IMPACTS OF NEGLECTING LONGWAVE SCATTERING AND THE METHODS OF REDUCING THESE UNCERTAINTIES IN MODEL SIMULATIONS CONTAINING CLOUDS
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Since clouds are spread widely around the world and heavily influence the Earth’s energy budget, people extensively use general circulation models (GCMs) to investigate the effect of clouds on the future climate. Since in the longwave spectrum, absorption leads the radiative transfer processes, to reduce computing time, radiative schemes in most GCMs only take absorption properties of clouds into account. This study investigates the issues of neglecting longwave scattering induced by clouds using satellite observations in 2010. Global simulations show that excluding longwave scattering overestimates upward flux at the top-of-atmosphere (TOA) by about 2.63 W/m^2 , which is about 10% of the TOA longwave (LW) cloud radiative effect, and underestimates downward flux at the surface by about 1.15 W/m^2 , corresponding to about 5% of the surface LW cloud radiative effect. Longwave scattering cools the atmosphere by about 0.018 K/day at the tropopause and heats about 0.028 K/day at the surface. The magnitude of the cloud radiative effect from neglecting longwave scattering is similar to the clear-sky radiative effect of doubling COv2. Spectral analysis shows that longwave scattering by ice clouds contributes over 40% of simulation biases in the 350- 500 cm^-1 band. For simpler calculations, the optical properties of ice clouds used in the Moderate Resolution Imaging Spectrometer (MODIS) Collection 6 cloud retrieval products are appropriately parameterized as a function of effective particle size. The overall coefficients of determination (R^2 ) of the corresponding fitting processes are larger than iii 0.9. By using these parameterized highly scattered ice cloud optical properties in an isothermal homogeneous cloud layer, the performance of various radiative transfer models is examined. The results show that 2-/4-stream approximations are relatively more efficient and are relatively more accurate than other approximation methods in comparisons of simulated cloud emissivity.
Kuo, Chia-Pang (2018). IMPACTS OF NEGLECTING LONGWAVE SCATTERING AND THE METHODS OF REDUCING THESE UNCERTAINTIES IN MODEL SIMULATIONS CONTAINING CLOUDS. Doctoral dissertation, Texas A & M University. Available electronically from