Effect of Scale Coupling Frequency on Simulated Climatology in the Uncoupled Superparameterized Community Atmosphere Model v. 3.0

Monday, 15 December 2014
Sungduk Yu, University of California Irvine, Irvine, CA, United States and Michael S Pritchard, University California Irvine, Carlsbad, CA, United States
Recent attempts to accelerate cloud superparameterization for climate simulation by using reduced cloud resolving model (CRM) extents unsatisfyingly amplify upper tropospheric temperature biases and liquid water condensate. The effect of scale coupling frequency, fscale, is investigated as a candidate strategy to remedy these biases by compensating for CRM-trapped subsidence. Significant sensitivity to fscale is found in the 10-year climatology of a superparameterized version of NCAR Community Atmosphere Model 3.0. Higher fscale improves both long wave cloud forcing (LWCF) and short wave cloud forcing (SWCF) in a systematic manner that scales quasi-linearly with increased fscale. In addition, a systematic quasi-linear mid-tropospheric warming associated with faster updraft is observed with higher fscale. These improvements suggest fscale is a useful tuning parameter in superparamtereized global climate models to improve mean state cloud forcing biases and can help remedy the too cold mid-troposphere thermal bias associated with the trapped subsidence when using reduced CRM setups. However, inconsistent LWCF response with ice water path (IWP), e.g. lower LWCF with more IWP, and confounding surface flux responses, e.g. increasing surface flux with fscale, need be further investigated.