Antarctic Clouds and Radiation within the NCAR Climate Models

Abstract

To evaluate and improve the treatment of clouds and radiation by the climate models of the National Center for Atmospheric Research (NCAR), simulations by the NCAR Community Climate Model version 3 (CCM3), as well as the recently released Community Atmosphere Model version 2 (CAM2), are examined. The Rasch and Kristjánsson prognostic cloud condensate scheme, which is now the standard scheme for CAM2, is included in a version of CCM3 and evaluated. Furthermore, the Rapid Radiative Transfer Model (RRTM), which alleviates the deficit in downward clear-sky longwave radiation, is also included in a version of CCM3. The new radiation scheme in CAM2 also alleviates the clear-sky longwave bias, although RRTM is not included. The impact of the changes is especially large over the interior of Antarctica. The changes induced by the introduction of the prognostic cloud scheme are found to have a much larger impact on the CCM3 simulations than do those from the introduction of RRTM. The introduction of the prognostic cloud scheme increases cloud emissivity in the upper troposphere, reduces cloud emissivity in the lower troposphere, and results in a better vertical distribution of cloud radiative properties over interior Antarctica. The climate simulations have a very large cold bias in the stratosphere, especially during summer. There are significant deficiencies in the simulation of Antarctic cloud radiative effects. The optical thickness of Antarctic clouds appears to be excessive. This contributes to a warm bias in surface temperature during winter and a deficit in downward shortwave radiation during summer. Some biases for Antarctica are larger for CCM3 with the prognostic cloud condensate scheme than with the standard diagnostic clouds. When the mixing ratio threshold for autoconversion from suspended ice cloud to falling precipitation is reduced toward a more realistic value, the Antarctic clouds are thinned and some of the biases are reduced. To improve the surface energy balance, not only must the radiative effects of clouds be improved, it is also necessary to improve the representation of sensible heat flux. Insufficient vertical resolution of the frequently very shallow, very stable surface boundary layer apparently contributes to an excessive heat flux from the atmosphere to the surface during winter. The representations of Antarctic clouds and radiation by the new NCAR CAM2 are not clearly improved compared to those of the earlier CCM3. For example, the surface albedo over Antarctica is descreased in CAM2 and Community Climate System Model version 2 (CCSM2) simulations in comparison to CCM3 simulations, contributing to a summer warm bias in tropospheric temperature for the former.