Synoptic aspects of two flash flood-inducing heavy rainfalls in southern Iran during 2019–2020

This study investigates the synoptic aspects of two heavy rainfall events that occurred in the southern half of Iran during the period 2019–2020. To this end, daily precipitation data from 114 synoptic stations scattered over southern, southwestern, and southeastern Iran were used to characterize the 24-h total precipitation, while the general extreme value (GEV) distribution was used to estimate the associated return periods. The geopotential height, u and v wind components, sea-level pressure, and atmospheric relative humidity data relative to 100 to 250 hPA levels were also used to analyze the synoptic aspects of the considered heavy rainfall. The statistics showed the 155-mm 24-h total rainfall occurred in Izeh station in March 2019 ranked first during the study period, while the 142-mm 24-h rainfall of Poldokhtar station ranked second. The estimated return period of the heavy rainfall of late March and early April 2019 in western Iran varies between 7 and 200 years across the studied stations. The estimated chance of recurring 155-mm 24-h heavy rainfall at Izeh station is 96 years, but it is 56 years for the 142-mm rainfall in Poldokhtar station. In the case of January 2020 heavy rainfall that occurred in southern and southeastern Iran, the estimated return period of the 176-mm 24-h total rainfall measured in Qeshm and Minab stations of Hormozgan province was 115 years, whereas it was 10 years for the 107 mm of rainfall at Rask station in Sistan and Baluchestan province. The synoptic analysis of the consecutive downpour of late March and early April 2019 that occurred in the west and southwest of Iran and the successive heavy rainfall of January 2020 that occurred in the southern and southeastern Iran was related to the blocking synoptic systems and the associated atmospheric turbulences established in these areas. In the heavy rainfall of March 2019, the formation of a strong cut-off low centered over Iraq sank the higher-latitude cold air into the system that in turn intensified it that favors absorbing intensive moisture from the Mediterranean Sea. The penetration of the system’s flanks into the Red Sea also aids the system to absorb hot and humid air from the low latitudes that fed consecutive days of heavy rainfall in southwestern Iran that lies in the forehead of the cut-off low. In the case of heavy rainfall of January 2020 that occurred in the south and southeast of Iran, a very deep trough was formed over the area due to the formation of an omega-shaped blocking system. The advection of cold air from the higher latitudes and the hot and moist air from the southern water bodies of Iran into the system causes intensive heavy rainfall in the area which is situated in front of the trough.