The Slab Boundary Layer Model Underpinning the Boundary Layer Control of Translating Tropical Cyclones: Comparison with an Idealized Three-Dimensional Numerical Simulation

Abstract

An oft quoted benchmark for understanding the pattern of boundary-layer induced vertical motion in a translating tropical cyclone is based on approximate solutions for a steady slab boundary layer beneath a prescribed, uniformly translating axisymmetric vortex in gradient wind balance. The fidelity of this framework is investigated here in the context of a recent, idealized, three-dimensional, convection-permitting, numerical model simulation of a translating tropical cyclone in a deep uniform flow. These new analyses show that the slab boundary layer model captures rather well the spatial structure of the inner-core wind asymmetries in the boundary-layer, but overestimates the magnitude of these asymmetries. We investigate also to what extent the flow emanating from the boundary layer influences the pattern of deep convection in the intensifying vortex in the numerical simulation. In this simulation, the location of maximum ascent during the mature stage is displaced azimuthally upstream in the mid-troposphere compared with that at the boundary layer top, consistent with the differential rotation in a warm cored vortex where the azimuthal velocity decreases with height. This vertical-motion tilt pattern is a robust feature also of innermost arcs of subsidence inside the eye region. During the period of intensification and maturity, the strength of ascent in the numerical simulation is larger in the mid-troposphere than at the boundary layer top as would be expected in a region of buoyant updraughts. However, this velocity difference is reversed in sign during the later decay period, suggesting the decay is linked to the inability of deep convection to remove mass at the rate that it is being expelled by the boundary layer. Some implications of these findings are discussed.