PNNL

Abstract

The realism of the Madden-Julian Oscillation (MJO) produced by the MM5 regional model is investigated using systematic and standardized statistical diagnostics. This study expands on previous work by choosing a larger domain and a different input dataset (ECMWF ERA-40 reanalysis) to force the initial and boundary conditions of the model. Results show that upper- and lower-level zonal winds display the correct MJO structure, phase speed (8 m s-1) and space-time power spectrum. However, the simulated free atmosphere moisture, Outgoing Longwave Radation (OLR) and precipitation do not exhibit any clear MJO signal. Yet, the boundary layer moisture, the moist static energy and the atmospheric instability, measured using a moist static energy instability index have clear MJO signals. A significant finding of this study is the ability of MM5 to simulate a realistic MJO phase speed in the winds without reproducing the MJO wind-convection coupling or a realistic propagation in the free atmosphere water vapor. This study suggests that the convergence of boundary layer moisture and the discharge and recharge of the moist static energy and atmospheric instability may be responsible for controlling the speed of propagation of the MJO circulation.