Thermodynamic Characteristics of Southwest Vortex Rainstorm in Southern Shaanxi

Abstract

Using daily 700 hPa geopotential height charts from October 2013 to October 2022， Southwest Vortex （SWV） annual data， European Centre for Medium-Range Weather Forecasts （ECMWF） ERA5 reanalysis data， and precipitation data from stations in Shaanxi Province， a statistical and diagnostic analysis was conducted on individual cases of heavy rain induced by the Southwest Vortex in southern Shaanxi.The results reveal the following insights： （1） Over the course of 10 years， there were a total of 119 days with heavy rain， among which 38 days were associated with heavy rain caused by the Southwest Vortex， accounting for about one-third （32%） of the total heavy rain days.These events were mostly observed from May to September， with the highest frequency in June.Statistically， the precipitation tended to start at night and end during the day.（2） The Southwest Vortex influencing southern Shaanxi originates primarily from the basin vortex， and typically， its movement eastward by 12 to 24 hours can lead to heavy rain in the region.The heavy rain area associated with the Southwest Vortex is mainly located in the northeast quadrant of the 700 hPa vortex center， to the south of the shear line， in the area with a large gradient of potential pseudo-equivalent potential temperature， at the intersection of the 500 hPa westerly trough and the outer southwest flow of the subtropical high， corresponding to the region of strong divergence at the 200 hPa level.（3） The study of the vertical structure of the Southwest Vortex indicates that the strong convergence center at 700 hPa is located to the east of the positive vorticity center.This region corresponds well with the heavy rainfall area.Strong divergence under high-altitude jet streams leads to air quality adjustment， lower-level convergence， and frontal genesis.（4） There are three moisture transport channels： one comes from the warm sea surface of the western Bay of Bengal， the second originates from the warm sea surface in the eastern Bay of Bengal， and the third derives from the South China Sea.During heavy rain periods， the cyclonic-like vorticity， divergence， moisture flux divergence generated by the terrain in the Qinba Mountain region combined with the systematic vorticity， divergence， and moisture flux divergence， enhancing the lower-level convergence.This is also an important factor contributing to the formation of heavy rain induced by the Southwest Vortex in southern Shaanxi.

Keywords

• southwest vortex
• terrain vorticity
• frontogenesis
• convective instability
• moisture tracing