What controls the size of a tropical cyclone?

It is still an open question, which processes control the radial extent of a tropical cyclone. It may possibly be a unique function of the radius of the initial perturbation. In this case, the tropical cyclone could not be interpreted as an attractor of the atmospheric dynamical system. Instead, the tropical cyclone could only be interpreted as a transient cyclone that either decays after some time, or changes its form.

This question is investigated within the axisymmetric cloud resolving model HURMOD. The simulations show that a steady-state tropical cyclone develops after a time period of about 60 days. The numerical solution exhibits the dynamical character of a fixed point attractor. As the model solutions are insensitive to the hydrostatic approximation, it is possible to derive a similarity relation, which establishes a link between the size of the tropical cyclone and various model parameters. According to this similarity relation, a change in size is related to certain parameter modifications. The relation delivers the individual factors, by which the parameters must be multiplied to reach a certain size change in terms of a self-similar solution. However, the relation does not reveal, which of the parameters control the tropical cyclone size. To gather information about this, we vary each of these parameters individually in HURMOD. For the control over the radius of maximum wind (RMW), the vertical and horizontal mixing lengths, the surface transfer coefficients, the Coriolis parameter, and the timescale for Newtonian cooling turn out to be important. In contrast, the radius of minimum tangential wind (as a measure of the size of the outflow anticyclone) does not depend sensitively on the mixing lengths. The considered radii do not always change in the way as predicted by the similarity law, when only one parameter is varied. Moreover, we find that the size of the steady-state tropical cyclone is also sensitive to other parameters, which do not enter the similarity relation. These are the sea surface temperature, the temperature stratification, and the tropopause height.