Channel Selection of Millimeter/Submillimeter Wave for Temperature and Humidity Sounding

The weather systems generated over tropical oceans are difficult to be detected by conventional observation, so the satellite remote sensing becomes the most effective way to monitor them. Geostationary Meteorological Satellites can observe the earth with high temporal resolution and is desirable for severe weather monitoring. But the spatial resolution for microwave instrument on board GEO could be poor because the satellites are in their high altitude orbits. To improve the spatial resolution, millimeter/submillimeter wave sounders are considered. Passive microwave sounding from geosynchronous orbit is first studied in the mid 1970s. During the early 1990s, it's suggested that the use of submillimeter wavelength water vapor and oxygen bands can significantly reduce the antenna size and costs, while retaining good spatial resolution. Based on this notion, a practical submillimeter wave geosynchronous microwave (GEM) sounder and imager is developed by the Geosynchronous Microwave Sounder Working Group (GMSWG). During the mid 1990s aperture synthesis is proposed to geostationary microwave imaging and sounding. The Geo STAR (Geostationary Synthetic Thinned Aperture Radiometer) and GAS (Geostationary Atmospheric Sounder) concept are put forward by America and Europe respectively based on this technique. In order to explore the potential applicability of millimeter/submillimeter sounders in tropical observation, weight functions for O₂ and H₂O absorption bands are calculated based on the atmosphere profiles of tropical region. The peak distribution of weight functions show that the far wings of 118 GHz can be chosen to measure the atmospheric temperature in lower troposphere, and for the temperature of the upper level, the far wings of 425 GHz are more suitable. In terms of humidity observation, the 183 GHz and 380 GHz channel combinations can be used for the middle and upper levels. Based on the analyses above, considering industry capability as well, eleven temperature channel combination including three channels around 118 GHz and eight channels around 425 GHz, eight humidity channel combination including three channels around 183 GHz and five channels around 380 GHz are recommended as the microwave observation channels for the geostationary orbit in the future. Furthermore, the differences of brightness temperature in different bandwidth range of O₂ and H₂O absorption bands are calculated using the temperature and water vapor profiles. Through this bandwidth selection test, retrieval results of temperature and humidity are statistically analyzed.It shows that in the temperature detection channel, 5.3 GHz or less and 4 GHz or less can be set as the bandwidth range for 118 GHz and 425 GHz respectively. As for the humidity detection channel, 183 GHz with a range of bandwidth 8.5 GHz or less and 380 GHz with a range of bandwidth 15 GHz avoiding 360.498 GHz is recommended, which is the oxygen absorption channel.