A numerical study of convective-scale downdrafts in the outer core of tropical cyclones in vertically varying environmental flows

Abstract

The characteristics of convective-scale downdrafts in the outer core of tropical cyclones in the lower- and upper-layer vertical wind shear (VWS) are investigated based on two high-resolution idealized numerical experiments. Four types of outer-core downdrafts, originating from the lower troposphere, the midtroposphere, the upper level, and the tropopause, respectively, are found. The downdrafts originating from the lower and midtroposphere can penetrate down near the surface, and those originating from the tropopause in upper-layer VWS tend to penetrate more downward than in lower-layer VWS. Downdrafts tend to be located in the more upwind portion of the downshear-right quadrant in lower-layer VWS than in upper-layer VWS. The frequency of downdrafts outside and upwind of the parent updraft increases with the increasing downdraft top height. Vertical momentum budgets indicate that downward-oriented buoyancy due to the evaporational cooling of rainwater and precipitation drag mainly contribute to the occurrence of low-level downdrafts, and the midlevel and upper-level downdrafts originate due to precipitation drag and are strengthened by the downward-oriented, buoyancy-induced perturbation pressure gradient. The processes governing the downdrafts from the tropopause are different between the two experiments. More icy-type particles are produced and transported outward at upper levels in the lower-layer shear experiment, resulting in larger downward-oriented buoyancy due to the sublimational cooling of icy-type particles and contributing to the development/maintenance of the downdraft from the tropopause in that experiment. However, the downward-oriented perturbation pressure gradient leads to the development/maintenance of the downdraft from the tropopause in the upper-layer shear experiment.