Radiance and Jacobian intercomparison of radiative transfer models appliedto HIRS and AMSU channels

The goals of this study are the evaluation of current fast radiative transf er models (RTMs) and line-by-line (LBL) models. The intercomparison focuses on the modeling of 11 representative sounding channels routinely used at n umerical weather prediction centers: 7 HIRS (High-resolution Infrared Sound er) and 4 AMSU (advanced microwave sounding unit) channels. Interest in thi s topic was evident by the participation of 24 scientists from 16 instituti ons. An ensemble of 42 diverse atmospheres was used and results compiled fo r 19 infrared models and 10 microwave models, including several LBL RTMs. F or the first time, not only radiances but also Jacobians (of temperature, w ater vapor, and ozone) were compared to various LBL models for many,channel s. In the infrared, LBL models typically agree to within 0.05-0.15 K (stand ard deviation) in terms of top-of-the-atmosphere brightness temperature (BT ) Individual differences up to 0.5 K still exist, systematic in some channe ls, and linked to the type of atmosphere in others. The best fast models em ulate LBL BTs to within 0.25 K, but no model achieves this desirable level of success for all channels. The ozone modeling is particularly challenging . In the microwave, fast models generally do quite well against the LBL mod el to which they were tuned. However, significant differences were noted am ong LBL models. Extending the intercomparison to the Jacobians proved very useful in detecting subtle or more obvious modeling errors. In addition, to tal and single gas optical depths were calculated, which provided additiona l insight on the nature of differences.