An improved bulk air-sea surface flux algorithm, including spray-mediated transfer

Abstract

A bulk air-sea flux algorithm couples the ocean and the lower atmosphere through flux boundary conditions and can be used in various analyses and in numerical models. The algorithm described here has two features not present in any other existing bulk flux algorithm. First, it has a new air-sea drag relation. Here, for wind speeds above about 9 m s-1, the friction velocity u*, which is related to the square root of the surface stress, is linearly related to UN10, the neutral-stability 10 m wind speed. When extrapolated to hurricane-strength winds, this drag relation has better properties than relations formulated in terms of a drag coefficient or a roughness length. The second unique feature of this flux algorithm is that it recognizes two routes by which heat and moisture cross the air-sea interface: one is the interfacial route, which is controlled by molecular processes right at the air-sea interface; the second is the spray-mediated route, which is governed by microphysical processes at the surface of sea spray droplets. Through microphysical theory and our analysis of 4000 sets of eddy-covariance measurements of the scalar fluxes, we separate the measured fluxes into the interfacial and spray contributions and thereby produce the only spray flux algorithm tested and validated against oceanic data. Because all components of our flux algorithm are physics-based and validated with data for winds up to 25 m s-1, one application is extrapolating this algorithm to hurricane-strength winds, where sea spray plays a dominant role in scalar transfer.