Study on the Mechanism of Warm-Sector Torrential Rain in Guangxi under Atmospheric Rivers Background

Abstract

Based on multi-source observation data and ERA5 reanalysis data， the characteristics of accompanying Atmospheric Rivers （ARs） in the Guangxi warm-sector torrential rain cases during the summer of 2010-2022 （June-August） are statistically analyzed.And the typical warm-sector torrential rain process under the background of ARs in Guangxi occurred in 2 -4 June 2022 are analyzed from the perspectives of thermal and dynamic based on the diagnosis of wave activity flux， horizontal frontogenesis and adiabatic heating.The results showed that： （1） Most warm-sector torrential rain cases are accompanied by ARs.When the ARs pass through Guangxi and remain below 1000 kg·m-1·s-1， the increase of ARs intensity is conducive to the warm-sector torrential rain.In most cases， the ARs are in the direction of southwest to northeast， and the direction angle are between 15° and 65° when ARs pass Guangxi.（2） During the typical warm-sector torrential rain process， the East Siberian blocking and the Northeast cold vortex are extremely active， resulting in the suppression of the Western Pacific Subtropical High （WPSH） and its southerly position which keep the ARs maintain in the Bay of Bengal， South China Sea， South China and north tropical Pacific， providing sufficient water vapor for the occurrence and development of warm-sector torrential rain.The increase of the pressure gradient caused by the maintenance of the WPSH and the eastward movement of the vortex， the acceleration of the monsoon flow at night jointly strengthen the local ARs.（3） The enhancement of ARs at night promotes the local water vapor convergence and vertical transport， cause the wet layer thickened and atmospheric precipitable water increase continuously， which is conducive to the increase of precipitation efficiency.At the same time， the continuous warm and moist transport is conducive to the establishment and maintenance of unstable stratification in the lower level， and the convective unstable structure runs through the whole precipitation process.（4） The convergent uplift and lateral friction of mountain topography promote the upward movement and vertical vorticity development， which， on the one hand， is conducive to the uplift of the warm and moist air accumulated in the front of the mountain triggering convection.On the other hand， it is conducive to the maintenance of convective system and cause more water vapor to condense into rain.In addition， the continuous frontogenesis forcing caused by the accumulation of warm and moist air is also conducive to the maintenance and intensification of precipitation.（5） The strong upward movement under the influence of ARs causes a large amount of water vapor to condense continuously and release latent heat which in turn strengthens the upward movement.The convection continues to develop and strengthen under this positive feedback mechanism.

Keywords

• warm-sector torrential rain
• atmospheric rivers
• convective instability
• latent heat