Parameterizations of Cloud-Radiation Interactions Based on Detailed Cloud Microphysics
Abstract
Cloud-radiation effects still account for much of the variation among leading global climate models in sensitivity to greenhouse gases. A single-column model (SCM) allows results of different cloud-radiation parameterizations to be compared directly with measurements. The relevant fields include cloud altitude, cloud amount, liquid and ice content, particle size spectra, and radiative fluxes at the surface and the top of the atmosphere. Comparisons with data from the Atmospheric Radiation Measurement (ARM) Program show conclusively that prognostic cloud algorithms with detailed microphysics are far more realistic than simpler approaches. Long-term comparisons of SCM quantities strongly modulated by clouds, such as monthly mean downwelling surface shortwave radiation, clearly demonstrate the superiority of parameterizations based on comprehensive treatments of cloud microphysics and radiative interactions. The single-column model was run at the ARM Southern Great Plains, Tropical Western Pacific, and North Slope of Alaska sites using forcing data derived from operational numerical weather prediction models. Our results indicate that atmospheric radiative fluxes are sensitive to the scheme used to specify the ice particle effective radius by up to 30 W m-2 on a daily time scale and up to 5 W m-2 on a seasonal time scale. Differing treatments of ice particle fallout have a significant effect on the amount and location of high cirrus clouds. An unexpected finding was that the variance of the modeled ice particle effective radius at a given level is considerably smaller than that suggested by ARM cloud radar measurements. Our results indicate that this theoretical underestimate of the ice particle effective radius variance can have effects on modeled radiative fluxes comparable in magnitude to those cited above for sensitivity to the mean values of ice particle effective radius.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AGUFM.A61C0095I
- Keywords:
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- 0320 Cloud physics and chemistry;
- 0360 Transmission and scattering of radiation;
- 1610 Atmosphere (0315;
- 0325);
- 3359 Radiative processes