Comparison of three parameterizations of subgrid-scale feedback physics for sea spray contributions to near-surface heat fluxes in hurricanes

Air-sea interaction is unquestionably a critical aspect of hurricane structure and evolution. Recent experiments with high resolution (km-scale) nested models have shown strong sensitivity to the representation of air sea fluxes - both the direct (interfacial) transfers and contributions of sea spray. With the advent of fully coupled (air-wave-ocean) models, it becomes possible to represent the surface fluxes in more physical detail (as opposed to simple wind-speed driven bulk flux algorithms). Recent theoretical work has led to a new representation of sea spray droplet flux that is driven by the fundamental processes associated with blowing large droplets off the tops of breaking waves. In today's high-resolution numerical models sea spray effects are not represented explicitly but are input at modifications to bulk flux relationships applied to the lowest atmospheric grid level. These bulk flux relationships assume a subgrid-scale structure for the scalar profiles (essentially Monin-Obukhov similarity). The actual contribution of the droplet spectrum to the sensible and latent heat fluxes is computed assuming the droplets fall through this specified temperature/humidity environment. However, this is complicated by coupling of the droplet heat fluxes to the mean temperature and humidity profiles near the surface. Thus, the heat and moisture transferred by the droplets modifies the local environment that drives these transfers - a process referred to a feedback. Feedback has little or no effect on the total enthalpy flux carried by the sea spray but it affects the partition of sensible versus latent heat (Bowen ratio). In this paper we will describe and compare three representations (one old and two new) of this subgrid-scale feedback process.