An Energetic Perspective of Hurricane Patricia (2015) as a Heat Engine through Isentropic Analysis

In this study, we set out to examine the energetics of Hurricane Patricia (2015) as a heat engine. Hurricane Patricia was poorly forecasted by all operational models and set numerous records including maximum SFMR estimated 10 m wind speeds and fastest observed intensification rate (Rogers et al. 2017). Using the Penn State WRF-EnKF convection-permitting regional hurricane analysis and forecast system assimilating all conventional observations and airborne Doppler radar observations, high resolution 1 km horizontal grid spacing, and a newly proposed surface drag parameterization of Chen and Yu (2016), in which C_d decreases for wind speeds greater than hurricane force, we are able to greatly improve the prediction of Hurricane Patricia. As a result of our success in capturing this historic intensification, we examine Patricia as a heat engine through isentropic analysis conducted on the deterministic forecast launched from the EnKF analysis mean, which will serve as our CNTL.

As a result of identified strong forecast sensitivity to the parameterization of the surface fluxes, which are believed to have profound impacts on the hurricane heat engine, we also conduct an ensemble forecast to investigate the impact of the uncertainty in the coefficients used to represent the exchange of enthalpy and momentum between the atmosphere and ocean at high wind speeds on forecasts of Hurricane Patricia. In addition to highlighting the influence of C_d/C_k uncertainty on tropical cyclone intensity, we examine how changes to C_d/C_k impact the storm structure and the tropical cyclone heat engine. To do so, we again employ the use of isentropic analysis and seek to highlight the impact of the exchange coefficients on the energetics of the tropical cyclone system that directly influence the intensification process. In this study, we argue that in addition to the importance of model horizontal resolution and accurate inner-core and environmental initial conditions, uncertainty in the surface drag and enthalpy coefficients can result in large forecast error and uncertainty by causing significant changes to the tropical cyclone heat engine, thus limiting the current practical predictability of tropical cyclones.