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Abstract
The Ice Cloud Imager (ICI) will be launched on the next generation of EUMETSAT polar-orbiting weather satellites and make passive observations between 183 and 664 GHz which are sensitive to scattering from cloud ice. These observations have the potential to improve weather forecasts through direct assimilation using "all-sky" methods which have been successfully applied to microwave observations up to 200 GHz in current operational systems. This requires sufficiently accurate representations of cloud ice in both numerical weather prediction (NWP) and radiative transfer models. In this study, atmospheric fields from a high-resolution NWP model are used to drive radiative transfer simulations using the Atmospheric Radiative Transfer Simulator (ARTS) and a recently released database of cloud ice optical properties. The simulations are evaluated using measurements between 89 and 874 GHz from five case studies of ice and mixed-phase clouds observed by the Facility for Airborne Atmospheric Measurements (FAAM) BAe-146 research aircraft. The simulations are strongly sensitive to the assumed cloud ice optical properties, but by choosing an appropriate ice crystal model it is possible to simulate realistic brightness temperatures over the full range of sub-millimetre frequencies. This suggests that sub-millimetre observations have the potential to be assimilated into NWP models using the all-sky method.
Highlights
The generation of EUMETSAT polar-orbiting weather satellites (EPS-SG) is planned for launch from 2022, and satellite B will carry the novel Ice Cloud Imager (ICI)
The difference between simulated and observed brightness temperatures at each location along the flight tracks is considered. This is indicative of the first-guess departures that might be seen within an all-sky assimilation system, and it will include differences due to mis-location of cloud features within the numerical weather prediction (NWP) model as well as errors in the cloud microphysics and radiative transfer calculations
The results presented in the previous section demonstrate that it is possible to simulate realistic sub-millimetre brightness temperatures for cloudy scenes using atmospheric fields from a state-of-the art high-resolution NWP model and an accurate radiative transfer model with a realistic representation of cloud ice scattering
Summary
The generation of EUMETSAT polar-orbiting weather satellites (EPS-SG) is planned for launch from 2022, and satellite B will carry the novel Ice Cloud Imager (ICI). The observed radiances have the potential to be directly assimilated in to numerical weather prediction (NWP) models using “all-sky” methods, which have been successfully demonstrated for existing microwave sensors [2]. Satellite observations of cloud and precipitation can improve forecast performance because they can be used by data assimilation systems to infer temperature, humidity and winds. Extending all-sky assimilation to sub-millimetre wavelengths will better constrain ice cloud microphysics [3] and has the potential to further improve forecasts by providing sensitivity to thinner ice clouds which do not cause significant scattering at existing microwave frequencies. A key requirement for all-sky assimilation is for both the forecast model and the radiative transfer model used as the observation operator to have a sufficiently-accurate representation of clouds and precipitation in order to simulate realistic top-of-atmosphere radiances or brightness temperatures
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