Potential Vorticity Diagnosis of a Simulated Hurricane. Part II: Quasi-Balanced Contributions to Forced Secondary Circulations

Abstract

Although the forced secondary circulations (FSCs) associated with hurricane-like vortices have been previously examined, understanding is still limited to idealized, axisymmetric flows and forcing functions. In this study, the individual contributions of latent heating, frictional, and dry dynamical processes to the FSCs of a hurricane vortex are separated in order to examine how a hurricane can intensify against the destructive action of vertical shear and how a warm-cored eye forms. This is achieved by applying a potential vorticity (PV) inversion and quasi-balanced omega equations system to a cloud-resolving simulation of Hurricane Andrew (1992) during its mature stage with the finest grid size of 6 km. It is shown that the latent heating FSC, tilting outward with height, acts to oppose the shear-forced vertical tilt of the storm, and part of the upward mass fluxes near the top of the eyewall is detrained inward, causing the convergence aloft and subsidence warming in the hurricane eye. The friction FSC is similar to that of the Ekman pumping with its peak upward motion occurring near the top of the planetary boundary layer (PBL) in the eye. About 40% of the PBL convergence is related to surface friction and the rest to latent heating in the eyewall. In contrast, the dry dynamical forcing is determined by vertical shear and system-relative flow. When an axisymmetric balanced vortex is subjected to westerly shear, a deep countershear FSC appears across the inner-core region with the rising (sinking) motion downshear (upshear) and easterly sheared horizontal flows in the vertical. The shear FSC is shown to reduce the destructive roles of the large-scale shear imposed, as much as 40%, including its forced vertical tilt. Moreover, the shear FSC intensity is near-linearly proportional to the shear magnitude, and the wavenumber-1 vertical motion asymmetry can be considered as the integrated effects of the shear FSCs from all the tropospheric layers. The shear FSC can be attributed to the Laplacian of thermal advection and the temporal and spatial variations of centrifugal force in the quasi-balanced omega equation, and confirms the previous finding of the development of wavenumber-1 cloud asymmetries in hurricanes. Hurricane eye dynamics are presented by synthesizing the latent heating FSC with previous studies. The authors propose to separate the eye formation from maintenance processes. The upper-level inward mass detrainment forces the subsidence warming (and the formation of an eye), the surface pressure fall, and increased rotation in the eyewall. This increased rotation will induce an additional vertical pressure gradient force to balance the net buoyancy generated by the subsidence warming for the maintenance of the hurricane eye. In this sense, the negative vertical shear in tangential wind in the eyewall should be considered as being forced by the subsidence warming, and maintained by the rotation in the eyewall.