The nocturnal boundary layer transition from weakly to very stable. Part II: Numerical simulation with a second-order model

Observations of the vertical and temporal structure of the nocturnal boundary layer before and after a transition from the weakly to the very stable regime have been presented in Part I. Here, similar transitions are investigated using a one-dimensional second-order closure numerical model, with an energy budget solved at the surface. The transition is driven by a decreasing mean wind at the top of the domain, and simulations with different cloud covers and surface thermal properties are considered. The time of the transition depends on the wind speed at the top of the domain and on the "coupling strength" between the surface and the atmosphere, which is affected by the cloud cover and surface thermal properties. The vertical profiles and temporal evolutions of the terms of the budgets of turbulent kinetic energy (TKE), heat flux and temperature variance are presented. Of these, only TKE budget presents the same dominant terms in both regimes. Absolute heat flux in the model is proportional to the cube of the wind speed in the very stable regime.