Numerical studies of tropical convection

Idealized numerical model experiments are presented to investigate
the convective generation of vertical vorticity in a tropical
depression. The calculations are motivated by observations made
during the recent PREDICT field experiment to study tropical
cyclogenesis, and by a desire to understand the aggregation of
vorticity debris produced by deep convection in models of tropical
cyclogenesis to form a monopole vortex. One aim is to isolate and
quantify the effects of low to mid level dry air on convective
cells that form within a depression and, in particular, on the
generation of vertical vorticity in these cells. Another aim is to
isolate the effects of a unidirectional boundary layer wind profile
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. A third aim is to isolate the effects of a
vortex boundary-layer wind profile on tropical deep convection,
focussing especially on the morphology of vertical vorticity that
develops. The growing convective updraughts, that are initiated by
a near surface thermal perturbation, amplify locally the ambient
rotation at low levels by more than an order of magnitude and this
vorticity persists long after the updraught has decayed, supporting
the results of an earlier study. The results of calculations with
dry air aloft do not support a common perception that the dry air
produces stronger downdraughts. In calculations where the vertical
wind shear changes sign at some level near the top of the boundary
layer, as occurs in warm-cored disturbances such as tropical
depressions or tropical cyclones, it was found that 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 updraught rotation with height. This feature is quite
unlike the structure in a typical middle-latitude `supercell'
storm. These results provide an essential first step to
understanding the interaction between deep convective elements in a
tropical depression or tropical cyclone. An increase in the
magnitude of boundary-layer shear was found to have the dual effect
of weakening the development of the initial thermal, which is
detrimental to vertical vorticity production by stretching and
tilting, while at the same time increasing the magnitude of
horizontal vorticity that can be tilted. The results provide a
basis for appraising a recent conjecture concerning the role of
storm splitting in explaining the contraction of the eyewall in
tropical cyclones.