@misc{81265, author = {Niels Bormann}, title = {Investigating the use of Lambertian reflection in the assimilation of surface-sensitive microwave sounding radiances over snow and sea-ice}, 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.
}, year = {2021}, journal = {ECMWF Technical Memoranda}, number = {886}, month = {09/2021}, publisher = {ECMWF}, url = {https://www.ecmwf.int/node/20180}, doi = {10.21957/hzcsrx8tt}, language = {eng}, }