We have investigated the sensitivity of modeled radiative fluxes to the specification of cloud microphysical parameterizations using a single-column model and measurements from the Atmospheric Radiation Measurement Program. The single-column model was run for the three month period of June-August, 2000 at the ARM Southern Great Plains and Tropical West Pacific sites. At each site the model was forced using 0-24 hour forecast products from the NCEP GSM. Several model runs were performed, each with a different package of cloud microphysical parameterizations. The temporal evolution of modeled cloud amount as well as surface radiative fluxes from a control run compare well with ARM measurements at the Southern Great Plains site. The realism of the control run results at the Tropical West Pacific site is difficult to evaluate due to the limited spatial resolution of ARM observations.

Our results indicate that surface and top-of-atmosphere radiative fluxes are sensitive to the scheme used to specify the ice particle effective radius. We also found that the inclusion of ice particle fallout can have a significant effect on the amount and location of high cirrus clouds. A particularly interesting finding from the model results was that the variance of the modeled ice particle effective radius is considerably smaller than that suggested by ARM cloud radar measurements at the Southern Great Plains site. This was the case for all four parameterizations of ice particle effective radius that were tested. Our preliminary results suggest that this underestimate of the ice particle effective radius variance can have significant effects on the modeled radiative fluxes due to the highly non-linear nature of cloud-radiation interactions. At the meeting we will present quantitative results from SCM simulations in addition to investigating any differences between model sensitivities at the two sites.