Abstract
Abstract. Relative humidity (RH) measurements in ice clouds are essential for determining ice crystal 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 co-located 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. A 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 airborne and a ground-based DAR system by using appropriate radar receiver characteristics. Sets of multi-frequency synthetic observation of attenuated reflectivities are then exploited to retrieve profiles of water vapour density by fitting the line shape at different levels. A total of 10 d of A-Train observations are used to test the measurement technique performance for different combinations of tones when sampling ice clouds globally. Results show that water vapour densities can be derived at the level 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-borne system (at 500 m vertical resolution and with circa 5 km integration lengths) with four tones in the upper wing of the absorption line. Deploying ground-based DAR system at high latitudes and high altitudes is highly recommended to test the findings of this work in the field.
Highlights
Adequate understanding of the cloud and precipitation processes that contribute to Earth’s water and energy cycle is required before significant progress can occur in our ability to predict future climate scenarios
Future space-borne cloud and precipitation radars are expected to be at the centre of such a revolution (The Decadal Survey, 2017), enhancing the view depicted in the past 20 years by the Tropical Rainfall Measuring Mission (TRMM) Ku-band Precipitation radar (Kummerow et al, 1998), the Global Precipitation Mission (GPM) Dualfrequency (Ku–Ka) Precipitation Radar (Skofronick-Jackson et al, 2016) and the CloudSat W-band Cloud Profiling Radar (Tanelli et al, 2007)
While the first Doppler radar is expected to be launched on board the EarthCARE satellite in 2021 (Illingworth et al, 2015), innovative radar concepts have been studied in the past decade, ranging from multi-wavelength radars proposed, e.g. as payloads of the Aerosol/Cloud/Ecosystems (ACE) mission and the Polar
Summary
Adequate understanding of the cloud and precipitation processes that contribute to Earth’s water and energy cycle is required before significant progress can occur in our ability to predict future climate scenarios. 2019), and this figure is certainly in reach for ground locations hosting a remote sensing observatory This highlights that in order to retrieve useful information for ice cloud studies water vapour densities must be retrieved within ∼ 3 %– 5 % or better – this in order to account for the previously mentioned additional uncertainty due to temperature – for a range of values between 0.5 and 5 g m−3. Our strategy is to exploit the novel retrieval model proposed in Roy et al (2018) in assessing the precision of DAR techniques in profiling ice clouds both from a ground and a space-borne perspective This will allow us to draw some conclusions on the potential of such observations for ice studies.
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