Revision of the microwave coefficient files in the IFS

TitleRevision of the microwave coefficient files in the IFS
Publication TypeMiscellaneous
Year of Publication2015
AuthorsLupu, C, Geer, A, Bormann, N
Secondary TitleTechnical Memorandum

The radiative transfer model RTTOV is an important component of the assimilation of radiance observations and is used in conjunction with a number of regression coefficient files generated for a given instrument and its channels. RTTOV-11 is the latest version and comes with a new set of radiative transfer coefficient files based on 54 fixed vertical pressure levels and the latest underlying spectroscopic parameters. This memorandum summarises the evaluation of RTTOV-11 microwave radiative transfer coefficient files for use in the operational assimilation system at ECMWF. Results show some improvement in the observation fits at first-guess. The change to 54 levels ensures smooth varying layer thicknesses (in particular at the top of the atmosphere) and improves the first-guess fits for ATMS and AMSU-A stratospheric channels. Small changes are noticeable in the mean temperature analyses above 100 hPa. The new specification of the 22 GHz water vapor line width and the inclusion of ozone for channels near 183 GHz for SSMIS, lead to a reduction in the mean bias corrections and reflect better agreement with the observations. The impact of using the newly released set of RTTOV-11 microwave coefficient files on medium-range forecasts is overall neutral. The results presented here suggested that it would be beneficial for the ECMWF operational system to use the most up-to-date version of the microwave coefficient files released with the latest version of RTTOV-11 and this will become operational with IFS cycle 41r2.