Climatic analysis of effective jet streams frequency on extreme precipitations in west of Iran

Abstract

In this study, the frequency of effective jet streams was analyzed in extreme and widespread precipitations in the west of Iran. For this purpose, the daily precipitation of 69 synoptic and climatic stations over 18,624 days (1961–2010) were selected. Then, 119 days of extreme and widespread precipitation in the study area were chosen based on generalized distribution for conducting related reviews and analyses. The frequency of jet streams in the geographical location from 0° to 120°E and −10° to 80°N were reviewed at four levels (250, 300, 400 and 500 hPa). Due to the large volume of information, only the highest and lowest levels (250 and 500 hPa) in relation to the surface were considered. According to the results, the highest frequency of jet stream was observed at 250 hPa. The second quarter of the jet stream core lay over the west of Iran (which is associated with increasing positive vorticity as well as upper-level divergence and lower-level convergence of the atmosphere). In general, the extension of jet stream up to 500 hPa indicated an unstable layer thickness, which can cause extreme and widespread precipitation in the west of Iran. The results of selected days based on cluster analysis and Lund correlation revealed that in rainy days, the wind speed was more than 50 m/s and the subtropical jet stream speed was over 40 m/s, leading to extreme precipitation in the west of Iran. HIGHLIGHTS The role of subtropical jet stream in developing extreme precipitations is more than 70% in Western Iran.; The spatial analysis of subtropical jet stream frequency in occurrence of widespread and extreme precipitations in Western Iran.; The spatial analysis of jet streams frequency at the level of 250 hPa indicated that the highest frequency of jet streams belongs to the range from the south of the Red Sea to the south of the Mediterranean Sea.; The synoptic analysis of jet streams of representative days of extreme precipitation.; The results of the selected days obtained from cluster analysis and Lund correlation indicated that on precipitation days, the wind speed was more than 50 m/s and the speed of subtropical jet stream was over 40 m/s, making the atmosphere unstable and eventually causing extreme precipitation in the west of Iran.;

Keywords

• atmospheric instability
• extreme precipitation
• jet stream
• west of iran