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Abstract

Emanuel's theory of hurricane potential intensity (E-PI) makes use of the assumption that slantwise convective instability vanishes in a steady-state vortex of a tropical cyclone. In the framework of an extended mathematical potential intensity model it is shown that relaxing this assumption and including an eye results in a larger maximum wind speed compared to that of the predictions made by E-PI. Previous studies by Bryan and Rotunno demonstrate that the effect of unbalanced flow considerably contributes to maximum winds in excess of E-PI ("superintensity"). The authors argue that the proposed mechanism induced by convective instability provides another possible explanation for simulated and observed tropical cyclones exceeding E-PI in addition to flow imbalance. Further evidence for the relevance of conditional instability in mature tropical cyclones to superintensity is given by the fact that convective available potential energy arises in numerical simulations of tropical cyclones. This is demonstrated by means of an axisymmetric cloud model that is in qualitative agreement with the analytical model. These simulations reveal a dependence of superintensity on the amount of CAPE outside the eyewall and also reproduce the decrease in superintensity with increased horizontal diffusion as found in previous studies.