What controls the size of a tropical cyclone? Investigations with an axisymmetric model

Abstract

Control mechanisms of tropical cyclone size are investigated with the axisymmetric cloud model HURMOD. In agreement with preceding HURMOD studies, the model results exhibit the existence of a fixed‐point attractor associated with a tropical cyclone. From the non‐dimensionalized model equations, a similarity law is derived, which relates six model parameters to the horizontal extent of the tropical cyclone in a steady state. Each parameter is associated with one of the following processes: planetary rotation, condensation time‐scale, radiative relaxation time‐scale, horizontal diffusion, vertical diffusion, and surface transfer. Individual variation of the parameters reveals that the radius of maximum horizontal wind speed is very sensitive to the Coriolis parameter, the mixing lengths and the radiative relaxation time‐scale, whereas the radius of minimum tangential wind merely depends on the Coriolis parameter, the surface transfer coefficients, and the radiative relaxation time‐scale. The increase of the radius of maximum horizontal wind speed with vertical eddy diffusivity goes along with an enhancement of the overturning mass flux within the secondary circulation. This agrees with the Hadley cell theory by Held and Hou who also emphasized the relevance of vertical diffusion. The strengthening of the overturning mass flux follows a power law similar to that found in an ocean modelling study. Moreover, TC size is found to be sensitive to environmental conditions comprising the prescribed sea‐surface temperature, tropopause height and static stability, while the sensitivity with respect to relative humidity is relatively small in the model. When the torque due to the upper sponge layer is switched off, the model exactly fulfills in the long run the steady state angular momentum budget equation.