Simulations of extreme rainfall in the Yarlung Tsangbo Grand Canyon, China
Authors: Chen Xuelong , Cao Dianbin , Zhang Qiang , Xu Xin 
Published: July 2024

The Yarlung Tsangbo Grand Canyon (YGC), one of the world&#8217;s deepest canyons, is located in the southeastern Tibetan Plateau (SETP). The YGC exhibits the highest frequency of convective activity in China. Due to frequent rainstorms in the wet season, natural disasters such as landslides and debris flows frequently occur, and often block traffic corridors. Thus, understanding the relationship between water vapor changes, convective cloud activity, and extreme rainfall events in the YGC is critical. A comprehensive observation network for water vapor variations, cloud activity, local circulation, and land-air interactions in the YGC was installed to help us to determine the relationship between the water vapor transport and heavy precipitation in the YGC and the physical process that determines the precipitation intensity, especially for cases of strong precipitation.
We analyzed 35 years observation data of daily precipitation to objectively classify the weather systems responsible for the heavy precipitation. Hierarchical clustering method divided the atmospheric circulation of the regional heavy precipitation into two representative patterns: the Tibetan Plateau vortex type (TPVT, accounting for 56.6% of the heavy precipitation events) and the mid-latitude trough type (MLTT&#65292;43.4%). The comprehensive analysis of the two patterns shows a clear connection between the heavy precipitation and positive vorticity anomaly, moisture convergence and the southeastward shift of the westerly jet core. Specifically, TPVT heavy precipitation events are caused by potential vorticity dry-to-wet processes during its eastward movement, while MLTT events are associated with the intrusion of deeply extratropical trough-ridge circulations into the SETP.
We used the Weather Research and Forecasting (WRF) model to simulate the water vapor flux during extreme rainfall events. The general shortcoming of the WRF precipitation simulation nudged with the European Centre for Medium-Range Weather Forecasts&#8217; reanalysis dataset version 5 (ERA5), is that it cannot capture strong rainfall period. We tested many WRF parameterization schemes at a 1 km grid resolution. The Multiscale Kain-Fritsch (MSKF) scheme outperforms explicit calculations (NO_CU) in capturing heavy precipitation events accurately, thanks to its enhanced treatment of cloud-radiation interactions. The incorporation of the Turbulence Orographic Form Drag (TOFD) scheme significantly improves precipitation simulation accuracy. This is particularly evident in the better representation of local circulation. A noticeable improvement in near surface wind speeds and the vertical profile of horizontal winds was presented by the TOFD scheme. It was found that when an optimized combination of parameterization schemes in WRF can better capture the variations in the wind and water vapor concentration in the YGC channel, the model produced the best simulation results for extreme rainfall in the YGC.
These analyses have help us understanding the impacts of YGC valley on the water vapor transport and extreme rainfall outbreak mechanism.