Radiance and Jacobian intercomparison of radiative transfer models applied to HIRS and AMSU channels

Abstract

Abstract. The goals of this study are the evaluation of current fast radiative transfer models (RTMs) and line-by-line (LBL) models. The intercomparison focuses on the modeling of 11 representative sounding channels routinely used at numerical weather prediction centers: 7 HIRS (High-resolution Infrared Sounder) and 4 AMSU (advanced microwave sounding unit) channels. Interest in this topic was evident by the participation of 24 scientists from 16 institutions. An ensemble of 42 diverse atmospheres was used and results compiled for 19 infrared models and 10 microwave models, including several LBL RTMs. For the first time, not only radiances but also Jacobians (of temperature, water vapor, and ozone) were compared to various LBL models for many channels. In the infrared, LBL models typically agree to within 0.05-0.15 K (standard deviation) in terms of top-of-the-atmosphere brightness temperature (BT). Individual differences up to 0.5 K still exist, systematic in some channels, and linked to the type of atmosphere in others.The best fast models emulate 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 model to which they were tuned. However, significant differences were noted among LBL models. Extending the intercomparison to the Jacobians proved very useful in detecting subtle or more obvious modeling errors. In addition, total and single gas optical depths were calculated, which provided additional insight on the nature of differences.

Intercomparison of general purpose clear sky atmospheric radiative transfer models for the millimeter/submillimeter spectral range

“…The bias found for AMSU channel 18 was 0.37 K compared to the reference model of the study by Garand et al [2001]. Bias here means the mean (over a set of atmospheric states) of the difference between brightness temperatures from ARTS and from the reference model, and the criteria for the quality of the agreement are the ones stated in the latter study.…”

“…[111] Note that in an independent exercise [John et al, 2002], ARTS was found to be in very good agreement with a large group of operational radiative transfer models for this sensor type which have been compared in the work of Garand et al [2001]. The bias found for AMSU channel 18 was 0.37 K compared to the reference model of the study by Garand et al [2001].…”

“…Similar recent intercomparison studies are Tjemkes et al [2003], von Clarmann et al [2003b], Garand et al [2001]. However, the studies by von Clarmann et al [2003b] and Tjemkes et al [2003] focused on the infrared wavelength range, and the study by Garand et al [2001] focused exclusively on models for the Advanced Television Infrared Observation Satellite (TIROS) Operational Vertical Sounder (ATOVS) instruments flown on polar orbiting meteorological satellites (POES) of NASA.…”

“…More details and derivations can be found in books by authors like, e.g., Goody and Yung [1989] or Janssen [1993].…”

An equivalent neutral wind observation operator for variational assimilation of scatterometer ocean surface wind data

“…This can be done analytically or by numerically estimating the Jacobian using finite differences by applying small perturbations to the background state as input to the nonlinear operator. This perturbation method has been applied previously to radiative transfer (Chevallier and Mahfouf, 2001;Garand et al, 2001) and cloud models (Fillion and Mahfouf, 2003) for the sensitivity characterization of parametrization schemes. Rüdiger et al (2010) have also tested this approach for the assimilation of remotely sensed observations of vegetation biomass into a land surface model.…”