Abstract
The summer days between 1961 and 2017 are divided into two categories: extreme precipitation days (EPDs) and general precipitation days (GPDs). Compared to GPDs, on EPDs, South China (SC) is affected by a local trough in the lower troposphere and the East Asian subtropical westerly jet (EAWJ) is strengthened in the upper troposphere. The anomalous extension of the western Pacific subtropical high (WPSH) in the South China Sea (SCS) is a key of EPDs. The reason for the extension of WPSH is due to a Kelvin wave-induced Ekman divergence (WIED) response to the SST anomaly zonal contrast between the tropical Indian Ocean (TIO) and tropical Pacific Ocean (TPO). The vertical velocity gradient ∂ω/∂p anomaly in accord with Sverdrup vorticity balance plays a secondary reinforcing role. Utilizing the quasi-geostrophic omega (QG ω) equation, the quantitative contributions of the dynamic term ωD and thermodynamic term ωQ are examined. On EPDs, the ωQ term over SC has the largest contribution to total ω, accounting for 46.2%, while the ωD term accounts for only 16.7%. The variation of ωQ is the main contributor to the variation of ω within different precipitation ranges, with time and with vertical levels. Water vapor plays a crucial role in adiabatic heating. The moisture budget analysis suggests that water vapor transport is significantly converted into precipitable water from 3 days before EPDs, increasing the humidity of the atmosphere and providing favorable conditions for adiabatic heating.
Published Version
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