SHDOM Correlated k-Distribution Programs

Three k-distribution programs are distributed with SHDOM to include the effects of molecular absorption in a broadband spectral integration. These programs read one atmospheric profile file (with pressure, temperature, water vapor, and ozone profiles) and produce an SHDOM CKD file. An SHDOM CKD file contains absorption coefficient profiles for each "k" in each spectral band in the k-distribution, along with some other information (see shdom.txt). The example Unix script run_kdist_les has examples of using CKD programs, the wavenumber and wavelength ranges of the spectral bands, and the Rayleigh molecular scattering coefficients for each band (input to propgen).

Two of the programs, ckdlwrrtm and ckdswrrtm, are derived from the longwave and shortwave Rapid Radiative Transfer Models (RRTM) developed at Atmospheric and Environmental Research, Inc, principally by Eli Mlawer (see http://www.rtweb.aer.com). The longwave CKD program is from the original RRTM version, not the new version first released in 2002. The longwave RRTM reference is:

Mlawer, E.J., S.J. Taubman, P.D. Brown, M.J. Iacono and S.A. Clough: RRTM, a validated correlated-k model for the longwave. J. Geophys. Res., 102, 16,663-16,682, 1997
Both the longwave and shortwave CKD programs do not exactly match the original RRTM algorithms because the SHDOM k-distribution model is different. The SHDOM model is that the temperature profile and scattering properties are independent of the "k" within each spectral band. The radiative transfer results based on ckdlwrrtm and ckdswrrtm do agree closely with the RRTM programs (fluxes within 1 - 2 W/m^2).

The third program, ckdfu, makes an SHDOM CKD file for both the shortwave and longwave, and is derived from Fu and Liou's original correlated k-distribution described in the article:

Fu, Qiang and K. N. Liou, 1992: On the Correlated k-Distribution Method for Radiative Transfer in Nonhomogeneous Atmospheres. J. Atmos. Sci., 49, 2139-2156.
This version is not the much upgraded "Fu and Liou" k-distribution used by the CERES program at NASA Langley. This older k-distribution parameterization has a number of deficiencies, including using one band for the ultraviolet and the visible, using the outdated Roberts et al. (1976) water vapor continuum, and missing trace absorbing species. Thus it is considerably less accurate than the RRTM based programs, however, it is substantially cheaper to run because it has fewer k's, and is appropriate for some theoretical experiments (e.g. where differences between SHDOM runs are being investigated). The size of the k-distributions in the three programs is summarized below.
Program Shortwave Longwave
Bands k's Bands k's
ckdlwrrtm 16 256
ckdswrrtm 14 224
ckdfu 6 54 12 67
The number of k's is the total number of monochromatic radiative transfer calculations that SHDOM must perform across the longwave or shortwave.

Input Parameters

The inputs are the same for all three CKD programs.
Parameter Description
ATMFILE Atmospheric profile file name
OUTFILE CKD file for SHDOM
UMCO2 Concentration of CO2 in ppmv
UMCH4 Concentration of CH4 in ppmv
UMN2O Concentration of N2O in ppmv

Atmospheric File Format

The atmospheric file specifies the profile of temperature, pressure, water vapor and ozone. Each row of the file contains an atmospheric level, starting at the top of the atmosphere and ending at the surface. As an example, the US standard atmosphere is included in the SHDOM distribution (usatm.dat). The format is ascii text with six columns:
  Height  Pressure  Temperature  Air_density  Vapor_density  Ozone_density
   (km)     (mb)       (K)        (g/m^3)        (g/m^3)        (g/m^3)      

CKD Output File

There are three sections of the output file.
  1. The band information: number of bands and for each band the wavenumber range (cm^-1), solar flux (W/m^2), number of g's (or k's), and delta g values (weights).
  2. The atmosphere information: the concentration of CO2, CH4, N2O, and for each level (from the top down) the height (km), pressure (mb), temperature (K), mixing ratio of water vapor and ozone.
  3. The absorption coefficients: each line contains the band number, level number (from top down), and k's for the g's.