Sensitivity of microwave radiances at 85-183 GHz to precipitating ice particles

Abstract

The response of passive microwave observations to ice particle scattering and surface emissivity has been studied for frequencies at 85, 150, and 183 GHz. Using two- and three-dimensional simulations of different precipitation events, we found that a channel around 150 GHz generally exhibits the strongest scattering signature due to precipitation-sized ice particles. This channel is only moderately affected by variations in surface emissivity. The sensitivity of the 150 GHz channel to variations in cloud water is about 25 (Kg -1 m3). In comparison, the sensitivity to changes in surface emissivity is about 5 K/(10%) for cold, dry atmospheres and less than 1 K/(10%) for typical midlatitude atmospheres. Channels at around 85 GHz are much stronger affected by variable surface emissivity (sensitivities are up to 15 K/(10%) and show on average a 2 to 2.5 times smaller scattering signature (10 Kg-1 m3). The sensitivity of a water vapor sounding channel at 183-7 GHz to ice particle scattering strongly depends on the environmental conditions but is in general about a factor of 1.5 to 2.5 smaller than at 150 GHz. This is due to the contribution of water vapor in and above the cloud to total emission, masking the scattering signal. The weighting functions of channels at 183-3 and 183-1 GHz peak too high up in the atmosphere and only show a weak or negligible response to the precipitation events under investigation. Based on these results, frequencies around 150 GHz, possibly in combination with channels at 85 and 183-7 GHz seem most appropriate for the detection and retrieval of precipitation properties at middle and high latitudes.