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Abstract Two existing moisture mode theories of the MJO, one emphasizing boundary layer moisture asymmetry (MA) and the other emphasizing column-integrated moist static energy (MSE) tendency asymmetry (TA), were validated with the diagnosis of observational data during 1979–2012. A total of 2343 MJO days are selected. While all these days show a clear phase leading of the boundary layer moisture, 20% of these days do not show a positive column-integrated MSE tendency in front of MJO convection (non-TA). A further MSE budget analysis indicates that the difference between the non-TA composite and the TA composite lies in the zonal extent of anomalously vertical overturning circulation in front of the MJO convection. A background mean precipitation modulation mechanism is proposed to explain the distinctive circulation responses. Dependent on the MJO location, an anomalous Gill response to the heating is greatly modulated by the seasonal mean and ENSO induced precipitation fields. Despite the negative MSE tendency in front of MJO convection in the non-TA group, the system continues moving eastward due to the effect of the boundary layer moistening, which promotes a convectively unstable stratification ahead of MJO convection. The analysis result suggests that the first type of moisture mode theories, the moisture asymmetry mechanism, appears more robust, particularly over the eastern Maritime Continent and western Pacific. 

Abstract A ~14‐hr long‐lived spiral rainband in Typhoon Longwang (2005) produced catastrophic rainfall in Fujian Province of China on 2 October 2005. In this study, the effects of terrain and landmass near Fujian on the structure and propagation of this rainband are investigated through high‐resolution numerical simulations. Results show that although the terrain and landmass near Fujian played a marginal role in the formation of the rainband, both greatly affected the structure and propagation of the rainband. Namely, convection in the upwind sector of the rainband tended to be maintained and locked up near the coastline in the control experiment with both the terrain and landmass near Fujian retained, but shrank more inland with the terrain near Fujian flattened, and further inland with the landmass near Fujian replaced by the virtual ocean. It is found that due to the land‐sea surface roughness contrast, the upstream tangential winds from ocean would be substantially decelerated over land and thus induced a local subgradient force onshore near the coastline. The radially inward agradient force and the subsequent surface friction helped maintain the moisture convergence, and thus convection and the cold pool in the upwind sector of the rainband near the coastline. Although the orographic lifting and blocking effects were found to be marginal to the moisture convergence in the rainband, the terrains near Fujian enhanced the deceleration of surface winds, enhancing the effect of land‐sea surface roughness contrast on low‐level moisture convergence and thus the lockup of the upwind sector of the rainband.