Tropical cyclone convection: the effects of ambient vertical vorticity

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Abstract

We present idealized numerical model experiments to isolate and quantify the influence of ambient vertical vorticity on the dynamics of deep convection, such as that in the inner-core region of a tropical depression. The vertical vorticity is represented either by a uniform horizontal shear, a uniform solid-body rotation, or a combination of both. We show that the growing convective cells amplify locally the ambient rotation at low levels by more than an order of magnitude and that this vorticity, which is produced by the stretching of existing ambient vorticity, persists long after the initial updraught has decayed. Significant amplification of vorticity occurs even for a background rotation rate typical of the undisturbed tropical atmosphere and even for clouds of only moderate vertical extent. This finding has important implications for tropical cyclogenesis and provides a basis for a unified theory of tropical cyclogenesis and tropical cyclone intensification.

The presence of ambient vertical vorticity reduces the updraught strength, slightly more when the vorticity is associated with horizontal shear than when it is associated with solid-body rotation on account of enhanced entrainment. The reduction of the updraught strength can be attributed to the reduction of the lateral inflow by the centrifugal and Coriolis forces. Despite the significant amplification of vorticity, the sum of the centrifugal and Coriolis forces is mostly small compared with the lateral pressure gradient. Thus, at the level of background rotation studied, the Rossby elasticity effects in the updraught postulated by Montgomery et al. are not large, but may be important for higher levels of background rotation. The simulations ignore several processes that are likely to be important in reality, such as ambient vertical shear and surface friction, but they provide important benchmark calculations for interpreting the additional complexity arising from the inclusion of these effects. Copyright © 2011 Royal Meteorological Society

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