The 94-GHz radar dim band: Relevance to ice cloud properties and CloudSat

Details of the microphysics are shown to be responsible for a region of ice cloud which, when probed from above, has decreasing radar reflectivity (Ze) downwards at 94 GHz but increasing Ze at 9.7 GHz. This 94-GHz radar dim band is found to be due to the combination of ice particle aggregation and non-Rayleigh scattering effects. Observations and model calculations indicate that it occurs when the particle size distribution (PSD) broadens such that its slope, as derived from fitted PSD, decreases below about 15 cm-1, or equivalently, to a median volume diameter exceeding 0.25 cm. Dimming occurs at temperatures (T) primarily between -5 and 0°C but can occur at -30°C or below in convectively-generated ice clouds (anvils). The dimming effect may produce an appreciable low bias in the ice water content (IWC) retrieved from Ze measured by CloudSat’s 94-GHz radar. Methods to estimate the IWC in the dim band are proposed.