Super-parameterization of roll vortices in tropical cyclone models

The primary effects of roll vortices on the boundary layer (BL) fluxes are inherently non-local and non-gradient and hence cannot be captured by standard parameterizations used in present tropical cyclone models. A new approach to the parameterization of roll vortices in the BL will be discussed. It resembles a recently emerged approach called “super-parameterization” of complex cloud processes in a GCM. The idea is to use a 2D cloud-resolving model in each column of a large-scale model to explicitly represent small-scale and mesoscale processes and interactions among them. Similarly, our method can be called super-parameterization of roll vortices in the BL, which includes a 2D high-resolution non-hydrostatic model that explicitly resolves roll vortices coupled with a 3D system of equations representing the large-scale (mesoscale) flow. The idea is based on a scale separation with the following assumptions: a) roll vortices and the large-scale flow have remarkably different horizontal scales across the background (mean) flow, and b) roll vortices are realized in the form of “2D” rolls aligned with the background (mean) wind. These differences in the characteristic spatial scales allow for the decomposition of the full 3-D equation system, representing a tropical cyclone model, into two coupled equation systems for the mean large-scale (or mesoscale) flow and convective scale motions (roll vortices) in it. An example of the application of this approach to a tropical cyclone model will be presented.