Wave and Wind Direction Effects on Ocean Surface Emissivity in High Wind Conditions

Understanding how and why wave and wind direction affects the ocean surface emissivity is key to being able to accurately retrieve wind speed measurements from microwave radiometers. Wave and wind direction can influence the development of the sea state (amount of white-capping, foam, and sea spray) that microwave radiometers depend on for relating brightness temperature to wind speed. This study aims to identify how wave and wind direction can change the ocean surface emissivity measured under similar wind speeds. Our focus will be on high wind conditions, particularly in tropical cyclones. To investigate these effects, measurements from the Stepped-Frequency Microwave Radiometer (SFMR) are used. The SFMR is an airborne C-band instrument flown on the National Oceanic and Atmospheric Administration (NOAA) WP-3D and U.S. Air Force C-130J hurricane hunter aircraft. A forward radiative transfer model and an inversion algorithm use the SFMR brightness temperature measurements to obtain a surface wind speed estimate. 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 and wind direction on off-nadir (high incidence angle) measurements can be investigated using data 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 incidence 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 off-nadir SFMR brightness temperature measurements and are attributed to the wind direction. 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. Further analysis is also performed to understand the effects that wind fetch may have on the sea state and thus the brightness temperature measurements. This research improves our understanding of the relationship between surface wind speed, ocean surface emissivity (brightness temperature), wind direction, wave direction, and incidence angle.