Progress of All-Sky SEVIRI Radiance Assimilation in NCEP NAMRR System

A summary of recent progress toward the assimilation of all-sky radiance (ASR) products from Meteosat SEVIRI (Spinning Enhanced Visible and Infrared Imager) observations into an hourly-updated version of NCEP's North American Mesoscale (NAM) analysis and forecast system will be presented. The NCEP Grid-point Statistical Interpolation (GSI) analysis uses the NASA/GMAO overcast cloudy scheme to directly assimilate cloud-affected infrared radiances from AIRS polar orbiter data. This scheme was extended to make use of the ASR products generated by EUMETSAT from Meteosat SEVIRI data. The analysis control variables of GSI have been extended to include the cloud-top pressure, which is diagnosed from the observations in overcast scenes, not taken from model background. The cloud-top pressure is adjusted simultaneously with atmospheric temperature and humidity during the variational minimization. In this approach, clouds are assumed to be single-layer blackbodies and the background cloud-affected radiances are computed in GSI using cloud-top pressure, cloud fraction and the clear-sky radiance computed from the Community Radiative Transfer Model (CRTM). In the current configuration, six channels including two water vapor channels around 6.2μm and 7.3μm, and four IR channels around 8.5, 11.2, 12.3 and 13.3μm are used to define initial estimates of cloud-top pressure and cloud fraction. However, only two water vapor channels 6.2 and 7.3 μm are assimilated in GSI.

The test for assimilating ASR SEVIRI data is conducted over a NAM-sized Europe and Africa domain with 12 km parent and 4 km nest resolution. The large domain and hourly assimilation afforded by NAMRR [NAM Rapid Refresh] together ensure the most complete use of SEVIRI observations. About 10% more data were assimilated per hour from ASR SEVIRI due to the application of a stringent quality control, compared with the original clear-sky (CSR) SEVIRI assimilation. The sensitivity of static and height-dependent background error of cloud-top pressure for assimilating cloud-affect radiance will be discussed. We also will present the preliminary impact of ASR SEVIRI assimilation on 4 km nest forecast of storms over Lake Victoria.