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

Shahab Shaffie,Gholamali Mozaffari,Younes Khosravi

doi:10.2166/wcc.2020.284

Shahab Shaffie, Gholamali Mozaffari + Show 1 more

Open Access

https://doi.org/10.2166/wcc.2020.284

Copy DOI

Abstract

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 precipitation is mainly governed by various factors such as jet stream
Since the geographic distribution pattern of rainfall and its intensity on a planetary scale are affected by the pressure system patterns, the location of Intertropical Convergence Zone (ITCZ), as well as the position and expansion of ocean currents, this study aims to identify another significant factor in the occurrence of extreme rainfall by the spatial analysis of the frequency of influential jet streams in the occurrence of extreme precipitation in the west of Iran
More than 70% of place of formation as well as establishment of jet streams passage affecting extreme precipitation in the west of Iran were located in this area, whose main direction has been over the Red Sea

Summary

Introduction

As defined by the World Meteorological Organization (WMO), whenever the speed of these strips exceeds 30 m/s, the jet stream is created (Kaviani & Alijani ). The speed of jet stream cores declines from its center to the outside where this reduction toward poles is cyclonic or positive, while it is anticyclonic or negative toward the equator. The speed cores are located in westerly winds of equatorial margins, and the other is over the polar front of tropical regions which moves more than in other areas. They create two relatively distinct contexts called polar front jet stream and subtropical jet stream. Polar front jet stream occurs in the mid and higher latitudes in the polar front and at a height of [9,10,11,12] km, in the parallels of [20–30], at the height of [12,13,14] km, and forms discontinuous circles around the earth (Masoodian & Mohammadi )

Objectives

Methods

Results

Conclusion