Impact of various features of CRTM in GEOS-5

NASA’s GEOS-5 global atmospheric data assimilation and forecast system currently uses the Community Radiative Transfer Model (CRTM) for assimilating satellite radiances. The newly released CRTM version 2.02 contains two transmittance models: ODAS (Optical Depth in Absorber Space) is equivalent to the default algorithm - Compact OPTRAM in previous releases; and the alternative ODPS (Optical Depth in Pressure Space) model, which is similar to the RTTOV-type transmittance algorithm. Several important features are incorporated in OPDS model: (1) CO2 is a user optional input variable absorber; (2) up to six user input variable absorbers (H2O, CO2, O3, N2O, CO and CH4) can be included for hyper-spectral sensors; (3) OPTRAN algorithm is used for water vapor line absorption; (4) water vapor continua transmittance is treated separately. In addition to the transmittance model updates, a bi-directional reflectance distribution function (BRDF) is implemented in this version to account for reflected solar radiation in affected short-wave infrared channels, and the reflectivity for down-welling infrared over all surfaces has been changed from Lambertian (isotropic) to specular (non-isotropic). These new features in CRTM, especially the changes in the transmittance algorithm for water vapor channels, are expected to impact assimilation results from satellite radiances. There are two objectives in this study. The first objective is to investigate the impact of ODPS and ODAS in CRTM on the assimilation of satellite radiances, analysis and forecast. The second objective is to examine the impact of ODPS and ODAS on the effectiveness of the assimilation of water vapor channels from AIRS in GEOS-5. Five GEOS-5 experiments have been designed for impact studies. The assessment metrics to evaluate the impacts are: the fit of the background and analysis profiles to Rawinsonde observations; the innovation statistics for satellite radiances; the anomaly correlation score for the 5-day forecast of the 500 hPa height over various geographical regions; and the impact on the 24-hour forecast measured using the GEOS-5 adjoint-based technique. In this presentation, the experiment configuration, results and conclusions regarding the impact of CRTM and the effectiveness of the assimilation of water vapor channels from AIRS in GEOS-5 are summarized and discussed.