Greg Hakim

Numerical experiments are performed to determine the mean state of an axisymmetric hurricane in statistical equilibrium. Most earlier studies used a damping scheme on the temperature field as a parameterization of radiative cooling, which we demonstrate yields storms that have little convection outside the eyewall and do not achieve statistical equilibrium. Here we explicitly simulate the effects of infrared radiation, which permits the storm to achieve radiative--convective equilibrium with its environment.

Beginning from a state of rest, a superintense storm develops after 10 days with a maximum surface wind speed in excess of 100 m/s. This transient storm weakens and is replaced by an equilibrium storm that lasts over 400 days with a time-mean maximum wind speed that compares closely with an estimate of the maximum potential intensity (MPI). The main assumptions of MPI theory are found to be consistent with the properties of the equilibrium storm, but the thermodynamic cycle does not resemble a Carnot cycle. Maximum radiative cooling is found in the midtropophere outside the storm, where convective clouds detrain into the dry layer of storm-outflow subsidence, producing a large vertical gradient in water vapor and cloud water.

Sensitivity experiments reveal that the results are robust to changes in the pre-storm thermodynamic sounding, ambient rotation, turbulent mixing, and details in the radiative heating field. We conclude that (1) the undisturbed tropical atmosphere is unstable to axisymmetric hurricanes, (2) MPI theory accurately bounds the time-mean storm intensity (but not transient fluctuations), and (3) the time-mean storm intensity is insensitive to turbulent mixing in the radial direction.