Tropical Cyclone Convection: The Effect of Boundary Layer Wind Profiles

Idealized numerical model experiments are presented to investigate aspects of deep convection in tropical disturbances, including the effects of a uni-directional boundary layer wind profile on storm structure, especially on vertical vorticity production. A second set of experiments are presented also to study the effects of a more realistic vortex boundary-layer wind profile on tropical deep convection, focussing on the morphology of vertical vorticity that is generated. 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.

In warm-cored disturbances such as tropical depressions or tropical cyclones, the vertical shear and horizontal vorticity change sign at some level near the top of the boundary layer so that, unlike in the typical middle-latitude ‘supercell' storm, the tilting of horizontal vorticity by a convective updraught leads not only to dipole patterns of vertical vorticity, but also to a reversal in sign of the vorticity dipole with height. This finding has implications for understanding the merger of convectively-induced vorticity anomalies during tropical cyclone evolution.

In experiments with a more realistic vortex boundary-layer wind profile, deep convection that forms in an environment with low-level vertical shear and cyclonic vertical vorticity develops vertical-vorticity dipole structures in which the cyclonic gyre is favoured and persists longer than the anticyclonic one.

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.