Orographic Effects on the Propagation and Rainfall Modification Associated with the 2007-08 Madden-Julian Oscillation Over the New Guinea Mountains

In this study, orographic effects on the propagation and rainfall modification mechanisms of the MJO 2007-08 when it passes over the New Guinea Mountains are investigated. This event is simulated by the Advanced Research Weather Research and Forecasting (WRF) model with a single domain of 5 km grid resolution. The numerical simulation with higher resolution can capture the MJO than some previous numerical studies with lower resolutions. The simulated results are then analyzed, which compare well with observations. It is found that the MJO goes through three stages when it passes over the New Guinea Mountains, namely blocking, splitting and merging stages. In the blocking stage, the convection associated with the MJO is weakened and the MJO is stalled by the New Guinea Mountains during which the eastward propagation is not continuous, and the rainfall is significantly reduced. During the splitting stage, the MJO is divided into two parts while the overall convective system moves southeastward across the island. In the merging stage, the split convective systems come together and continues moving eastward after passing over the New Guinea Mountains.