Revisiting the WISHE mechanism of hurricane intensification

As defined in the online AMS Glossary of Meteorology, the wind-induced surface heat exchange (WISHE) is a hypothesis for the amplification of certain atmospheric circulations, including tropical cyclones, polar lows, and the Madden–Julian oscillation. The mechanism involves a positive feedback between the circulation and heat fluxes from the sea surface, with stronger circulation giving rise to larger surface fluxes of heat, which are then quickly redistributed aloft by convection, in turn strengthening the circulation. In this theory, emphasis is placed on the surface fluxes as the principal rate-limiting process; convection serves only to redistribute heat. This hypothesis was first proposed in Rotunno and Emanuel (1987) and has continuously been refined (e.g. Emanuel1989, 1997, 2003).

Through a series of convection-permitting high-resolution ensemble simulations with the Weather Research and Forecast (WRF) model for both idealized and real-world tropical cyclones, this study re-examines the importance of the WISHE mechanism of hurricane intensification. We show that the feedback between surface wind and surface enthalpy flux has an important influence on tropical cyclone evolution in terms of timing and rate of intensification as well as final intensity, even though, as with any classical instability mechanism, such a feedback is not strictly necessary.

When the wind speed is artificially capped in idealized numerical ensemble experiments, storm development is slowed and storms achieve a smaller final intensity, in some cases not developing at all. For example, in idealized simulations with the surface wind speed for calculating the surface enthalpy flux artificially limited to 10 m/s, the peak growth rate is less than half of the reference uncapped simulations while the final intensity in terms of maximum surface wind is reduced by at least 30%. Further capping the wind speed to 5 m/s results in hardly any development.

We further performed capped simulations for two real-world storms, and found that the quality of the simulations is strongly compromised without the WISHE feedback mechanism. For example, artificially capping the surface wind limited to 10 m/s in the simulation of Katrina (2005) just before it entered the Gulf of Mexico led to a delay of the storm's intensification by about 4 days and reduced the peak intensity to less than the strength of a category-1 hurricane instead of a strong category-5 storm occurred in nature.