Characteristics Of Convective-scale Downdrafts In The Outer Core Of Sheared Tropical Cyclones

The characteristics of convective-scale downdrafts and surface cold pools in the outer core of sheared tropical cyclones(TCs)are investigated based on high-resolution idealized numerical simulations.The downdrafts under various environmental vertical wind shears(VWS)have similar traits.Different from previous observational studies,the proportion of the downdrafts originating from the tropopause is largest of the four types of convective-scale downdrafts identified in the outer core.The frequency of downdrafts outside and upwind of the parent updraft increases with increasing downdraft top height.Vertical momentum budgets indicate that downward-oriented thermal buoyancy(TB)due to the evaporational cooling of rainwater and precipitation drag mainly contribute to the occurrence of low-level downdrafts.The midlevel and upper-level downdrafts originate due to the precipitation drag of raindrops,graupel and snow particles,and are strengthened by the downward-oriented,buoyancy-induced perturbation pressure gradient acceleration(BPGA)that is enhanced on the side with a stronger horizontal gradient of upward velocity.Although the effect of precipitation drag is limited,larger downward-oriented TB due to the sublimational cooling of icy-type particles and the downward-oriented BPGA contribute to the development/maintenance of the downdrafts from the tropopause.The dynamics of downdraft initiation also vary markedly in the experiments with different VWS magnitudes and profiles.The core of the parent updraft in the lower-layer shear experiment tends to align vertically due to the interaction between cold pools and low-level shear,less conducive to the formation of low-level downdrafts.Finally,the horizontal location of low-level downdrafts is further away from the bottom of their parent updraft in the upperlayer shear experiment than in the lower-layer shear experiment.However,More icy-type particles are produced and transported outward at upper levels because of the active convective cell in the lower-layer and weak shear experiments,resulting in larger downward-oriented TB due to the sublimational cooling of icy-type particles.On the other hand,the vortex-scale pressure perturbation is likely to veil the updraft-induced BPGA in the two experiments.As a result,the negative TB mainly contributes to the development/maintenance of the downdraft from the tropopause in weak and lower-layer VWS.In strong and upper-layer VWS experiments,on the contrary,the downward-oriented TB is less significant because of weaker updraft,and the downdrafts from tropopause originate due to a more signifcant BPGA field on the outward side of the updraft.In addition,The height of the downward-oriented BPGA field caused by the ambient buoyant updraft core is deeper than the negative TB field.The downdrafts from the tropopause hence tend to develop more deeply in the strong and upperlayer shear experiments than in the weak and the lower-layer VWS experiments.The characteristics of cold pools closely linked to the downdrafts are examined here.Convective-type cold pools are closely associated with the outer-core convective cells under VWS of different magnitudes,and the total number of cold pools and the frequency of convective-type cold pools both decrease as shear increases.The intensity of nonconvectivetype cold pools is stronger than convective-type cold pools because of deeper depth in nonconvective-type cold pools.However,convective-type cold pools show larger negative potential temperature perturbations but smaller negative equivalent potential temperature and water vapor mixing ratio perturbations,indicating that the contribution to the negative equivalent potential temperature perturbation near the ground by the dry air transported by convective-scale downdrafts is greater than that by evaporation.Budgets of surface perturbation pressure indicate that in-cloud warming due to latent heat release leads to a pressure decrease,while subcloud evaporational cooling and water loading are conducive to high pressure near the surface in cold pools.Moreover,the active convection in convective-type cold pools results in more significant precipitation drag and subcloud evaporation than that in nonconvective-type cold pools.