precipitation has been analyzed using environmental to circulation approach. Based on the 99th percentile index, 109 days of the extreme and pervasive precipitation was selected from the IRIMO data base ( including 33 synoptic, climatic and rain gage stations during 15992 days from Farvardin, 1st, 1340 to Dey, 11th,1383). Jet stream frequencies and their locations have been detected over extensive area from 0Â° to 120Â° E and 0Â° to 80Â° N in four levels ( 250, 300, 400 and 500 H.Pa ) at 00:00, 06:00, 12:00 and 18:00 UTC.
The results of this research indicate that except the 250 H.Pa level that jet stream at 06:00 have considerable frequency, over 300 and 400 H.Pa levels and at 18:00 they observe over study area. At the same time, the plots of average speed of jet stream in conforming with the maximum frequency of jet stream on one hand and on the other hand is simultaneous with occurring the maximum speed of jet stream over study area that indicate area have fourth quarter (positive vortices and upper divergence) which is located on the Khazar western coastal. This can result in expansion of air mass at upper level or ascending vertical motion. Generally, the elongation of jet streams mostly until 500 H.Pa level at 18:00 indicates the thickness of instability layer that can create the extreme and pervasive precipitation over Khazar western coastal.