Impact of different diffusion schemes on simulated rainfall: Land-ocean contrast

The impact of vertical diffusion schemes on simulated precipitation has been investigated using two diffusion schemes, the Mellor and Yamada level 2 (MY) and the Holtslag and Boville (HB) schemes in the Center for Climate System Research/National Institute for Environmental Studies (CCSR NIES) model. The results indicate the HB scheme reproduced strong vertical mixing over land, especially in summer hemisphere, which has apparent diurnal variation owing to strong vertical mixing in the convective boundary layer. The MY scheme displayed relatively weak vertical mixing over land but strong mixing over the ocean. The MY scheme tends to reproduce strongly the local mixing by wind shear compared to the HB scheme. The variations in the thermodynamic feature were due to the difference in the planetary boundary layer scheme results in the warmer and drier air over land and relatively colder and wetter air over the ocean. Such an apparent contrast over land and ocean leads to a horizontal gradient in temperature and changes the large‐scale circulation. The change in sea level pressure over the western equatorial Pacific, induced by the HB scheme, results in changes in the zonal and meridional pressure gradients between the equator and subtropical region, which is the main driving force of the low‐level divergence. A weakening of low‐level easterly winds owing to changes in the zonal and meridional pressure gradients has a great influence on the rising motion in the western Pacific, together with the low‐level moisture content advected toward the off‐equatorial area.