Moist Potential Vorticity Diagnosis of the Tropical Cyclone Boundary Layer: Influence of Roll Vortices

Abstract

Physical processes determining the dynamic and thermodynamic structure of a tropical cyclone boundary layer (TCBL) are quite different from anywhere else in the atmospheric boundary layer due to the substantial contribution of latent heating and frictional convergence. These processes regulate the radial and vertical distributions of momentum and enthalpy fluxes that are closely related to storm development and intensification. Our current understanding of TCBL is limited by the number of observations in this region, and a majority of the observational studies assume an axisymmetric structure. Three-dimensional observations and numerical studies show that substantial asymmetric structure exists in the TCBL. This study investigates the link between the asymmetric structure and small-scale processes using a Moist Potential Vorticity (MPV) framework. The simulated TCBL is uniquely characterized as a region of negative MPV with a robust and coherent layer of high-magnitude negative MPV embedded within, referred to as the Potential Vorticity Minimum Layer (PVML). The PVML can interact with the local flow anomalies such as those associated with roll vortices provided they are vertically collocated. The small-scale dynamical processes set the thermodynamic structure inside the TCBL and this interplay modulates the height of the PVML. Since the height of the PVML combines information about the local wind and thermal structures using a materially conserved variable, it is a valuable proxy to study the evolving 'topography' of a simulated TCBL.