How is the Two-Regime Stable Boundary Layer Reproduced by the Different Turbulence Parametrizations in the Weather Research and Forecasting Model?

Five planetary-boundary-layer parametrizations of the Weather Research and Forecasting model are compared with respect to their ability to simulate the very stable and the weakly stable regimes of the stable boundary layer. This is performed for single column models where the large-scale mechanical forcing is represented by geostrophic wind speeds ranging from 0.5 to 12 m $backslash$$$backslash$$$$backslash$backslash$hbox {s}$backslash$^{-1}$backslash$$$backslash$$s-1. The performance of the models is assessed by contrasting the relationships they produce between the turbulence velocity scale and the mean wind speed, between potential temperature gradient and the mean wind speed, and between the flux and gradient Richardson numbers. The level-2.5 Mellor--Yamada--Nakanishi--Niino parametrization simulates the very stable regime the best, mainly because its heat eddy diffusivity decreases with respect to the momentum eddy diffusivity as the stability increases, while the same is not true for the other parametrizations considered.