Improving MJO simulation by enhancing the interaction between boundary layer convergence and lower tropospheric heating

It has been a great challenge for global weather and climate models to simulate realistic Madden–Julian Oscillation (MJO) while keeping global energy and water balance unaffected. This work demonstrates that, in the Nanjing University of Information Science and Technology Earth System Model, enhanced boundary layer (BL) convergence feedback to the lower tropospheric heating in both the modified Tidtke (TDK) and relaxed Arakawa–Schubert (RAS) convective schemes have significantly improved the quality of MJO simulation in terms of both the eastward propagation and three-dimensional dynamic and thermodynamic structures. The modifications to the TDK and RAS schemes include (a) a BL depth-dependent convective inhibition, and (b) a bottom-heavy diffusivity in the shallow convection scheme. To understand how these modifications improved the MJO simulation, we applied dynamics-oriented diagnostics to reveal the critical role of the interaction between the lower-tropospheric heating and the BL convergence. The modified schemes enhance the lower-tropospheric diabatic heating to the east of the MJO convective center, which leads to increased Kelvin wave easterly winds. The strengthened MJO easterly winds reinforce the BL moisture convergence to the east of the MJO center and therefore result in increased upward transports of moisture and heat from the BL to the free atmosphere, which further moisten and destabilize the lower troposphere and thereby increase the lower-tropospheric heating. The positive feedback between the BL convergence and lower tropospheric heating improves MJO eddy available potential energy generation to the east of major convection and promotes MJO eastward propagation. The results indicate that correct simulation of the heating induced by shallow and/or congestus clouds and its interaction with BL dynamics is critical for realistic simulation of the MJO as suggested by the trio-interaction theory.