Improvement Of The Mellor–Yamada–Nakanishi–Niino Planetary Boundary Layer Scheme Based On Observational Data In China

The Mellor–Yamada–Nakanishi–Niino(MYNN) planetary boundary layer(PBL) scheme is a second-order turbulence closure model that is an improved version of the Mellor–Yamada scheme referencing large-eddy simulation(LES). It has good performance in the simulation of PBL structures and evolution, particularly over the ocean surface. However, when used with complex underlying surfaces, overestimation of the turbulent momentum flux and the sensible heat flux occurs at several observational points. On the basis of observational data of surface fluxes in China, we attempt to improve the MYNN parameterization by tuning the parameters and representation of the turbulence scale. Closure constants and empirical expressions in the diagnostic equation are first tuned. An additional term of temperature turbulent flux is considered in the potential temperature prognostic equation to improve the surface heat flux parameterization. The modified MYNN scheme is incorporated into a three-dimensional mesoscale model and is evaluated with various underlying surface observational data. Amelioration of the surface turbulent fluxes is confirmed at five observational sites in China over different land-use types. The two heavy fog cases happened on 17 March 2009 at Yellow sea and 4 December 2011 in north China, respectively. As a result, the modified MYNN achieves better simulation of the cloud water mixing ratio in PBL. The simulated cloud water mixing ratio agrees well with observation illustrated by the FY-3 satellite imagery. More stable stratification is confirmed in the fog area with the modified scheme. The vertical profiles of potential temperature and specific humidity also show a good result in accordance with observational data.