Analyses on the altitude variation of radiation contributions for submillimeter channel sets

This study investigated variation in the upwelling radiation contribution to typical remote sensing channels of temperature (50-60 GHz, 118 GHz, and 424 GHz) and water vapor (22 GHz, 183 GHz, and 380 GHz) with altitude to evaluate their sounding abilities and obtain channel sets for sounding of lower atmosphere with high resolution. The gaseous attenuation described in Recommendation ITU-R P.676-11 and atmosphere profiles illustrated in Recommendation ITU-R P.835-6 were applied to calculate the upwelling radiation based on radiative transfer theory. The variation of brightness temperature contribution (Tb contribution vs. altitude) with both channels and profiles was analyzed in detail. The Tb contribution altitude at microwave channels was lower than that at submillimeter channels, making higher frequency channels suitable for remote sensing of the middle and upper atmosphere. Offset frequencies were selected to extend the sounding range: offsets of up to 1 GHz for the 118 GHz channel and up to 10 GHz for the 183 GHz channel enable sounding of the Earth's atmosphere. Differences in the Tb contribution altitudes to different profiles were more significant for water vapor than temperature channels because of the variable distribution of water vapor in the atmosphere. Therefore, the sounding ability of one channel varies for different profiles. An offset frequency of 10 GHz for the 380 GHz channel is sufficient for sounding the Earth's atmosphere in high latitude winter. Sea surface radiation calculations indicated that it can be neglected for the 380 GHz and 424 GHz channels even for the high latitude winter.