ANALYTICAL TROPICAL CYCLONE ASYMMETRIC CIRCULATION FOR BAROTROPIC MODEL INITIAL CONDITIONS

The flow field around a tropical cyclone can be decomposed into the sy mmetric vortex, an environmental steering flow, and an asymmetric circ ulation that is the residual after the first two components are remove d. In a barotropic model with a quiescent environment, the asymmetric circulation is associated with a propagation vector that is defined to be the difference between the storm-motion vector and the steering fl ow. An analytical specification derived by Carr for the asymmetric cir culation and the corresponding propagation vector is proposed for use in the initial conditions of dynamical track-prediction models. One wi dely used method for estimating the environmental steering flow has be en to average the wind components in radial bands centered on the stor m position. It is shown that the portion of the asymmetric circulation that is included within these radial band averages results in a syste matic rotation of the propagation vectors defined relative to the stee ring flow, which is quite similar to that calculated by compositing ob servations. Thus, some method other than the radial band-average estim ate of the steering will be required in the wind-field decomposition t o separate the large-scale environment from the influence of the tropi cal cyclone. It is demonstrated that inclusion of the analytically gen erated asymmetric circulation in the initial conditions of a barotropi c track-prediction model leads to a propagation vector with the correc t speed and direction. It is proposed that inclusion of the asymmetric circulation would eliminate much of the initial slow bias in dynamica l track-prediction models that include only the symmetric vortex circu lation and thus improve the tropical cyclone track forecasts.