Speaker

Benjamin Barr, Postdoctoral Fellow, Woods Hole Oceanographic Institution

Seastate-Dependent Sea Spray Heat Fluxes and Impacts on Tropical Cyclone Structure and Intensity Using Fully Coupled Atmosphere-Wave-Ocean Model Simulations

Abstract

Air-sea fluxes of sensible and latent heat are fundamental to the energetics of tropical cyclones (TCs) and their intensity. The contributions of sea spray to air-sea heat fluxes and the impacts of spray heat fluxes on TC structure and intensity are not well understood due to the extreme difficulty of taking measurements of spray in high winds, to the complexity of spray generation and heat transfer physics, and to the difficulty of representing seastate-dependent spray processes in numerical TC forecast models. The role of ocean surface waves in spray generation is largely parameterized in models using surface winds, which means that the dynamics of the complex, non-equilibrium seastates found in TCs are not accounted for. In this work, we aim to improve both understanding and modeling of seastate-dependent sea spray heat fluxes and their interactions with TCs by 1) developing an improved parameterization for seastate-dependent air-sea heat fluxes with spray for use in fully coupled atmosphere-wave-ocean (AWO) models, 2) implementing it in the Unified Wave Interface-Coupled Model (UWIN-CM), a fully coupled AWO regional model, and 3) performing UWIN-CM model experiments with spray for a diverse set of TCs. We find that seastate-dependent spray generation differs strongly from the traditional wind-based approach, especially during TC landfall where coastal wave shoaling amplifies wave breaking and dissipation. In UWIN-CM TC simulations, the new seastate-dependent spray physics modifies both surface heat fluxes and storm structure, affecting TC intensity in both positive and negative ways. In our simulations, spray evaporative cooling dominates in boundary layers (BLs) of tropical storms and weak hurricanes (i.e., when peak azimuthal-mean 10-m windspeed is below 30-40 m/s), which hinders intensification. If TCs intensify beyond this early stage, increased wave breaking and spray generation produce positive sensible heat fluxes under the eyewall as well as increasing latent heat fluxes, which should promote eyewall deep convection and intensification. However, this strengthening effect is offset by structural inefficiency due to the evaporatively-cooled BL inflow, and spray continues to impede intensification overall. Finally, if TC intensification continues, spray heating of BL air under the eyewall can become strong enough to overwhelm the influence of structural inefficiency, and spray invigorates eyewall deep convection and promotes intensification.