Sensitivity experiments on the role of moisture in the eastward propagation of MJO

In this study, we employ the nudging assimilation in the Weather Research and Forecasting (WRF) model to conduct a set of sensitivity experiments on the role of moisture in the Madden–Julian Oscillation (MJO) eastward propagation, focusing on the typical case of 30–60 days filtered component in the tropical atmosphere from the Indian Ocean to the western Pacific Ocean during September–November 2004. Using 11 different cumulus parameterization schemes, the simulation results show that the ability of the regional climate model in simulating the MJO eastward propagation is sensitive to the cumulus parameterization schemes: a suitable scheme can well reproduce the MJO eastward propagation characteristics, while most schemes show no skill for the MJO eastward propagation. When the simulated moisture (specific humidity) in the model domain is nudged toward the observational values, the low-frequency evolution of the tropical zonal winds in the lower troposphere exhibit MJO features well, and the low-frequency phase of moisture is ahead of the zonal winds by about 6–7 days, which suggests that the atmospheric moisture distribution is the key factor for the eastward propagation of the MJO, and the effect of tropical wet convection is taken via affecting the atmospheric instability. However, when the atmospheric temperature assimilation is conducted, there is less improvement in the skill of MJO simulation in the lower troposphere than the moisture nudging assimilation.