Passive Microwave Observations - An Introduction
The papers presented in this session mostly describe early attempts to correlate observed SEASAT-1 and Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) radiances or brightness temperatures with sea surface temperature (SST), near-surface winds, cloud droplets, atmospheric water vapor and rainfall rates. Although the correlations were clearly evidenced in all cases, these initial efforts have not yet defined the ultimate accuracies with which the aforementioned geophysical parameters may be inferred from SMMR brightness temperatures. Such definition was hampered by some peculiarities in the SMMR performance, unexpected on the basis of prelaunch calibration studies, including much smaller polarization differences over oceans (ca. 50K versus ca. 70K at most wavelengths) and asymmetric polarization mixing during the scan period. Since the geophysical parameter retrieval algorithms were designed, for the most part, to accept absolute radiances from the SMMR, it is not surprising to see that large offsets occur in the retrieved parameters when using input radiances which are apparently not properly calibrated, or that larger-than-expected deviations in the trends are observed after the offsets have been subtracted from the retrieved values when the polarization mixing has not been properly removed. A proper procedure for removing this mixing has been identified, but not yet applied to the data used in studying the trends in the retrievals. If the polarization difference discrepancy can be solved, offsets in the retrieved parameters may also be reduced. If not, it would appear that systematic offsets in the retrieved parameters can still be removed, but without the benefit of a satisfactory theoretical model to support the procedure.