Role of radiative transfer in the modeled mesoscale development of summertime arctic stratus

Improvements have been made in the treatment of radiation in Version 5 of the Pennsylvania State University-National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5) to simulate boundary layer stratus observed during the summertime Arctic Stratus Experiment of 1980. Shortwave radiation is treated using a two-stream, delta-Eddington approximation developed for Version 2 of the NCAR Community Climate Model. This code offers many improvements over the original radiative transfer code developed by Dudhia [1989], including a more detailed treatment of surface albedo, solar absorption by ozone, and improved treatment of liquid, ice, and mixed-phase clouds. Longwave radiative calculations are performed with the broadband radiative transfer code currently employed in Version 3 of the European Centre for Medium-Range Weather Forecasts (EC3) model. Improvements offered by EC3 include longwave radiative absorption by ozone and trace gases and the explicit radiative treatment of mixed-phase clouds. The importance of radiation in the formation of low-level Arctic stratus and the evolution of an anticyclone is illustrated with three simulations: (1) baseline simulation using the Dudhia [1989] radiative transfer, (2) improved radiative transfer (i.e., CCM2 shortwave and EC3 longwave), and (3) radiative transfer neglected. The area extent of low clouds is reduced toward observed values when improved radiative transfer is implemented. The temperature, moisture, and cloud water profiles show significant sensitivity to the treatment of radiative transfer as well. Comparisons between observations and model results show that the new radiation package improves the area extent of cloud cover and the quality of the simulated surface radiative fluxes. The importance of radiative cooling in the evolution of a Beaufort Sea anticyclone is demonstrated.

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