## 2 Principle of Passive Microwave Remote Sensing from Space  
Passive microwave instruments onboard satellites may have up to two dozen bands (also called channels) for soundings and imaging [12]. The sounding channels are for vertical profiles of atmospheric temperatures and water vapor as well as clouds. The imaging channels are for surface parameters and integrated column values of atmospheric parameters. Passive microwave channels are selected based on the fundamental properties of the Earth and its atmosphere. In general, atmospheric opacity is low at low frequencies and increases at higher frequencies, due to water vapor absorption and also increased absorption and scattering by clouds and precipitation. There is a relatively weak water vapor absorption line at 22.235 GHz and a strong water vapor absorption line at 183.31 GHz. The strongest water vapor absorption is at 183.31 GHz and the absorption decreases away from the absorption line. Using multiple water vapor channels, the passive microwave sensor can acquire atmospheric water vapor at different heights. The weaker the absorption, the lower the atmospheric layers that can be observed. ATMS has five water vapor sounding channels at 183.31 \(\pm\) 7.0, 183.31 \(\pm\) 4.5, 183.31 \(\pm\) 3.0, 183.31 \(\pm\) 1.8, and 183.31 \(\pm\) 1.0 GHz that are sensitive to water vapor between 100 and 800 hPa in general. The channel at 183.31 \(\pm\) 7.0 GHz can be sensitive to water vapor in the surface boundary layer. Over high latitudes, where TPW is very low, this sensitivity can extend to surface parameters, such as skin temperature and surface emissivity. ATMS channel 1 at 23.8 GHz provides useful information about the total precipitable water vapor and water vapor in the surface boundary layer. Because of the higher scattering at ATMS channel 17 (165 GHz) than channel 16 (88 GHz), these channels can be used to characterize precipitating clouds, for example, rain vs. snow. Multiple oxygen absorption lines are located between 50 and 60 GHz (called the 60-GHz band hereafter) and close to 118 GHz. The 60-GHz band is less affected by clouds and wide enough to allow multiple channels to measure atmospheric temperature profiles. The 118-GHz oxygen absorption line cannot provide the same quality of information on temperatures as the oxygen absorption lines within 50-60 GHz, because the spectral absorption line is narrower and the effects of clouds are stronger than at the 60-GHz band. However, because of the shorter wavelength, smaller fields of view can be achieved at 118 GHz, so it is of interest for small-scale features having a large amplitude, where ATMS has 13 channels within 50-60 GHz for measuring the atmospheric temperature profile. Microwave observations have good impact on the quality of the numerical weather prediction analyses (initial states) and forecasts because they can mostly measure temperatures above and below clouds, whereas infrared sensors have very limited capability in penetrating clouds. It has been shown that high-impact weather systems are often under cloudy conditions. Therefore, techniques that can provide information inside and below clouds are most valuable for NWP to capture newly developing weather systems. In addition, the radiation source function within microwave frequencies is a quasi-linear function of the temperature, which directly relates to the atmospheric or surface temperature. Passive microwave measurement for a given channel onboard satellites depends on multiple atmospheric and surface parameters. For example, the brightness temperature of ATMS channel 1 at 23.8 GHz depends on the atmospheric profiles of temperature and water vapor as well as the surface temperature and emissivity. Under cloudy conditions, it depends on the cloud water content as well. Sea surface emissivity is a function of the sea surface wind and sea surface temperature. Snow surface emissivity depends on snow density, depth, and grain size. The snow surface roughness and ice layer complicate the emissivity modelling.