Abstract
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.
Highlights
The Madden Julian Oscillation (MJO) is characterized by slowly eastward-propagating precipitation and circulation anomalies that are largely confined between 15°S and 15°N, which result in 20–70 days oscillations in the tropics
Wang and Lee (2017) revealed that the eastward propagation in global climate model (GCM) is led by the lower tropospheric heating and eddy available potential energy (EAPE) generation to the east of MJO precipitation center, suggesting that MJO simulation in GCMs may critically depend on the diabatic heating released in shallow and congestus clouds
The diagnostics include (1) the horizontal structure of boundary layer moisture convergence (BLMC) that leads the propagation of MJO precipitation by about 5 days; (2) the horizontal structure of 850 hPa zonal wind and its equatorial asymmetry (Kelvin wave easterly vs. Rossby wave westerly intensity), which result from the interaction between wave dynamics and convective heating; (3)
Summary
The Madden Julian Oscillation (MJO) is characterized by slowly eastward-propagating precipitation and circulation anomalies that are largely confined between 15°S and 15°N, which result in 20–70 days oscillations in the tropics. Using a general trio-interaction model with the simplified Betts–Miller cumulus parameterization scheme (Wang and Chen 2017), it was shown that the BL dynamics is a primary driver for the eastward propagation of the MJO system, and the moisture feedback and wave feedback can slow down the eastward propagation. Wang and Lee (2017) revealed that the eastward propagation in GCMs is led by the lower tropospheric heating and EAPE generation to the east of MJO precipitation center, suggesting that MJO simulation in GCMs may critically depend on the diabatic heating released in shallow and congestus clouds. The modified Tokioka constraint will be used to improve the MJO simulation without degrading other aspects of the global climate simulation under the singlecloud convective scheme. The sea ice model resolution is about 1° latitude by longitude with four sea ice layers and one snow layer on the top of the ice surface
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