A Non-Dimensional Index for Characterizing the Transition of Turbulence Regimes in Stable Atmospheric Boundary Layers

The transition from moderate to weak turbulence regimes remains a grand challenge for stable boundary layer parameterizations in weather and climate models. In this study, a critical horizontal Froude number (approximate to 0.28) is proposed to characterize such a transition, which corresponds to the development of quasi two-dimensional pancake vortices. Traditionally defined stability parameters corresponding to the critical horizontal Froude number are estimated and are consistent with values in the literature. The critical horizontal Froude number can recover previously used height- and site-dependent mean wind speed thresholds. These findings offer a way to constrain the validity range of Monin-Obukhov similarity theory in numerical models for weather and pollutants dispersion. Plain Language Summary The transition from moderate to weak turbulence regimes in stable atmospheric boundary layers presents a grand challenge for numerical models. Our results show that such a transition occurs when the horizontal Froude number is equal to a critical value of about 0.28, which is physically connected to the development of quasi two-dimensional pancake vortices. These findings can help improve turbulence parameterizations in numerical models for weather and pollutants dispersion.