Tropical convection: the effects of ambient vertical and horizontal vorticity

We present a series of idealized numerical model experiments to investigate aspects of deep convection in tropical depressions, including the effects of boundary-layer wind structure on storm structure, especially on vertical vorticity production and updraught splitting, and the combined effects of horizontal and vertical shear on vertical vorticity production, with and without background rotation. In warm-cored disturbances such as tropical depressions, 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 to a reversal also in sign of the updraught rotation with height. This finding has implications for understanding the merger of convectively induced vorticity anomalies during vortex evolution. Ambient cyclonic horizontal shear and/or cyclonic vertical vorticity favour amplification of the cyclonically rotating gyre of the dipole. Consistent with an earlier study, storm splitting occurs in environments with pure horizontal shear as well as pure vertical shear, but the morphology of splitting is different. In both situations, splitting is found to require a relatively unstable sounding and relatively strong wind shear.