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• Ground based radiometer has been used to calculate cloud attenuation. • Significant cloud attenuation has been observed over Kolkata in monsoon at 30 GHz. • August shows the worst month scenario due to high value of LWP. • Cloud attenuation has been compared with rain attenuation. The objective of this study is to investigate cloud attenuation at 30 GHz frequency using ground-based microwave radiometric observations at a tropical location, Kolkata. At higher frequencies and lower elevation angles, cloud attenuation is of major concern at a tropical location. The location experiences high value of liquid water path (LWP), which is responsible for cloud attenuation, during the Indian summer monsoon (ISM) and pre-monsoon season. Significant amount of cloud attenuation has been observed during monsoon season at 30 GHz. Two years observations of exceedance probability of cloud attenuation and worst month statistics are presented. The variation of cloud attenuation with frequencies for different elevation angles has also been investigated. The seasonal and diurnal patterns of cloud attenuation are examined. Cloud attenuation, inferred from radiometric measurements before rain commencement, has been compared to rain attenuation at Ku-band. Exceedance probabilities of cloud and rain attenuation have been compared. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]