A Minimal Three-Dimensional Tropical Cyclone Model

We will describe a new three-dimensional numerical model designed for basic studies of tropical cyclone behaviour, including motion and intensity change as well as the interaction between a tropical cyclone and other systems such as upper-level troughs. The model, which is formulated in sigma-coordinates, has three vertical levels, one characterizing a shallow boundary layer and the other two representing the upper and lower troposphere, respectively. The model has three options for treating cumulus convection on the sub-grid scale and a simple scheme for the explicit release of latent heat on the grid scale. The cumulus parameterization schemes are described in a paper by Roger Smith at this conference. It is shown that a realistic hurricane-like vortex develops from a weak initial vortex using all three cumulus schemes and even in the case where only the explicit release of latent heat is included. In each case there is a period of gestation during which the moisture in the inner core region increases on account of surface moisture fluxes, followed by a period of rapid deepening. Thereafter, the model storm slowly fills. The inclusion of parameterized convection reduces the gestation period. Precipitation from the convection scheme dominates that due to the explicit release of latent heat in the early stages of development, but this situation is reversed as the vortex matures. These findings are similar to those of Baik et al. In a paper Monthly Weather Review in 1990, who used the Betts-Miller parameterization scheme in an axisymmetric model with eleven levels in the vertical. One difference between the model results using different convection schemes is the length of the gestation period, another difference is the maximum intensity attained.