Effective control parameters in a deep convection scheme for improved simulation of the Madden–Julian oscillation

Abstract

This work seeks to find the most effective parameters in a deep convection scheme (relaxed Arakawa–Schubert scheme) of the National Centers of Environmental Prediction Climate Forecast System model for improved simulation of the Madden–Julian Oscillation (MJO). A suite of sensitivity experiments are performed by changing physical components such as the relaxation parameter of mass flux for adjustment of the environment, the evaporation rate from large-scale precipitation, the moisture trigger threshold using relative humidity of the boundary layer, and the fraction of re-evaporation of convective (subgrid-scale) rainfall. Among them, the last two parameters are found to produce a significant improvement. Increasing the strength of these two parameters reduces light rainfall that inhibits complete formation of the tropical convective system or supplies more moisture that help increase a potential energy to large-scale environment in the lower troposphere (especially at 700 hPa), leading to moisture preconditioning favorable for further development and eastward propagation of the MJO. In a more humid environment, more organized MJO structure (i.e., space–time spectral signal, eastward propagation, and tilted vertical structure) is produced.