Improved Broadband Emissivity Parameterization for Water Vapor Cooling Rate Calculations

Reference transmissivities based on line-by-line calculations have been computed for a wide range of homogeneous paths of water vapor. A new approach is employed in which wideband emissivities are directly fitted to the line-by-line reference calculations without using the intermediate step of narrowband models. A significant improvement in accuracy is obtained over previous schemes. Compared with line-by-line computed fluxes and cooling rates (without continuum absorption) for the standard middle-latitude summer (MLS) profile, the maximum error in fluxes 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 reformulated the treatment of the water vapor continuum by producing a new parameterization based on the semiempirical model of Cough et al. This results in ∼7.5 W m−2 difference in calculated radiative fluxes at the tropopause, and maximum difference in fluxes can approach 15 W m−2 in the troposphere for the MLS atmosphere.