Comparison between cross-track and conical scanning microwave window channels near 90 GHz

The principal objective of this study was to determine the angular characteristics of the cross-track 92-GHz window channel of the SSM/T-2 microwave water vapor radiometer (T-2) over the ocean surface and to relate measurements from this instrument to corresponding 85-GHz window channel measurements from the conical scanning SSM/I imager. The conical scanner views at constant incidence angle and fixed polarizations, whereas the cross-track instrument scans across incidence angles with changing polarization. A model, based on radiative transfer calculations and Fresnel surface parameterization, successfully interrelated signals from the two radiometers as a function of T-2 scan angle for a significant fraction of the oceanic measurements. This confirmed the angular dependence model and provided a general relationship between 92-GHz SSM/T-2 and 85-GHz SSM/I signals, which is applicable in the absence of depolarization by rain, clouds, or severe sea surface roughness. Intercomparison between instruments, based on surface modeling, may be useful for instrumental calibration, it may assist in evaluation of microwave transmission models, and it does provide a validity test for ocean surface emissivity parameterization and cloud clearing procedures.