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

Abstract:

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, f_scale, is investigated as a candidate strategy to remedy these biases by compensating for CRM-trapped subsidence. Significant sensitivity to f_scale is found in the 10-year climatology of a superparameterized version of NCAR Community Atmosphere Model 3.0. Higher f_scale improves both long wave cloud forcing (LWCF) and short wave cloud forcing (SWCF) in a systematic manner that scales quasi-linearly with increased f_scale. In addition, a systematic quasi-linear mid-tropospheric warming associated with faster updraft is observed with higher f_scale. These improvements suggest f_scale 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 f_scale, need be further investigated.