The Axisymmetric Slab Boundary Layer Model Underpinning the Boundary Layer Control of Tropical Cyclones

The dynamics of the slab boundary layer model for an axisymmetric vortex are re-examined in the light of the continued relevance of this model as a benchmark for understanding aspects of tropical cyclone structure. Of particular relevance is the control the boundary layer has on the location of deep convection. Using two different methods to solve for the steady nonlinear boundary layer, it is shown that in relatively narrow vortices, the location of maximum ascent out of the boundary layer lies inside the radius of maximum gradient wind. In contrast, as the breadth of the gradient wind profile increases, the location of maximum ascent moves outwards beyond that of the maximum gradient wind. These new findings help to understand airborne Doppler radar observations of certain structural differences between rapidly developing and mature storms. Shown also is the propensity of the boundary layer of broad vortices to produce secondary maxima of tangential wind and ascent out of the boundary layer well beyond the radius of maximum gradient wind. This intrinsic boundary layer feature has relevance to understanding secondary eyewall formation. Some implications of the new calculations to translating vortices and to the theory of potential intensity are discussed also.