Evaluation of differential absorption radars in the 183 GHz band
for profiling water vapour in ice clouds
Abstract. Relative humidity (RH) measurements in ice clouds are essential for determining the ice crystals growth processes and rates. A differential absorption radar (DAR) system with several frequency channels within the 183.3 GHz water vapour absorption band is proposed for measuring RH within ice clouds. Here, the performance of a DAR system is evaluated by applying a DAR simulator to A-Train observations in combination with collocated European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis. Observations from the CloudSat W-band radar and from the CALIPSO lidar are converted first into ice microphysical properties and then coupled with ECMWF temperature and relative humidity profiles in order to compute scattering properties at any frequency within the 183.3 GHz band. Self-similar Rayleigh Gans approximation is used to model the ice crystal scattering properties. The radar reflectivities are computed both for a space-borne and a ground-based DAR system by using appropriate radar receiver characteristics. Sets of multi-frequency synthetic observation of attenuated reflectivities are then used to retrieve profile of water vapour density by fitting the line shape at different levels. 10 days of A-Train observations are used to test the measurement technique performance for different combination of tones when sampling ice clouds globally. Results show that that water vapour densities can be derived with accuracies that can enable ice process studies (i.e. better than 3 %) both from a ground-based system (at the minute temporal scale and with circa 100 m vertical resolution) and from a space/airborne system (at 500 m vertical resolution and with circa 5 km integration lengths) with four tones in the right wing of the absorption line. A ground-based DAR system to be deployed at high latitude/high altitudes is highly recommended to test the findings of this work in the field.