Comparison of two types of persistent heavy rainfall events during sixteen warm seasons in the Sichuan Basin

Based on hourly precipitation from national surface stations,persistent heavy rainfall events(PHREs)over the Sichuan Basin(SCB)are explored during the warm season(May to September)from 2000 to 2015 to compare synoptic circulations and maintenance mechanisms between different PHRE types.There are two main types of PHREs:one is characterized by a rain belt west of 106°E over the SCB(WSB-PHREs),and the other features a rain belt east of 106°E over the SCB(ESB-PHREs).In total,there are 18 ESB-PHREs and 10 WSB-PHREs during the study period.Overall,the rain belts of WSB-PHREs are along the terrain distribution east of the Tibetan Plateau,while the precipitation intensity of ESB-PHREs is stronger.For the two types of PHREs,the shortwave trough over the SCB and the western Pacific subtropical high act as their favorable background environments,particularly for ESB-PHREs.The water vapor of WSB-PHREs is mainly transported from the South China Sea,whereas for ESB-PHREs the South China Sea and Bay of Bengal are their main moisture sources.The composite vorticity budgets of southwest vortices during their mature stage indicate that the convergence effect is a dominant factor for maintaining the two types of PHREs,and the strong vertical vorticity advection is also favorable,but the relative contribution of vertical advection is larger for WSB-PHREs.

Seasonal evolution of the dominant modes of the Eurasian snowpack and atmospheric circulation from autumn to the subsequent spring and the associated surface heat budget

This study investigates the seasonal evolution of the dominant modes of the Eurasian snowpack and atmospheric circulation from autumn to the subsequent spring using snow water equivalent （SWE）, snow cover frequency （SCF）, and 500 hPa geopotential height data. It is found that the Eurasian SWE/SCF and circulation dominant modes are stably coupled from autumn to the subsequent spring.The temporal coherence of the seasonal evolution of the dominant modes is examined.The seasonal evolution of the Eurasian circulation and SWE dominant modes exhibit good coherence, whereas the evolution of the Eurasian SCF dominant mode is incoherent during the autumn-winter transition season. This incoherence is associated with a sign-change in the SCF anomalies in Europe during the autumn-winter transition season, which is related to the wind anomalies over Europe. In addition, the surface heat budget associated with the Eurasian SWE/SCF and circulation dominant modes is analyzed. The sensible heat flux （SHF） related to the wind-induced thermal advection dominates the surface heat budget from autumn to the subsequent spring, with the largest effect during winter. The surface net shortwave radiation is mainly modulated by snow cover rather than cloud cover, which is estimated to be as important as, or likely superior to, the SHF for the surface heat budget during spring.The NCEP-NCAR surface heat flux reanalysis data demonstrate a consistency with the SWE/SCF and air temperature observational data, indicating a good capability for snow-atmosphere interaction analysis.

Mechanisms governing the evolution of a long-lived northwest vortex that caused a series of disasters in Northwest China

The northwest vortex(NWV)is a type of mesoscale vortex that appears with a relatively high frequency in Northwest China.To further the understanding of the NWV’s evolution,in this study,the moisture and circulation budgets of a long-lived NWV(~132 h)that appeared in early August 2019 were calculated.This vortex induced a series of torrential rainfall events in Northwest China and Mongolia,which caused severe transmission line faults and urban waterlogging.Synoptic analyses indicate that the NWV was generated in a favorable background environment characterized by notable upper-level divergence and strong mid-level warm advection.The moisture budget shows that the East China Sea and Bohai Sea acted as the main moisture sources for the NWV-associated precipitation,and the water vapor was transported into the rainfall regions mainly by easterly and southeasterly winds.The circulation budget indicates that,during the developing stage,convergence-related vertical stretching was a dominant factor for the NWV’s development;whereas,the vortex’s displacement from regions with stronger cyclonic vorticity to those with weaker cyclonic vorticity mainly decelerated its development.In the decaying stage,divergence-related vertical shrinking and the net export of cyclonic vorticity due to the eddy flow’s transport resulted in the NWV’s dissipation.