Performing such passive microwave measurements at a suite of frequencies is essential to separate the effect of water vapor from those of clouds, precipitation or changes in the radiance of the land, ocean or ice below. Scientifically, global water vapor profiles are essential to the numerical weather prediction of rainfall and drought, and help constrain such predictions in general. Two different types of microwave observations are used, those in transparent bands within which the water vapor absorption stands out (1) against the colder ocean background (ocean partially reflects the extremely cold cosmic background radiation), or (2) against that of cold, low-emissivity land. No profile information is usually retrieved, only an estimate of the column integrated abundance. The suite of frequencies most often used for this includes a subset of 18.7, 19.35, 22.2, 23.8, 31.4, 36.5, 37.0, 85.5 and 89 GHz.1
Additionally, passive microwave observations of water vapor are important because weather radars measure only the reflectivity of water/ice droplets in the atmosphere. Extraction of useful information from radar reflectivity measurements relies greatly on knowledge of the droplets’ size distribution, which requires complex and costly multiband radar measurements to directly measure.
Passive microwave instruments, on the other hand, directly measure the total quantity of liquid water as well as water vapor and other variables. Such radiometers can herald impending weather events by measuring the presence of water vapor in advance of cloud formation, and then detect the formation of liquid water droplets well in advance of detection by rain radars. Moreover, when used in conjunction with weather radars, passive radiometers provide a high degree of precision in the measurement of the path- or area-averaged quantities being observed that serve to calibrate the radar’s signal. In this manner the radiometer is able to facilitate the radar’s capability to provide high resolution measurements.2 Passive measurements from EESS satellites near the water vapor absorption line at 22.2 GHz are essential not only for measuring atmospheric water vapor but also for reducing error in other geophysical parameters due to the presence of water vapor, especially in moist atmospheres. For example, the accuracy in measuring sea surface wind speed, sea surface temperature, liquid cloud water or precipitation would significantly degrade if the 22 GHz water vapor channel was not present or unusable due to RFI contamination.
1 National Research Council, Spectrum Management for Science in the 21st Century, The National Academies Press, Washington, D.C., 2010, p. 29.
2 Ibid, pp. 24-25.