Latent Heating Effect on the Intensification of Southwest Vortices Over the Downstream Area of the Second‐Step Terrain in China

Abstract

AbstractThe eastward displacement of Southwest Vortices (SWVs) profoundly impacts the precipitation in the middle and lower reaches of the Yangtze River Basin (YRB) in summer. At present, the feedback mechanism of latent heating on the persistent eastward movement of SWVs under the influence of the orographic effect of the second‐step terrain in China remains unclear. Therefore, this study focused on clarifying the relationship between orographic effects and the mechanisms governing the persistent eastward movements of SWVs, as well as the feedback effect of latent heating on the eastward‐moving SWVs. In this study, it is revealed that discontinuous maintenance of closed vortex circulation occurs when eastward‐moving SWVs pass over the second‐step terrain region. Specifically, eastward‐moving SWVs first experience dissolution on the windward side, followed by substantial reconstruction of closed vortex circulation on the leeward side when passing over the second‐step terrain region. Moreover, the reconstruction of closed vortex circulation on the leeward side is closely associated with the feedback effect of latent heating, and the microphysical property of cold clouds identified during the vortex intensification determines the dual‐peak feature observed in the vertical constitution of the total latent heating, which is related to two major phase‐transition processes: the condensation of water vapor into liquid‐phase hydrometeors (LCP_vapor) process and the desublimation of water vapor into ice‐phase hydrometeors (LSP_vapor) process. From a potential vorticity (PV) perspective, latent heating promotes the genesis of PV anomalies, which accordingly facilitates the maintenance of a positive “vorticity tower” (i.e., total budget of vorticity) via the tilting effect and the horizontal advection effect of local vorticity. Further sensitivity experiments of latent heating indicated that latent heating associated with the LCP_vapor process fulfills a more important role in sustaining the positive “vorticity tower” than does that associated with the LSP_vapor process during the vortex intensification.