The dependence of nadir ocean surface emissivity on wind vector as measured with microwave radiometer

Abstract

Global brightness temperature observations of TOPEX/Poseidon microwave radiometer (TMR) at 18, 21, and 37 GHz have been collocated with near-simultaneous SeaWinds wind vector data as well as with monthly sea surface temperature and salinity products. The combined data allow us to study the dependence of zenith-directed ocean surface emissivity, at each frequency, upon both wind speed and direction. Results show a clear two-branch wind speed dependence; weak and linear below 7 m s with an increase in sensitivity above that point. The observed emissivity also depends on the angle between the wind direction and TMR's antenna polarization orientation, providing satellite confirmation of aircraft-derived results. There is little change in these wind vector dependencies with frequency.

Combined Wind Vector and Sea State Impact on Ocean Nadir-Viewing Ku- and C-Band Radar Cross-Sections

“…Combined with theoretical investigations [ 7 - 10 ], these results have suggested that nadir-looking radiometer directional sensitivity follows from the azimuthal anisotropy of the spatial curvature spectrum of short-gravity and capillary waves. Such expected signatures were further documented from TOPEX radiometer data [ 11 ]. Intuitively, from reciprocity between emissivity and reflectivity, such a directional sensitivity might also be expected on nadir radar cross section measurements.…”

Verification of Remotely Sensed Sea Surface Winds in Hurricanes

“…We again question what effect, if any, this variability might have on surface wind speed retrievals. Tran et al (2002) indicate that at 18 GHz and nadir incidence, the wind direction modulation of excess emissivity has a maximum magnitude of ϳ1.5 ϫ 10 Ϫ3 that occurs at around 11.5 m s Ϫ1 , with a leveling off or a slight decrease at higher wind speeds. Assuming a negligible frequency dependence, this corresponds to a ⌬T B of around 0.5 K, for which it should be noted is near the level of the SFMR's inherent instrument noise (see appendix B).…”

“…The profiles used to compute the transmission are labeled as follows: 1000-1004 mb and Ͻ995 mb storms are from data compiled by Sheets (1969); eyewall data from Frank (1977); eye data from Jordan (1957); Jordan mean tropical data from Jordan (1958) SFMR retrieval algorithm does not currently take this into account since the contribution of ocean surface roughness to the emissivity is assumed to be negligible at zero incidence. Previous studies (e.g., Tran et al 2002) have documented a modulation of the wind-induced excess emissivity by the orientation between satellite radiometers and the local surface wind vector (i.e., the angular difference between antenna polarization direction and sea surface roughness elements). At wind speeds above 10 m s Ϫ1 , as much as a 10% difference between cross-wind and up-/downwind induced excess emissivity has been observed.…”

Ocean water vapor and cloud liquid water trends from 1992 to 2005 TOPEX Microwave Radiometer data

“…The 18 and 37 GHz channels provided corrections for the effects of sea surface emissivity variations (due to surface winds) and cloud liquid, respectively. Because of the nadir viewing geometry, polarization effects were minimal, with a small effect due to wind direction dependency of the sea surface emissivity [ Tran et al , 2002]. All three TMR channels operated in a single linear polarization mode throughout the mission.…”