An Initialization Scheme of Hurricane Models by Vortex Specification

Abstract

A scheme is presented to improve the representation of a tropical cyclone in the initial condition of a high-resolution hurricane model. In the proposed method, a crudely resolved tropical cyclone in the large-scale analysis is replaced by a vortex that is properly specified for use in the prediction model. Appropriate filters are used to remove the vortex from the large-scale analysis so that a smooth environmental field remains. The new specified bogus vortex takes the form of a deviation from this environmental held so that it can be easily merged with the latter field at the correct position. The specified vortex consists of both axisymmetric and asymmetric components. The symmetric component is generated by the time integration of an axisymmetric version of the hurricane prediction model. This ensures dynamical and thermodynamical consistency in the vortex structure, including the moisture field, and also compatibility of the vortex with the resolution and physics of the hurricane model. In the course of the integration of the axisymmetric model, the tangential wind component is gradually forced to a target wind profile determined from observational information and empirical knowledge. This makes the symmetric vortex a good approximation to the corresponding real tropical cyclone. The symmetric flow thus produced is used to generate an asymmetric wind field by the time integration of a simplified barotropic vorticity equation, including the beta effect. The asymmetric wind field, which can make a significant contribution to the vortex motion, is then added to the symmetric flow. After merging the specified vortex with the environmental flow, the mass field is diagnosed from the divergence equation with an appropriately controlled time tendency. The wind field remains unchanged at this step of initialization. Since the vortex specified by the proposed method is well adapted to the hurricane prediction model, problems of initial adjustment and false spinup of the model vortex, a long-standing difficulty in the dynamical prediction of tropical cyclones, are alleviated. It is anticipated that the improvement of the initial conditions can reduce the error in hurricane track forecasting and extend the feasibility of tropical cyclone forecasting to intensity change.