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

Abstract:

This work seeks for the most effective parameters in a deep convection scheme (relaxed Arakawa–Schubert convection scheme) of the National Centers for Environmental Prediction Climate Forecast System model for the improved simulation of the Madden–Julian Oscillation (MJO). A suite of sensitivity experiments are performed in a coupled free run format. Two parameters are found to lead to a significant improvement: a threshold of relative humidity of the boundary layer and a fraction of re-evaporation of convective precipitation. Increasing the strength of these two parameters increases a supply of water vapor and condensate from deep cumulus ensembles to environment in the lower troposphere (especially, 700 hPa), and reduces light rainfall amount that prevents complete organization of convective system. Under a more humid environment (or positive moisture–convection feedback), the space–time spectral signal, eastward propagation, tilted vertical structure of dynamic and thermodynamic variables associated with the MJO are more comparable to observations and a recharge–discharge theory.