Empirical Model for Sea Spray Production

Parameterizations of sea spray aerosol (SSA) production still show one to two orders of magnitude uncertainty. It has been found that parameterizations in terms of variables, like the Reynolds number, which represent a joint measure of wind and wave state forcing as well as influence of the seawater properties, such as viscosity and surface tension governed by sea surface temperature (SST), salinity, and surfactants, better account for the variance of SSA observations than parameterizations based on wind speed, U₁₀, alone. Results from a recent experiment on-board FLoating Instrument Platform (FLIP) have shown that the microwave ocean surface emissivity e (or brightness temperature T_B) from radiometric measurements is another variable that reliably represents the composite sea state of the ocean interface and the associated SSA production F. FLIP observations afforded the derivation of a direct empirical relationship between F of super-micron SSA and the degree of polarization of ocean surface microwave emissivity e. A direct empirical expression F(δe) that uses satellite-based radiometric observations can provide alternative estimates of F at any region on the globe in any season. The availability of such data can help constrain physically-based parameterizations of SSA flux. This study aims to verify the performance and extend the range of applicability of the empirical expression F(de) developed from the FLIP data. This is done by combining de at the ocean surface from WindSat sensor and measurements of F obtained with direct eddy covariance method. A radiative transfer model (RTM) was developed to estimate e at the ocean surface from WindSat T_B measured at the top of the atmosphere (TOA). Measurements of SSA flux were made with a collocated sonic anemometer and Compact Lightweight Aerosol Spectrometer Probe (CLASP) during the Waves Aerosol and Gas Exchange Study (WAGES) — a 3-year programme of continuous air-sea flux measurements on the research ship James Clark Ross operated by the British Antarctic Survey. WAGES data (around 4500 data points) were matched in time and space with high resolution WindSat T_Bs. Matching criteria of 30 and 60 minutes and collocation distances from 5 to 25 km produce from 200 to 600 satellite-in situ data pairs. This combined data set spans from the North Atlantic (~ 80^o N) to the Southern Ocean (~80^o S) and covers wide range of oceanic conditions. This data set is used to develop a new empirical expression F(δe) over expanded parameter space (e.g., U₁₀, SST, etc.). We present the RTM model developed at NRL to correct WindSat TOA TB data and obtain e at the ocean surface at horizontal (h) and vertical (v) polarizations. These e_h and e_v values are used to construct variable δe. Size segregated direct eddy covariance estimates of SSA flux of particles with radii between 0.2 and 7 μm at 80% relative humidity are used to construct total (size-integrated) flux F. Preliminary results on SSA flux from the new empirical expression F(δe) are reported and future developments of the project are outlined. ed.