An intercomparison of radiation codes for retrieving upper-tropospheric humidity in the 6.3-mu m band: A report from the first GVaP workshop

An intercomparison of radiation codes used in retrieving upper-tropospheric humidity (UTH) from observations in the v2 (6.3 mu m) water vapor absorpti on band was performed. This intercomparison is one part of a coordinated ef fort within the Global Energy and Water Cycle Experiment Water Vapor Projec t to assess our ability to monitor the distribution and variations of upper -tropospheric moisture from spaceborne sensors. A total of 23 different cod es, ranging from detailed line-by-line (LBL) models, to coarser-resolution narrowband (NB) models, to highly parameterized single-band (SB) models par ticipated in the study. Forward calculations were performed using a careful ly selected set of temperature and moisture profiles chosen to be represent ative of a wide range of atmospheric conditions. The LBL model calculations exhibited the greatest consistency with each other, typically agreeing to within 0.5 K in terms of the equivalent blackbody brightness temperature (T -b). The majority of NE and SE models agreed to within +/-1 K of the LBL mo dels, although a few older models exhibited systematic T-b biases in excess of 2 K. A discussion of the discrepancies between various models, their as sociation with differences in model physics (e.g., continuum absorption), a nd their implications for UTH retrieval and radiance assimilation is presen ted.