On the Relationship of a Low-Level Jet and the Formation of a Heavy-Rainfall-Producing Mesoscale Vortex over the Yangtze River Basin

Abstract

Dabie vortices (DBVs) are a type of heavy-rainfall-producing mesoscale vortices that appear with a high frequency around the Dabie Mountain over the Yangtze River Basin. For a long time, scholars have found that DBVs tend to form when a low-level jet (LLJ) appears in their neighboring regions. However, the underlying mechanisms of this phenomenon still remain vague. This study furthers the understanding of this type of event by conducting detailed analyses on a long-lived eastward-moving DBV that caused a severe flood in the 2020 summer. It is found that the LLJ in this event was belonged to a nocturnal LLJ type, with its maximum/minimum appeared around 2100/0600 UTC. The diurnal cycle of LLJ affected precipitation and intensity of the DBV notably: As the LLJ intensified, vortex’s precipitation and intensity both enhanced, and vice versa. The LLJ exerted two effects on the DBV’s formation that are opposite to each other. The more important effect is that the LLJ caused intense lower-level convergence around its northern terminus. This convergence directly produced cyclonic vorticity through vertical stretching, which dominates the DBV’s formation and enhances the convection-related upward cyclonic vorticity transport that acted as another favorable factor. The less important effect is that (i) the LLJ induced import of anticyclonic vorticity into the vortex’s central region, which decelerated the DBV’s formation; and (ii) the LLJ-related to strong ascending motions tilted horizontal vorticity into negative vertical vorticity, which reduced the growth rate of cyclonic vorticity.