Estimation of liquid water amount by a microwave radiometer

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The microwave radiative properties of water cloud was investigated by solving a radiative transfer equation in which the effect of scattering by cloud particles is taken into account. In the microwave band often used in cloud remote-sensing (0.8-3cm), clouds can be grouped into two categories. A cloud composed only of waterdrops smaller than about 300 μm in radius, which generally corresponds to non-precipitating cloud, shows a unique relationship between the integrated liquid water and the effective emissivity, almost independently of the size distribution of drops. For a cloud including drops larger than 300μm the above relationship strongly depends upon the size distribution. The relationship between the integrated liquid water and the effective emissivity was further studied for several microwave wavelengths. For an optically thinner cloud a short wavelength of 0.81cm can give a better and more sensitive estimation of integrated liquid water. For the cloud of large optical depth for which the microwave radiation at short wavelength becomes saturated, a combination of radiation at 0.81 and 2.0cm wavelengths can reduce the effect of size distribution and give the better estimation of liquid water amount.
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