Convective Asymmetry and Imbalance in Tropical Cyclones Generated by Random Winds on the Periodic F-Plane in a Cloud-System-Resolving Numerical Model

This paper examines convective asymmetry and imbalance in tropical cyclones (TCs) that emerge from random winds on the periodic f-plane in a cloud-system-resolving numerical model. The model is configured with warm-rain microphysics, a traditional representation of surface-fluxes, Smagorinsky-type eddy diffusion, and a basic parameterization of longwave radiation. Within the simulation set, the sea-surface temperature (T_s) ranges from 26 to 32 ^oC, and the Coriolis parameter f ranges from 10^-5 to 10^-4 s^-1. The number of TCs that develop in a simulation increases rapidly with f and ranges from 1 to 18. As a whole, the simulation set provides a diverse set of vortices that can be used for a meaningful statistical study.

It is shown that axisymmetric steady-state theory, generalized to permit gradient-wind and hydrostatic imbalance, provides a reasonably accurate value for the maximum wind speed V_max of a mature TC up to surprisingly high levels of inner-core convective asymmetry (CA). The smallest values of CA are found to coincide with the highest degrees of imbalance (largely connected to supergradient flow) near the radius of maximum wind. On the opposite end of the spectrum, an excessive level of CA coincides with substantial violation of the key theoretical assumption of slantwise convective neutrality in the main updraft of the basic state. A reliable curve-fit is obtained for the anticorrelation between a simple measure of CA and V_max normalized to a classic estimate of its balanced potential intensity that is based solely on environmental conditions and air-sea interaction parameters. The importance of imbalance in simulated TCs whose wind speeds are within observational limits is assessed.

This work is supported by NSF grant AGS-1101713.