Accounting for several infrared radiation processes in climate models

Abstract

Three aspects of longwave (LW) radiation processes are investigated using numerical experiments with the Canadian Atmospheric Global Climate Model version 4.3 (CanAM4.3). These are the overlapping LW and shortwave (SW) radiation, scattering by clouds, and specification of ocean emissivity. For the overlapping of solar and infrared spectra, using a single band scheme was compared against a method directly inputting solar energy. Offline calculations show that for high clouds using the single band can cause an overestimate of the downward LW flux, whereas a method that accounts for input solar energy in the LW yields results that are more accurate. Longwave scattering by clouds traps more infrared energy in the atmosphere and reduces the outgoing radiation to space. Simulations with CanAM4.3 show that cloud LW scattering can enhance the LW cooling rate above the tropopause and reduce it inside the troposphere, resulting in warmer temperatures, especially in the tropics and low latitudes. This implies a larger temperature gradient toward the polar region, which causes a strengthening of the Hadley circulation and shifting of the intertropical convergence zone (ITCZ). The increase in lower tropospheric temperature also affects the lower troposphere water vapor and precipitation. Sensitivity to the specification of ocean emissivity is examined by comparing a broadband scheme dependent on the surface wind and solar zenith angle against one that resolves the wavelength dependence. Experiments with CanAM4.3 show that the two oceanic emissivity schemes can produce over 1 W m-2 seasonal mean difference of the upward flux at the surface.