Statistical-Synoptic Analysis of April 2019 Heavy Rainfall in Doroud-Boroujerd Basin

The occurrence of catastrophic floods in March-April 2019 in Lorestan province was a clear example of heavy rains that caused very heavy damage to agricultural, urban, transportation, and communications infrastructure.  The purpose of this study is to investigate the March-April 2019 rainfall in the Droud-Borujerd catchment area in terms of statistical, synoptic, and dynamic characteristics. In this regard, from the data related to the daily station rainfall in March-April 2019 of Boroujerd and Doroud synoptic stations, NOAA climate Synoptic factor data for the 25th March and 2nd April 2019, and upper atmospheric data from the University of Wyoming database for the mentioned days of April 2019. The results of statistical analysis showed that the occurrence of rainfall in April 2019 was one of the heavy rains that in the first wave (25th March) 2019 was 15% of the total annual rainfall and in the second wave (2nd April 2019) 20% of the total annual rainfall was recorded. Analysis of synoptic patterns of these heavy rainfall events showed that a similar synoptic pattern produced these two waves of heavy rainfall in the region. On both days, the presence of a strong trough a height of 5500 and 5550 geopotential meters, on the east of the Mediterranean Sea that the western region of Iran located in the front of this trough, lead to heavy rainfall. In these two days, the omega index had reached a significant critical value (-0.2). Moisture injection in the study area was done by the interaction of two cyclonic systems (east of the Mediterranean Sea) and anti-cyclone (on the Gulf of Aden) and the source of moisture supply was the Mediterranean Sea, the Red Sea, and the Persian Gulf, respectively. High atmospheric instability indices did not confirm the existence of very severe instability in the region. Moderate instability in the lower levels of the atmosphere, which could not be extended to the upper level (Skew-T diagram), indicated that a global synoptic system was involved in the whole region and no local convection factor played a role