ON THE BOGUSSING OF TROPICAL CYCLONES IN NUMERICAL-MODELS - A COMPARISON OF VORTEX PROFILES

At the resolutions currently in use, and with the sparse oceanic data coverage, numerical analyses cannot adequately represent tropical cycl one circulations for use in numerical weather prediction models. In ma ny cases there is no circulation present at all. Most numerical weathe r prediction centers therefore employ a ''bogussing'' scheme to force a tropical cyclone vortex into the numerical analysis. The standard pr ocedure is to define a synthetic data distribution based on an analyti cally prescribed vortex, which is passed to the analysis scheme as a s et of high quality observations. In this study, four commonly used bog us vortices are examined by comparing resultant forecast tracks in an environment at rest, and in a background now that simulates a typical monsoon trough-subtropical ridge structure. There are three main findi ngs, each of which has significance for operational tropical cyclone t rack prediction. First, great care is needed in the choice of the char acteristics of the bogus vortex, such as the radius and magnitude of t he maximum wind. Second, the tropical cyclone trajectories can be very sensitive to their initial position in the idealised environment. Thi rd, the bogus vortex can substantially influence the environment, espe cially over longer time periods and for vortices of larger size.