Preprints
https://doi.org/10.5194/amt-2020-442
https://doi.org/10.5194/amt-2020-442

  20 Nov 2020

20 Nov 2020

Review status: a revised version of this preprint is currently under review for the journal AMT.

Introducing hydrometeor orientation into all-sky microwave/sub-millimeter assimilation

Vasileios Barlakas1, Alan J. Geer2, and Patrick Eriksson1 Vasileios Barlakas et al.
  • 1Department of Space, Earth and Environment, Chalmers University of Technology, Gothenburg, Sweden
  • 2European Center for Medium-Range Weather Forecasts, Reading, UK

Abstract. Numerical weather prediction systems still employ many simplifications when assimilating microwave radiances in all-sky conditions (clear sky, cloudy, and precipitation). For example, the orientation of ice hydrometeors is ignored, along with the polarization that this causes. We present a simple approach for approximating hydrometeor orientation, requiring minor adaption of software and no additional calculation burden. The approach is introduced in the RTTOV (Radiative Transfer for TOVS) forward operator and tested in the Integrated Forecast System (IFS) of the European Centre for Medium-Range Weather Forecasts (ECMWF). For the first time within a data assimilation (DA) context, this represents the ice-induced brightness temperature differences between vertical (V) and horizontal (H) polarization, the polarization difference (PD). The discrepancies in PD between observations and simulations decrease by an order of magnitude at 166.5 GHz, with maximum reductions of 10–15 K. The error distributions, which were previously highly skewed and therefore problematic for DA, are now roughly symmetrical. The approach is based on rescaling the extinction in V- and H-channels, which is quantified by the polarization ratio ρ. Using dual polarization observations from Global Precipitation Mission microwave imager (GMI), suitable value for ρ was found to be 1.5 and 1.4 at 89.0 and 166.5 GHz, respectively. The scheme was used for all the conical scanners assimilated at ECMWF, with broadly neutral impact on the forecast, but with an increased physical consistency between instruments that employ different polarizations. This opens the way towards representing hydrometeor orientation for cross-track sounders, and at frequencies above 183.0 GHz where the polarization can be even stronger.

Vasileios Barlakas et al.

 
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Status: final response (author comments only)
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Vasileios Barlakas et al.

Vasileios Barlakas et al.

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Short summary
Oriented non-spherical ice particles induce polarization that is ignored when cloud-sensitive satellite observations are used in numerical weather prediction systems. We present a simple approach for approximating particle orientation, requiring minor adaption of software and no additional calculation burden. With this approach, the system realistically simulates the observed polarization patterns, increasing the physical consistency between instruments with different polarizations.