Microphysics-Based Bulk Parameterizations of Enthalpy and Momentum Fluxes for Tropical Cyclones

Many coastal communities stand to benefit from improved tropical cyclone intensity forecasts, and an important step towards increasing the accuracy and decreasing the uncertainty of these forecasts is refining the parameterizations of the sub-grid scale processes that control intensification. While intensity forecasts have significantly improved since their inception, state-of-the-art models still rely on constant-valued enthalpy and momentum exchange coefficients to parameterize boundary layer fluxes, and these coefficients significantly impact the predicted intensification rate. This work presents new microphysics-based, bulk parameterizations for enthalpy and momentum exchange through the turbulent tropical cyclone boundary layer. These parameterizations are microphysics-based in order to more accurately estimate the contributions from sea spray, since sea spray is critically important for the exchange processes that govern intensification. Specifically, the spray-mediated flux parameterizations are developed by using the prognostic evaporation equations for saline drops together with direct numerical simulations of evaporating sea spray subject to extreme wind speeds. The direct numerical simulations consider a domain that is equally partitioned by volume between sea water and air and subject to a pressure gradient forcing. A volume of fluid interface tracking scheme is used in conjunction with an idealized phase-change module to capture the evolution of the surface and the spray. These parameterizations are computationally inexpensive, are functions of local meteorological conditions, and are constructed to be readily incorporated into intensity prediction systems. Improving enthalpy and momentum flux parameterizations by leveraging knowledge of the local conditions and the microphysics of sea spray evaporation could improve the forecasted intensification rates of tropical cyclones and provide local authorities with better information.