Radiometric estimation of water vapor content over Brazil

A multi-channel microwave radiometre (make: Radiometrics Corporation) is installed at Instituto Nacional de Pesquisas Espaciais–INPE, Brazil (22°S). The radiometric output of two channels of the radiometer in the form of brightness temperature at 23.834GHz and 30GHz, initially, were used to find out the ambient water vapor content and the non-precipitable cloud liquid water content. The necessary algorithm was developed for the purpose. The best results were obtained using the hinge frequency 23.834GHz and 30GHz pair having an r.m.s. error of only 2.64. The same methodology was then adopted exploiting 23.034GHz and 30GHz pair. In that case the r.m.s. error was 3.42. These results were then compared with those obtained over Kolkata (22°N), India, by using 22.234GHz and 31.4GHz radiometric data. This work conclusively suggests the use of a frequency should not be at the water vapor resonance line. Instead, while measuring the vapor content for separation of vapor and cloud liquid, one of them should be a few GHz left or right from the resonance line i.e., at 23.834GHz and the other one should be around 30GHz.