Tropical cyclone flow asymmetries induced by a uniform flow revisited

We investigate the hypothesized effects of a uniform flow on the structural evolution of a tropical cyclone using a simple idealized, three‐dimensional, convection‐permitting, numerical model. The study addresses three outstanding basic questions concerning the effects of moist convection on the azimuthal flow asymmetries and provides a bridge between the problem of tropical cyclone intensification in a quiescent environment and that in vertical shear over a deep tropospheric layer. At any instant of time, explicit deep convection in the model generates flow asymmetries that tend to mask the induced flow asymmetries predicted by the dry, slab boundary layer model of Shapiro, whose results are frequently invoked as a benchmark for characterizing the boundary layer‐induced vertical motion for a translating storm. In sets of ensemble experiments in which the initial low‐level moisture field is randomly perturbed, time‐averaged ensemble mean fields in the mature stage show a coherent asymmetry in the vertical motion rising into the eyewall and in the total (horizontal) wind speed just above the boundary layer. The maximum ascent occurs about 45° to the left of the vortex motion vector, broadly in support of Shapiro's results, in which it occurs ahead of the storm, and consistent with one earlier more complex numerical calculation by Frank and Ritchie. The total wind asymmetry just above the boundary layer has a maximum in the forward right sector, which is in contrast to the structure effectively prescribed by Shapiro based on an inviscid dry symmetric vortex translating in a uniform flow where, in an Earth‐relative frame, the maximum is on the right.