Mechanism Affecting the Regime Transition in the Stable Boundary Layer: A Single Column Model Study

Under very strong stratification it is well known that the numerical weather prediction models (NWPM) have great difficulties to reproduce the behavior of the flow of the atmospheric boundary layer. Generally, the stable boundary layer (SBL) shows different patterns for weak and strong dynamic stability, which characterizes a bi-stable equilibrium. When the turbulence is weak the first levels of the SBL can get disconnected from the upper levels, and flow variables would exhibit large vertical gradients. On the other hand, if turbulence is strong the levels stay connected and the flow becomes vertically homogeneous. The flow can switch from disconnected to connected if turbulence intensity increases, and it occurs in a suddenly way. In this work we investigate how the external parameters, i.e. wind and surface cooling controls the turbulence intensity near the ground, by the usage of a single column model. In the simulations, two different orders of closure are compared, and the dependence with the stability was prescribed by classical stability functions. The results show that the model is able to reproduce the transition, and sharp transitions are observed when a short-tail stability function are used. Furthermore, the role that turbulence and wind speed, near the surface, have over the boundary layer height is also investigated.