PNNL

Abstract

This study reports the Single-Column Model (SCM) part of the Atmospheric Radiation Measurement (ARM) / the Global Energy and Water-cycle Experiment (GEWEX) Cloud System Study (GCSS) joint SCM and Cloud-Resolving Model (CRM) Case 3 intercomparison study, with a focus on evaluation of cumulus parameterizations used in SCMs. Eight types of state-of-the-art cumulus parameterizations used in thirteen SCMs are evaluated under summertime midlatitude continental conditions using data collected at the ARM Southern Great Plains (SGP) site during the summer 1997 Intensive Operational Period (IOP). Cumulus mass fluxes estimated from ten CRMs are also used to diagnose problems in the SCMs. It is found that the cumulus parameterizations based on spectral cloud ensemble models are generally more active than those based on a bulk cloud model over midlatitude lands. Differences in the approaches that are used in determining the atmospheric instability in these two types of schemes are thought to be the main reason. Most models with the former category of the parameterizations consistently show large systematic warm/dry biases in the troposphere. The errors are relatively small in those with the bulk mass flux schemes except for a model with a non-penetrative convection scheme, which shows large systematic cold/moist biases in the troposphere. It is also shown that surface precipitation rates produced by the models are mainly from cumulus clouds and surface stratiform precipitation rates are significantly underestimated. Finally, sensitivity of the results to different forcing approaches is briefly discussed. Results from the National Center for Atmospheric Research?s Community Climate Model (CCM3) SCM with improved triggering and closure assumptions are also discussed.