Tuesday, 24 January 2012: 2:30 PM
The Application of Satellite Sea-Surface Salinity Observations to Operational Passive Microwave Radiometry
La Nouvelle A (New Orleans Convention Center )
Global quasi-near-real-time sea-surface salinity (SSS) observations from satellites provide the opportunity to improve operational passive microwave retrievals by reducing bias and uncertainty resulting from the use of climatological salinity values in the calculation of ocean surface emissivity. Climatological SSS values, e.g, the National Oceanic and Atmospheric Administration (NOAA) National Oceanographic Data Center's (NODC) World Ocean Atlas 2009, are highly sparse in time and space, even with the inclusion of the many observations from Argo floats. Consequently, the representativeness of the climatological values potentially is subject to significant bias, as well as uncertainty. From a passive microwave radiometry perspective, this issue of representativeness is further complicated by where in the water column that salinity was measured. For example, the World Ocean Atlas 2009 bins together all observations in the top 10 m of the water column. The depth of surface in situ observations typically ranges from 1 m to 5 m, with most Argo floats recording surface salinity at a depth of about 5 m, resulting in near-surface salinity observations. However, for microwave surface emissivity calculations, the depth of interest is less than 1 cm, i.e., skin salinity. Near-real-time processes with the potential to alter the skin SSS with respect to the listed climatological value include: evaporation, precipitation, convective mixing, mechanical mixing and advection. Significant governing factors are the strength of the vertical density gradient and the mechanical mixing, as well as proximity to salinity fronts. Satellite SSS observations observe the salinity within the ocean surface skin, providing better representativeness than in situ observations from that perspective, as well as near-simultaneous global coverage each mission cycle. Initial work discussed the role of salinity in determining ocean surface emissivity, depicting the complicated intertwined relationships of SSS, sea-surface temperature (SST), frequency, and incidence angle. Additional factors of surface roughness and foam contribute to surface emissivity, but these factors are not a function of salinity. Subsequent work highlighted global differences between the European Space Agency's Soil Moisture – Ocean Salinity (SMOS) mission's initial Level-3 SSS data (SMOS Barcelona Expert Centre, 2008) and the World Ocean Atlas 2009, depicting both salinity and surface brightness temperature differences at 11 GHz, representative of an operational passive microwave frequency. For that study, surface brightness temperature was computed assuming nadir view, flat sea, and no foam, employing NOAA's operational Community Radiative Transfer Model (CRTM) for the computation of ocean surface emissivity. This study extends the previous work by exploring additional significant frequencies while considering observation incident angles and polarizations relevant to operational instruments, such as AMSR-E, WindSat, TMI, SSMIS, AMSUA, and ATMS.