Passive microwave imagine of O2emissions using the 118.75-GHz(1−) resonance has been investigated for tropospheric and stratosphere remote sensing of atmospheric temperature and precipitation. An imaging millimeter-wave spectrometer (MTS) using eight double-sideband channels centered around the 118.75-GHz O2 resonance, and including a fixed-beam 53.65-GHz radiometer and video camera was constructed. The MTS collected data during 33 flights of the NASA ER-2 high-altitude aircraft in 1986 during the Genesis of Atlantic Lows Experiment (GALE) and the Cooperative Huntsvilie Meteorological Experiment (COHMEX), yielding the first high spatial resolution microwave images of atmospheric O2 brightness.
The isolated 118-GHz fine offers higher spatial resolution and precipitation sensitivity than O2 lines in the 5-mm band near 60 GHz. The brightness temperature perturbations of clouds in nonprecipitating regions are typically twice as large in the 118-GHz channels relative to comparable 60-GHz channels. However, observations over cirrus anvils show that the 118-GHz brightnesses are not adversely sensitive to some optically opaque cloud cover. Thus, these channels are expected to be useful for temperature sounding in the presence of clouds, although retrieval ambiguities can result from variations in the water vapor profile and surface emissivity. The demonstration of 118-GHz temperature profile retrievals is left for a subsequent paper.
Over deep convective precipitation, 118-GHz brightness temperature images are characterized by decreases of up to 200 K due to strong scattering in the storm core. The amplitudes of the 118-GHz brightness perturbations contain information on the altitude of the cell top. The shape of the 118-GHz spectrum is also suggested to contain altitude information by virtue of the various peaking altitudes of the 118-GHz weighting functions. Precipitation cells observed by the MTS sometimes appear in bands or rows, and have been accompanied by periodic radiance structures.