Multiscale Interactions in MJO Structure and Evolution: DYNAMO Field Campaign Case Study Using a Coupled WRF Model

Madden Julian Oscillation (MJO) manifest itself as a large precipitation envelop propagating slowly eastward at an average speed of around 5m/s. It has long been observed that a myriad of smaller scale precipitation structures are embedded/modulated by the MJO envelop, including eastward propagating Kelvin waves, westward propagating Equatorial Rossby waves and two-day waves. Observations also show stronger signals for eastward propagating waves than westward ones. Within the high-frequency disturbances, convection often organize themselves into mesoscale convective systems with different lifetimes and propagation directions. How these small-scale structures interact with, and contribute to MJO formation mechanisms remains unknow. These multiscale structures are simulated using the convection-resolving Weather Research and Forecasting (WRF) model coupled with the Regional Ocean Modeling System (ROMS), using case studies during the Dynamics of MJO (DYNAMO) field campaign in October and November, 2011. Compared with the Tropical Rainfall Measurement Mission (TRMM) satellite observations, the model simulations show two major discrepancies: 1) It rains too much/too easily during the MJO suppressed phase; 2) The westward propagating signals are too strong. These discrepancies show up more prominently over the Indian Ocean than at Maritime Continent. In this study, we carry out model sensitivity studies to systematically test relationships between MJO propagations, its internal structures, and its environments. It is found, from the strongest to weakest influence, that radiation interactions, mean-state grid nudging, surface fluxes, and microphysics impact MJO structures and mean surface rainfall. These sensitivities are tied together through heat and moisture balances in the atmosphere.