A Quasi-Equilibrium Theory for Tropical Cyclone Potential Intensity

Empirical and theoretical methods have been used to estimate the upper bound on tropical cyclone intensity as a function of their thermodynamic environments. These upper bound estimates have sometimes been referred to as potential intensity (PI). Perhaps the most well-known of these PI theories was developed by K. Emanuel in the 1980s, which has been modified since that time to include additional factors such as dissipative heating. Comparison of Emanuel's PI (EPI) theory to observations shows that it provides a reasonable estimate of the upper bound on tropical cyclone intensity, but tropical cyclones in nature rarely reach this upper bound. It is hypothesized that once a tropical disturbance becomes sufficiently organized it will approach a quasi-equilibrium intensity that is a fraction of the EPI value. The fractional reduction of the EPI can be estimated using the same basic mathematical formulation, but with modifications due to convective entrainment and ocean cooling. The magnitude of the entrainment rate is parameterized in terms of the environmental vertical wind shear, and the ocean cooling is estimated using a simple vertical mixing formulation. The modified EPI values will be compared with observations of tropical cyclone maximum winds and the impact on statistical intensity forecast models will be evaluated.

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