Large‐Eddy Simulation of Sea Spray Impacts on Fluxes in the High‐Wind Boundary Layer

Abstract

Although there have been considerable improvements in forecasting the track of tropical cyclones (TCs) over the past several decades, intensity forecasts remain challenging and improvements have lagged behind (Rogers et al., 2006). One challenge is that the understanding of air-sea exchange in high-wind regimes remains limited due to the logistical difficulty of taking measurements in these conditions. Further complications arise from a lack of knowledge on how the presence of sea spray affects the air-sea fluxes, and how this varies with wind speed (Veron, 2015). Improving parameterizations of air-sea fluxes in numerical models is of particular importance for reducing uncertainty in intensity forecasts (Sroka & Emanuel, 2021).Key parameters in air-sea exchange are the surface drag coefficient (C D ) and surface enthalpy flux coefficient (C K ), which are known to factor into the maximum storm potential energy and the maximum tangential wind speed (Emanuel, 1995). In numerical weather prediction (NWP) models C D and C K (or the individual heat and moisture coefficients, C H and C E , often assumed to be equal to C K ) are typically parameterized as functions of the 10-m wind speed (U 10 ). At high winds beyond roughly 30 ms −1 , however, the behavior of these flux coefficients are highly uncertain (Richter et al., 2016), and almost certainly not solely a function of wind speed. One factor in particular that is central to our understanding of the transfer of heat and momentum at high wind speeds is the presence of spray droplets. Spray mediation of air-sea fluxes has been the subject of many previous investigations, including theoretical, observational, and numerical modeling approaches, and a comprehensive review on the subject can be found in Veron (2015) or Sroka and Emanuel (2021). Although many numerical modeling studies have investigated the impact of spray on air-sea fluxes, most of these studies have relied on bulk estimates in which the net effect of spray is parameterized rather than handled explicitly.Here we investigate the impact of sea spray on the transfer coefficients for momentum, heat, and moisture, by performing simulations using a large-eddy simulation (LES) code coupled with a Lagrangian microphysical model. Paired simulations are performed with and without the presence of spray, across a range of wind speeds up