Exploring Swell Impact to Radiometer Derived Foam Fraction and Wind Speed

The potential impact of swell direction to WindSAT foam fraction and to WindSAT retrieved winds is explored. It has been found that foam fraction increases as the swell direction becomes opposite to the wind direction. This increase becomes more prominent as the WindSAT frequency channel increases given the same polarization (e.g. up to +.9% for the 37 GHz H-pol). WindSAT foam fraction derived from the V-polarization shows the least increase compared to H-polarization (+~.2%). The wind speed bias between WindSAT and model wind increases slightly (up to 0.5 m/s) as the swell direction becomes opposite to the wind direction. These results indicate that a swell traveling in the opposite direction of wind driven sea foam can disturb the surface roughness, enough to noticeably increase foam coverage. Additionally, the increase of WindSAT winds in such conditions indicates a residual dependence to swell, which should be appropriately addressed in the retrieval process.