The Effects of Critical Layers on Residual Layer Turbulence

Abstract The authors report results of a study of finescale turbulence structure in the portion of the nocturnal boundary layer known as the residual layer (RL). The study covers two nights during the Cooperative Atmosphere–Surface Exchange Study 1999 (CASES-99) field experiment that exhibit significant differences in turbulence, as indicated by the observed turbulence dissipation rates in the RL. The RL turbulence sometimes reaches intensities comparable to those in the underlying stable boundary layer. The commonly accepted concept of turbulence generation below critical values of the gradient Richardson number (Rig) is scale dependent: Ri values typically decrease with decreasing vertical scale size, so that critical Rig values (≈0.25) occur at vertical scales of only a few tens of meters. The very small scale for the occurrence of subcritical Ri poses problems for incorporating experimentally determined Rig -based methods in model closures in models with poor resolution. There appear to be two distinc...