Analytical tropical cyclone asymmetric circulation for barotropic model initial conditions

The flow field around a tropical cyclone can be decomposed into the symmetric vortex, an environmental steering flow, and an asymmetric circulation that is the residual after the first two components are removed. 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 flow. An analytical specification derived by Carr for the asymmetric circulation and the corresponding propagation vector is proposed for use in the initial conditions of dynamical track-prediction models. One widely used method for estimating the environmental steering flow has been to average the wind components in radial bands centered on the storm position. It is shown that the portion of the asymmetric circulation that is included within these radial band averages results in a systematic rotation of the propagation vectors defined relative to the steering flow, which is quite similar to that calculated by compositing observations. Thus, some method other than the radial band-average estimate of the steering will be required in the wind-field decomposition to separate the large-scale environment from the influence of the tropical cyclone. It is demonstrated that inclusion of the analytically generated asymmetric circulation in the initial conditions of a barotropic track-prediction model leads to a propagation vector with the correct speed and direction. It is proposed that inclusion of the asymmetric circulation would eliminate much of the initial slow bias in dynamical track-prediction models that include only the symmetric vortex circulation and thus improve the tropical cyclone track forecasts.