A numerical study of the October and November DYNAMO MJO events using a regional model

The first two months of the DYNAMO (Dynamics of the Madden-Julian Oscillation) field campaign observed convective initialization of two MJO events in the central equatorial Indian Ocean. To have better understanding of these MJO events, we perform large domain cloud-permitting numerical experiments using the Weather Research and Forecasting model. The model uses prescribed SST as the lower boundary condition, and the model domain covers nearly entirely tropical Indian Ocean and the maritime continent. We integrate the model continuously from the beginning of October to the middle of December 2011 at a horizontal grid spacing of 9 km with a set of numerical and physical schemes that conserve moisture and moist static energy reasonably well. Detailed comparison against observation and the reanalysis data indicates that our free simulation (without nudging or data assimilation) captures the overall development of two MJO events in many aspects, including surface rainfall, horizontal and vertical winds, and radiation fields. Analysis of the moist budget over the northern sounding array indicates a small residual, likely attributed to numerical inaccuracy of off-line calculations. Despite of overall agreement with observation, the model simulates a larger contribution of horizontal moisture advection to the moist budget than either sounding array data or the ECMWF-Interim dataset, among which the horizontal advection term also disagrees to some extent. To further explore the initialization mechanism of the two MJO events, sensitivity experiments are performed to examine the role of various physical processes, including air-sea interaction, extratropical influence, radiation, synoptic eddies, circumglobal propagating dry waves. Results from these experiments will also be discussed.