An Intercomparison of Radiation Codes for Retrieving Upper–Tropospheric Humidity in the 6.3–mm Band: A Report from the First GVaP Workshop

Abstract

An intercomparison of radiation codes used in retrieving upper?tropospheric humidity (UTH) from observations in the n2 (6.3 mm) water vapor absorption band was performed. This intercomparison is one part of a coordinated effort within the Global Energy and Water Cycle Experiment Water Vapor Project to assess our ability to monitor the distribution and variations of upper?tropospheric moisture from spaceborne sensors. A total of 23 different codes, ranging from detailed line?by?line (LBL) models, to coarser?resolution narrowband (NB) models, to highly parameterized single?band (SB) models participated in the study. Forward calculations were performed using a carefully selected set of temperature and moisture profiles chosen to be representative 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(Tb). The majority of NB and SB models agreed to within±1 K of the LBL models, although a few older models exhibited systematicTb biases in excess of 2 K. A discussion of the discrepancies between various models, their association with differences in model physics (e.g., continuum absorption), and their implications for UTH retrieval and radiance assimilation is presented.