Dynamical constraints on the intensity and size of tropical cyclones

This study investigates the rotational constraint on the intensity and size of tropical cyclones using a minimal, three-layer, axisymmetric tropical-cyclone model. Three sets of experiments will be described. In the first set of calculations, the same initial baroclinic vortex is spun up in a quiescent environment with different levels of background rotation. The rotation is characterized by the Coriolis parameter, f, which corresponds to latitudes ranging between 5°N and 37°N. It is found that the strongest vortices, as characterized by their final intensity, develop in environments with intermediate background rotation, a result that is in line with those of classical laboratory experiments by Turner and Lilly. Our results suggest also the existence of an optimum background rotation strength to obtain the largest storm as measured by the radius of gale-force winds.

Unlike the laboratory experiments, the rotation strength and the forcing strength are not independent in the tropical-cyclone model, because the air-sea moisture fluxes, which provide the main energy supply for a tropical cyclone, depend on the intensity. To pursue the analogy with the laboratory experiments further, a similar set of calculations is carried out with all moist processes excluded. In these calculations, the vortex is forced with a prescribed diabatic heating rate that is deduced from the first set of calculations. For this heating rate, there is no optimum background rotation rate for size or intensity within the range of realistic values of f, implying that the relationship between the forcing strength and rotation strength is an important additional constraint in tropical cyclones.

A third set of calculations is carried out in which moisture is included, but the breadth of the initial vortex is varied, keeping the initial intensity and the latitude the same. As the initial vortex is broadened at a fixed rotation rate, the initial intensification rate is decreased and the time of onset of rapid intensification is progressively increased, but the final vortex intensity is similar in all cases. This set of experiments demonstrates that the effects of the initial vortex are progressively forgotten. At any given time, the smaller the breadth of the initial profile, the smaller is the radius of gales, but the differences in the sizes of the different vortices decrease as time proceeds.

The relevance of these results to understanding tropical-cyclone dynamics will be discussed.