A new stable boundary-layer mixing scheme and its impact on the simulated East Asian summer monsoon

This paper investigates the impact of stable boundary-layer (SBL) mixing in a vertical diffusion package on the simulated climatology in a regional model. In contrast to previous studies, we focus on the sensitivity of the simulated climatology to the representation of SBL processes in the modelled atmosphere, paying particular attention to precipitation and associated large-scale patterns. The new SBL scheme, based on the bulk Richardson number between the surface layer and the top of the boundary layer and implemented in the Yonsei University (YSU) boundary-layer scheme, was evaluated against the local scheme in which the mixing coefficient is a function of the local Richardson number at a given model level. A statistical evaluation of a series of short-range forecast confirms that the boundary-layer structure is closer to the radiosonde observation when the new SBL scheme is used. In a regional climate framework, the results with the new SBL scheme in July 2006 demonstrate that modulating the subcloud structure with enhanced vertical mixing improves the simulated monsoon climatology by displacing the monsoonal precipitation southwards. Together with the local effects of the enhanced SBL mixing that warms and dries the boundary layer, the dynamical feedbacks accompanying strengthened moisture convergence results in enhanced precipitation towards what was observed. A ten-member ensemble of three-month June–July–August simulations for 1999–2008 shows that the revised SBL scheme improves the temperature and moisture profiles in the lower troposphere as well as the precipitation climatology. The interannual variation of seasonal precipitation is more realistic over both land and oceans. Copyright c ⃝ 2010 Royal Meteorological Society