IMPROVED BROAD-BAND EMISSIVITY PARAMETERIZATION FOR WATER-VAPOR COOLING RATE CALCULATIONS

Reference transmissivities based on line-by-line calculations have bee n computed for a wide range of homogeneous paths of water vapor. A new approach is employed in which wideband emissivities are directly fitt ed to the line-by-line reference calculations without using the interm ediate step of narrowband models. A significant improvement in accurac y is obtained over previous schemes. Compared with line-by-line comput ed fluxes and cooling rates (without continuum absorption) for the sta ndard middle-latitude summer (MLS) profile, the maximum error in fluxe s is 1.5 W m-2; agreement is within 1% in fluxes and within 0.11 K/day , or 5%, in cooling rate. Unlike most published water vapor continuum schemes, which use the Roberts et al. model, the authors have reformul ated the treatment of the water vapor continuum by producing a new par ameterization based on the semiempirical model of Clough et al. This r esults in approximately 7.5 W m-2 difference in calculated radiative f luxes at the tropopause, and maximum difference in fluxes can approach 15 W m-2 in the troposphere for the MLS atmosphere.