Improvement of the Mellor-Yamada Type Planetary Boundary Layer Scheme for Use in Mesoscale Models

Abstract

Atmospheric mesoscale models are highly complex and their performance varies widely depending on the models used. Turbulent transport within the boundary layer is especially difficult to analyze, but has a significant impact on mesoscale model applications. In this study, the Mellor-Yamada-Nakanishi-Niino planetary boundary layer model is improved. A new length scale and turbulent closure constants are calculated from two large eddy simulations. The modified MYNN model maintains the original’s accurate eddy coefficients and drastically improves the prediction of the momentum dissipation rate, length scale and stability functions. A 12-member WRF ensemble is used to validate the new model outside of the database on which it is based. The WRF results show that the new model improves the bias and mean absolute error of temperature and relative humidity. There is a significant change in the TKE and length scale predictions that motivates further study of the modified boundary layer scheme.