Tropical cyclone convection: the effects of a vortex boundary-layer wind profile on deep convection

摘要

We describe idealized numerical model experiments to study the effects of a vortex boundary-layer wind profile on the generation of vertical vorticity in tropical deep convection. Situations are considered in which there is either no vertical shear above the boundary layer or negative vertical shear appropriate to a warm-cored vortex. Deep convection growing in these environments develops dipole structures of vertical vorticity in which the cyclonic gyre is favoured and persists longer than the anticyclonic one. The orientation of the dipole at a particular height is determined partly by that of the ambient horizontal vortex lines, which rotate with height, and also by the vertical advection of vertical vorticity from below. An increase in the magnitude of boundary-layer shear enhances the distortion of the initial thermal, weakening its subsequent ascent rate. This effect is detrimental to vertical vorticity production by stretching but, because the increase in shear implies an increase in the magnitude of horizontal vorticity that can be tilted, its net effect on the vertical vorticity production by tilting cannot be foreseen. In the calculations described, the effect of the increased horizontal vorticity dominates, so that updraughts rising in stronger vertical shear have larger vertical vorticity maxima, despite smaller vertical velocity maxima. With negative vertical shear above the boundary layer, the vorticity dipole reverses in sign with height, as in a recent study of the effects of unidirectional shear. The results provide a basis for appraising a recently proposed conceptual model for the inward contraction of eyewall convection in tropical cyclones, as well as a starting point for developing an improved understanding of the formation of a vorticity monopole during tropical cyclogenesis.