Wave and Wind Direction Effects on Ocean Surface Emissivity Measurements

Measurements of surface winds by microwave instruments rely on an accurate retrieval of the ocean surface emissivity. However, it has not been known exactly how wave and wind direction effect the emissivity retrievals. To investigate these effects, measurements from the Stepped-Frequency Microwave Radiometer (SFMR) are used. The platforms for the SFMR are the National Oceanographic and Atmospheric Administration (NOAA) P-3 and Air Force C-130J hurricane hunter aircraft. The SFMR measures sea surface microwave brightness temperatures at six frequencies ranging from 4.7 to 7.2 GHz. Surface wind speed estimates are obtained from these brightness temperatures by using a retrieval algorithm that employs a geophysical model function relating surface emissivity and wind speed. The SFMR is designed to obtain a single nadir track of surface wind speeds directly beneath the aircraft during level flight and not when turning because of the complexity of the wave field and foam distribution when the SFMR views the surface off-nadir or during aircraft rolls. However, the effects of the wave field on the measurements can be investigated using measurements obtained during the 2008, 2014, and 2015 Atlantic hurricane seasons and the 2015 Winter Ocean Winds project. These campaigns have the advantage of collecting SFMR data in precipitation-free regions at various roll angles ranging from 10° to 60° from level flight. These data allow for an examination of conditions more typical of satellite observations, and also allow for calibration of the SFMR for off-nadir observations. Asymmetries are found in the wind-induced component of the brightness temperature measurements. It is shown that the asymmetries are a result of the effects on the whitecap and foam distribution by both wave and wind directions and their modifications to the surface wind stress. An analysis of the magnitude of the asymmetries is also performed to show the sensitivity of the changes to the brightness temperature measurements on the wind speed retrieval. This research will allow for improvements to the SFMR algorithm to account for the relationship between surface wind speed, brightness temperature, wind direction, wave direction, and incidence angle, and improve our understanding of ocean surface emissivity retrievals.