Estimation of sea surface foam coverage and effective foam layer thickness from satellite microwave measurements

Abstract

Sea surface foam commonly represents the accumulation of bubbles on the sea surface caused by breaking waves and plays a critical role in the air-sea interaction process and climate research. Both foam coverage and foam layer thickness have a profound effect on sea surface emissivity for moderate to high wind speeds due to the high emissivity of foam at microwave bands. However, there is a lack of models to estimate sea foam layer thickness due to the limited experimental data and constrained empirical formulations. To address this limitation, we propose a dual-channel (L- and Ka-bands) method that utilizes passive microwave satellite measurements to estimate sea surface foam coverage and effective foam layer thickness over the global ocean. This method uses microwave measurements from the Aquarius/SAC-D and Soil Moisture Active Passive (SMAP) missions, together with observations from the WindSat instrument, to convert the top-of-atmosphere (TOA) brightness temperature (TB) to sea surface TBs and then isolate the foam-induced emission component. Because sea surface emissivity is sensitive to foam distributions, a foam emissivity model was developed and implemented to derive the foam layer parameters (coverage and thickness) as functions of wind speed. The retrieved foam coverage shows strong agreement with independent observations, with root mean square differences (RMSDs) of 0.87 % against field measurements and 0.38 % against satellite data. In addition, the retrieved effective foam thickness also shows close consistency with hydrodynamic model expectations. The global distributions of foam coverage and effective layer thickness were further retrieved and analyzed in this study, which demonstrated high potential of the proposed models compared to the previous methods and suggested that the combined use of L- band and Ka-band measurements from a single platform could improve the accuracy of foam coverage and effective layer thickness over the global ocean.