The QME AERI LBLRTM: A Closure Experiment for Downwelling High Spectral Resolution Infrared Radiance

D. D. Turner, D. C. Tobin, S. A. Clough, P. D. Brown, R. G. Ellingson, E. J. Mlawer, R. O. Knuteson, H. E. Revercomb, T. R. Shippert, W. L. Smith, M. W. Shephard
2004 Journal of the Atmospheric Sciences  
Research funded by the U.S. Department of Energy's Atmospheric Radiation Measurement (ARM) program has led to significant improvements in longwave radiative transfer modeling over the last decade. These improvements, which have generally come in small incremental changes, were made primarily in the water vapor self-and foreign-broadened continuum and the water vapor absorption line parameters. These changes, when taken as a whole, result in up to a 6 W m Ϫ2 improvement in the modeled clear-sky
more » ... modeled clear-sky downwelling longwave radiative flux at the surface and significantly better agreement with spectral observations. This paper provides an overview of the history of ARM with regard to clear-sky longwave radiative transfer, and analyzes remaining related uncertainties in the ARM state-of-the-art Line-by-Line Radiative Transfer Model (LBLRTM). A quality measurement experiment (QME) for the downwelling infrared radiance at the ARM Southern Great Plains site has been ongoing since 1994. This experiment has three objectives: 1) to validate and improve the absorption models and spectral line parameters used in line-by-line radiative transfer models, 2) to assess the ability to define the atmospheric state, and 3) to assess the quality of the radiance observations that serve as ground truth for the model. Analysis of data from 1994 to 1997 made significant contributions to optimizing the QME, but is limited by small but significant uncertainties and deficiencies in the atmospheric state and radiance observations. This paper concentrates on the analysis of QME data from 1998 to 2001, wherein the data have been carefully selected to address the uncertainties in the 1994-97 dataset. Analysis of this newer dataset suggests that the representation of self-broadened water vapor continuum absorption is 3%-8% too strong in the 750-1000 cm Ϫ1 region. The dataset also provides information on the accuracy of the self-and foreignbroadened continuum absorption in the 1100-1300 cm Ϫ1 region. After accounting for these changes, remaining differences in modeled and observed downwelling clear-sky fluxes are less than 1.5 W m Ϫ2 over a wide range of atmospheric states. of the known physics, such as line-by-line models, are typically used to construct significantly faster radiation models to calculate radiative fluxes in GCMs. The lineby-line model used to build these faster models must be accurate, as even 1% changes in radiation are significant for climate (Ellingson and Wiscombe 1996) . Recognizing the importance of radiative transfer in climate modeling, the World Climate Research Program, the International Radiation Commission, and the U.S. Department of Energy (DOE) initiated an international radiative transfer model study. The initial focus of the Intercomparison of Radiation Codes in Climate Models (ICRCCM) was on clear-sky scenes, which are inherently easier to model than cloudy scenes. The differences in computed longwave radiative fluxes for the clear-sky atmospheres approached 70 W m Ϫ2 (Ellingson et al. 1991; Ellingson and Wiscombe 1996) . Even the line-by-line models differed by tens of watts per square
doi:10.1175/jas3300.1 fatcat:hmjyrj24sracrnv6ycphuwqnwq