Synoptic Analysis of the Impact of Cut-Off Lows in Heavy Rains in Iran

Nowadays, heavy rains are an important aspect of climate studies because of their dangers and damage. Cut-off lows are influential in the occurrence of heavy rainfall in Iran by creating atmospheric instability. In this regard, first, the climatic variables of temperature, precipitation, and geopotential height of 500 hPa were investigated in connection with the formation of cut-off low during the period 1986-2018. Then, to predict the changes of the cut-off low event in the future and its impact on heavy precipitation, ECMWF database ERA-Interim version and CMIP5 models with CORDEX exponential microscale approach under two scenarios of optimistic release RCP4.5 and pessimistic RCP8. 5 was used until 2099. The results showed that in the RCP4.5 scenario, the increase in precipitation was estimated to be at least 2.41 mm, and in the RCP8.5 scenario, the maximum precipitation decrease was estimated to be 91 mm. The surface pressure anomalies of 500 hPa will increase by 61.5 and 92.5 geopotential meters based on RCP4.5 and RCP8.5. The lowest frequency cut-off low events was in 1995 and 1996 with 10 events, and the highest event was in 1986 with 30 events. With the increase of air temperature and the advance of subtropical high pressure towards high latitudes, the place of formation and the path of movement cut-off low in 33 years have been extended towards northern latitudes, which indicate the effects of change of the climate in Iran. The existence of a significant positive relationship between the frequency cut-off low events and heavy rainfall was confirmed at the 5% error level. Moreover, the impact of cut-off low on the frequency of heavy rainfall in Iranian stations, especially the northern stations, has been more impressive.