Parameterization of oceanic whitecap fraction based on satellite observations

Abstract

In this study, the utility of satellite-based whitecap fraction ( W) data for the prediction of sea spray aerosol (SSA) emission rates is explored. More specifically, the study aims at evaluating how an account for natural variability of whitecaps in the W parameterization would affect SSA mass flux predictions when using a sea spray source function (SSSF) based on the discrete whitecap method. The starting point is a data set containing W data for 2006 together with matching wind speed U 10 and sea surface temperature (SST) T. Whitecap fraction W was estimated from observations of the ocean surface brightness temperature T B by satellite-borne radiometers at two frequencies (10 and 37ĝ€GHz). A global-scale assessment of the data set yielded approximately quadratic correlation between W and U 10. A new global W ( U 10) parameterization was developed and used to evaluate an intrinsic correlation between W and U 10 that could have been introduced while estimating W from T B. A regional-scale analysis over different seasons indicated significant differences of the coefficients of regional W ( U 10) relationships. The effect of SST on W is explicitly accounted for in a new W ( U 10, T) parameterization. The analysis of W values obtained with the new W ( U 10) and W ( U 10, T) parameterizations indicates that the influence of secondary factors on W is for the largest part embedded in the exponent of the wind speed dependence. In addition, the W ( U 10, T) parameterization is able to partially model the spread (or variability) of the satellite-based W data. The satellite-based parameterization W ( U 10, T) was applied in an SSSF to estimate the global SSA emission rate. The thus obtained SSA production rate for 2006 of 4.4ĝ€ × ĝ€1012ĝ€kgĝ€yearĝ'1 is within previously reported estimates, however with distinctly different spatial distribution.