Investigating role of tropical-extratropical interactions in formation of atmospheric rivers and supplying extreme-widespread precipitation moisture in Iran: A case study on March 25, 2019

Abstract

This study aims to investigate the role of tropical-extratropical interactions in the formation of atmospheric rivers as an important source of moisture in extreme-widespread precipitation of Iran. Extreme precipitation events are extracted based on the 95th percentile index from 1989 to 2019 in Iran. Then, the threshold of widespread precipitation is determined. A day is defined as having extreme-widespread precipitation if one-third of the selected stations have mostly extreme precipitation. Finally, 9 days with the highest extreme precipitation and spatial continuity are selected. The upper air data of the 9 selected days are extracted and, accordingly, synoptic charts are plotted. The used data include ERA5, which are extracted from the lower (800 and 850 hPa) and middle (500, 600 and 700 hPa) levels. The results indicate an interaction with tropical circulation patterns by penetration of extratropical circulation patterns into tropical and subtropical regions. The interaction between patterns could lead to the formation of atmospheric rivers with tropical origin from ITCZ, their transport to subtropical and extratropical regions and their moisture supply along the path by different moisture sources in tropical, subtropical and extratropical regions. The formed atmospheric rivers are divided into two categories, namely continental and oceanic, based on their formation origin. The continental atmospheric river is formed at the lower level and, in some cases, at the middle level, while the oceanic atmospheric river is only formed at the middle level. With the emergence of atmospheric rivers in Iran, there have been extreme-widespread precipitation events due to unstable conditions and rising atmosphere.