Ninth ARM Science Team Meeting Proceedings, San Antonio, Texas, March 22-26, 1999 1 Flexible, Longwave Radiative Transfer (FLRT) in Clear and Cloudy Atmospheres J. S. Delamere and K. Stamnes Geophysical Institute, University of Alaska, Fairbanks Fairbanks, Alaska E. J. Mlawer and S. A. Clough Atmospheric and Environmental Research, Inc. Cambridge, Massachusetts Introduction This paper introduces a flexible, longwave radiative transfer tool (FLRT), which can be used to create a correlated-k, multiple-scattering model for inhomogeneous atmospheres. The spectral bandwidths can be chosen by the user within the 10 to 3000 wavenumber range. FLRT provides a mechanism from which rapid radiative transfer models (RRTMs) can be generated. Rapid radiative transfer models permit accelerated calculations of radiances, fluxes and cooling rates without comprising accuracy. Such models have a variety of atmospheric radiative transfer applications. One application includes modeling radiance measurements in spectral channels of satellite or ground-based sensors. Features of the Flexible, Longwave Radiative Transfer Tool (FLRT) Radiative transfer computations in the infrared spectral region are notoriously demanding due to the complex line structures of the many radiatively active gases. Consequently, the absorption coefficients can vary rapidly across a small spectral interval and many monochromatic computations are required to produce spectrally-integrated quantities such as fluxes and cooling rates. Results from line-by-line models, which capture this variation in absorption coefficients, are extremely accurate but computationally prohibitive. The correlated-k method is an efficient, numerical procedure that substantially reduces the number of calculations in a spectral interval from a line-by-line model without compromising accuracy. This is accomplished by grouping, in ascending order by strength, the absorption coefficients within the wavenumber interval, which creates a smooth “k distribution.” A much smaller set of characteristic absorption coefficients κ j , where j represents a subinterval of the k distribution, can now be used in radiative transfer calculations for each homogeneous layer. The correlated-k method has been reviewed in detail in a number of papers (Arking and Grossman 1972; Goody et al. 1989; Lacis and Oinas 1991; Fu and Liou 1992; Mlawer et al. 1997). FLRT employs the techniques of the Mlawer et al. (1997) RRTM to create k distributions from absorption coefficients provided by the line-by-line radiative transfer model (LBLRTM) (Clough et al. 1992; Clough and Iacono 1995). Absorption due to water vapor, carbon dioxide, ozone, nitrous oxide,