Bimodal Representation of Convection for Extended Range Prediction with a Modified Kain-Fritsch Cumulus Scheme

A bimodal representation of cumulus convection is presented in a model hindcast study of two Madden Julian Oscillation (MJO) events that occurred in November 2011 during the Dynamics of the MJO (DYNAMO) research program observational period. The convection scheme modifies the Kain-Fritsch scheme treatment by adopting an assumed quasi-balance of updraft parcel buoyancy at the cloud base level. To improve the performance of the scheme for tropical convection as occurred in the DYNAMO case examined here, several further enhancements are described, including addition of a treatment of convective momentum transport, a modified representation of the rate of updraft-environment mixing, and formulation of the scheme as a two-component treatment, including a representation of convection forced by boundary layer turbulent plumes. Sensitivity tests designed to investigate the significance of various aspects of the scheme are carried out using a fully coupled ocean (HYCOM) / atmosphere (NAVGEM) / sea ice (CICE) model under development by the U.S. Navy as part of the National Earth System Prediction Capability (ESPC) effort. No single feature of the modified scheme was identified as key to representing the MJOs in this study; but rather the various modifications contributed in different ways to achieve a significantly improved representation of the MJO events.