A Simple Method for Simulating Tropical-Cyclone Boundary Layers

Abstract:

A method to simulate the boundary layer of tropical cyclones in an idealized manner is described. The method can be used either with planetary boundary layer (PBL) parameterization schemes or within large eddy simulations (LES). The key step is to include a term to the velocity equations representing the advection of angular momentum by the secondary circulation of a tropical cyclone. Using both single-column modeling and LES, model output is evaluated using dropsonde data and flight-level observations from tropical cyclones. Bulk properties of the flow (such as inflow-layer depths of ~1 km and surface inflow angles of ~25 degrees) compare well with observations only when the aforementioned mesoscale tendency terms are added; in contrast, traditional approaches without mesoscale tendency terms underestimate near-surface shear and overestimate boundary layer depth. Maximum effective eddy viscosity from LES is of-order 50 m² s^-1, consistent with recent observational studies. Analyses of temporal spectra demonstrate that the energetics of turbulence are captured well in LES, but only when grid spacing is approximately 10 m (or less). The method described here is proposed as an inexpensive way to evaluate boundary layer schemes for NWP models, and to generate realistic turbulence statistics for a variety of applications.