ARM Program Research Improves Longwave Radiative Transfer Models

Turner, D. D., National Oceanic and Atmospheric Administration

General Circulation and Single Column Models/Parameterizations

Radiative Processes

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, and W.L. Smith. 2004. Journal of Atmospheric Science, 61, 2657-2675.


Top panels: Examples of downwelling infrared radiance observed by the AERI for two different clear sky cases with different amounts of water vapor. Bottom panels: Differences between the AERI observations and calculations made using the LBLRTM at the start of the ARM Program (pre-ARM in black) and circa 2003 (red), showing the vast improvement in the ability to model downwelling infrared radiation. In terms of longwave radiative flux, the error in the downwelling flux was 6-8 W/m2 using the pre-ARM model, while the error is less than 1.5 W/m2 using the improved 2003 model.

As published in a series of recent journal articles, researchers sponsored by the DOE Atmospheric Radiation Measurement (ARM) Program have completed a major effort aimed at improving parameterizations of radiative transfer used in global climate models. Their development and validation of a line-by-line radiative transfer model (LBLRTM) resulted in a reduction in errors of longwave flux calculations from 6 to 8 W/m2 to less than 1.5 W/m2. This improvement reduces the uncertainties in model simulations of heating and cooling of the earth's atmosphere.

As one of four primary research groups within the ARM Program science team, the Instantaneous Flux (IRF) Working Group focuses on testing radiation parameterizations at the accuracy required for climate studies. One of the first areas they identified as a priority was improving clear-sky downwelling longwave radiance calculations produced by the LBLRTM. A well-validated LBLRTM could then be used to "build" rapid radiative transfer models or parameterizations for use in global climate models.

To achieve this goal, the group required observations with good radiometric "truth"—without reliable measurements at the outset, subsequent validation efforts would be essentially useless. The IRF Working Group identified the Atmospheric Emitted Radiance Interferometer—an instrument developed for the ARM Program that collects continuous measurements of downwelling infrared radiance at high spectral resolution with better than 1% accuracy—as an ideal candidate for providing these measurements. In addition, the validation effort required accurate observations of atmospheric state (i.e., profiles of water vapor and temperature) to use as input into the model. The research team conducted a Quality Measurement Experiment, or QME, to evaluate: (1) the accuracy of the AERI, (2) the accuracy of the input data used to drive the LBLRTM, and (3) the accuracy of the LBLRTM and its components. The QME effort led to improvements in all three areas.

Major progress was reported in 2003(a,b) as results became available from ARM field campaigns focused on water vapor. These campaigns were conducted to improve the water vapor observations used to drive the LBLRTM. More recently, research published in November and December 2004(c,d,e) describes the capabilities of the AERI and the comparisons conducted between the AERI observations and LBLRTM calculations. The original accuracy of "state-of-the-art" models in 1990 had longwave flux residuals on the order of 6 to 8 W/m2; improvements in the LBLRTM using the AERI observations reduced these flux errors to less than 1.5 W/m2. Involving scientists from a large number of organizations funded by ARM, this research spearheaded by the IRF Working Group represents a major accomplishment for the ARM Program.

(a) The Atmospheric Radiation Measurement Program's water vapor intensive observation periods: overview, initial accomplishments, and future challenges. H.E. Revercomb, D.D. Turner, D.C. Tobin, R.O. Knuteson, W.F. Feltz, J. Barnard, J. Bosenberg, S. Clough, D. Cook, R. Ferrare, J. Goldsmith, S. Gutman, R. Halthore, B. Lesht, J. Liljegren, H. Linne, J. Michalsky, V. Morris, W. Porch, S. Richardson, B. Schmid, M. Splitt, T. Vanhove, E. Westwater, and D. Whiteman. 2003. Bulletin of the American Meteorological Society, 84, 217-236, 2003

(b) Dry bias and variability in Vaisala radiosondes: The ARM experience. D.D. Turner, B.M. Lesht, S.A. Clough, J.C. Liljegren, H.E. Revercomb, and D.C. Tobin. 2003. Journal of Atmospheric and Oceanic Technology, 20, 117-132, 2003.

(c) Atmospheric Emitted Radiance Interferometer. Part I: Instrument Design. R. O. Knuteson, H. E. Revercomb, F. A. Best, N. C. Ciganovich, R. G. Dedecker, T. P. Dirkx, S. C. Ellington, W. F. Feltz, R. K. Garcia, H. B. Howell, W. L. Smith, J. F. Short, D. C. Tobin. 2004. Journal of Atmospheric and Oceanic Technology, 21, 1763-1776.

(d) Atmospheric Emitted Radiance Interferometer. Part II: Instrument Performance. R. O. Knuteson, H. E. Revercomb, F. A. Best, N. C. Ciganovich, R. G. Dedecker, T. P. Dirkx, S. C. Ellington, W. F. Feltz, R. K. Garcia, H. B. Howell, W. L. Smith, J. F. Short, D. C. Tobin. 2004. Journal of Atmospheric and Oceanic Technology, 21, 1777-1789.

(e) 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, and W.L. Smith. 2004. Journal of Atmospheric Science, 61, 2657-2675.