Investigating the Effects of Sea Spray on Surface Wind Stress and Waves under Hurricane Conditions

Tropical cyclone (TC) intensification is dependent upon the imbalance between the moist energy input and wind energy dissipation at the air-sea boundary layer. These energy and momentum exchanges are extremely challenging to measure under high wind conditions; consequently, accurate estimation of the transfers for forecast models are near impossible to obtain. Rigorous work has been performed in recent decades to understand the surface energy and momentum fluxes under such conditions, with some theoretical, but unverified, success. One collective conclusion is the presence of sea spray just above the sea surface plays a crucial role in surface fluxes under high winds. Here, we aim to make improvements to the accuracy of the surface momentum flux (stress) dependency on high wind speeds with focus on sea spray contributions.

We use a wind dependent flux model (Bourassa et al. 2006, AOI) altered to include sea spray effects based upon a new surface drag model derived from a recent wave tank study (Troitskaya et al. 2017, SR). This drag model incorporates newly quantified bag-breakup mechanism, which was determined the dominant spray generation mechanism at wind speeds > 20ms^-1. We perform regression analysis to obtain a best curve fit between the drag coefficient from the flux model and 10m wind speeds. We use the Simulating WAves Nearshore (SWAN) model, forced with H*Wind product blended with NARR data, to test the behavior of the new surface drag parameterization for two Gulf of Mexico storms: Ivan (2004) and Katrina (2005). The generated wave fields are validated with NDBC moored bouy data. After tuning for open ocean conditions, the anticipated surface stress improvements developed from these wave field hindcasts give insight on the sensitivity of surface stress and waves to the presence of sea spray under hurricane force winds. Our results will also notably assist the TC community in improving our understanding of processes impacting TC intensity.