Single-column models, arm observations, and GCM cloud-radiation schemes

Among the most serious sources of uncertainty in current general circulatio n models (GCMs) is the treatment of clouds and cloud-radiation interactions . We have used a single-column model (SCM) diagnostically to evaluate param eterizations against observations from the Atmospheric Radiation Measuremen t (ARM) Program. We find that schemes with explicit cloud water budgets and interactive radiative properties are potentially capable of matching obser vational data closely. In our SCM, using an interactive cloud droplet radiu s decreases the cloud optical thickness and cloud infrared emittance of hig h clouds, which acts to increase the downwelling surface shortwave flux and the outgoing longwave radiation. However, it is difficult to evaluate the realism of the vertical distribution of model-produced cloud extinction, cl oud emittance, cloud liquid water content and effective cloud droplet radiu s until high-quality observations of these quantities become more widely av ailable. We also find that in the SCM, cloud parameterizations often undere stimate the observed cloud amount, and that ARM observations indicate the p resence of clouds while the corresponding maximum relative humidity is less than 80%. This implies that the underlying concept of a critical gridpoint relative humidity of about 80% for cloud formation, as used in many GCM cl oud parameterizations, may need to be reexamined. (C) 1999 Elsevier Science Ltd. All rights reserved.