Investigating the use of Lambertian reflection in the assimilation of surface-sensitive microwave sounding radiances over snow and sea-ice
Title | Investigating the use of Lambertian reflection in the assimilation of surface-sensitive microwave sounding radiances over snow and sea-ice
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Technical memorandum
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Date Published |
09/2021
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Secondary Title |
ECMWF Technical Memoranda
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Number |
886
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Author | |
Publisher |
ECMWF
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Abstract | This paper investigates two aspects aimed at an increased use of surface-sensitive microwave sounding radiances for NWP, namely the assimilation of humidity-sounding radiances from ATMS over snow, as well as the use of Lambertian reflection in the radiative transfer modelling over snow and sea-ice surfaces. The Lambertian characteristics are modelled through a parametrization of the effective zenith angle used to calculate the down-welling radiation in the radiative transfer model. Using Lambertian rather than specular reflection significantly improves the forward modelling for 183 GHz humidity-sounding channels over snow and sea-ice in comparisons to observations, including a reduction of significant zenith-angle dependent biases. For temperature-sounding channels in the 50 GHz band, comparisons between observations and background equivalents also suggest improvements from treating snow and sea-ice surfaces as Lambertian or semi-Lambertian, but the characteristics appear more variable, and there are indications of other sources of error such as biases in the skin-temperature. For both spectral regions the improvement from using non-specular reflection is clearest for snow-covered land surfaces. Assimilation experiments show that the addition of 183 GHz humidity-sounding channels from ATMS over snow-covered land lead to a slight positive impact over the higher latitudes in winter. On top of this, replacing the specular surface assumption over snow and sea-ice with a fully Lambertian for 183 GHz channels on ATMS and semi-Lambertian for 50 Ghz channels on ATMS and AMSU-A means that more observations pass quality control, giving a neutral to slightly positive impact at higher latitudes in winter. Combining the two changes leads to small forecast benefits that are statistically significant up to day 3 at higher latitudes. |
URL | https://www.ecmwf.int/en/elibrary/81265-investigating-use-lambertian-reflection-assimilation-surface-sensitive-microwave |
DOI | 10.21957/hzcsrx8tt |
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