Observations of turbulence in the residual layer

We report results of a study of fine-scale turbulence structure in the portion of the nocturnal boundary layer known as the residual layer (RL). The observations were taken with the University of Colorado's CIRES (Cooperative Institute for Research in the Environmental Sciences) Tethered Lifting System (TLS), during the Cooperative Atmosphere-Surface Exchange Study 1999 (CASES-99) field experiment. We use data obtained by the TLS on two nights. The results show:

1) The RL may exhibit significant turbulence reaching intensities comparable to those in the underlying stable boundary layer. Such observations alter the current picture of the RL from that of a quiescent region with predominantly laminar flow to one of a dynamic region with rapidly changing small-scaled variations of turbulence.

We hypothesize that enhanced turbulence is related to upward-propagating atmospheric gravity waves generated by stably stratified flow over the low-relief terrain. During one night a presence of critical layers in the RL, generated by wind turning with height, caused deposition of upward-propagating wave momentum resulting in generation of intermittent turbulence.

2) The well-accepted concept of turbulence generation below a critical value of the gradient Richardson number (Ri) is, in fact, scale dependent: Ri values typically decrease with decreasing vertical scale size, so that critical Ri values (»0.25) occur at vertical scales of only a few tens of meters. This poses a problem for incorporating experimentally determined Ri values in model closures on models that have poor vertical resolution.

3) There appears to be two distinct turbulence ‘regimes' in the RL: a very weak but ever-present background turbulence level with minimal temporal and spatial structure, and a more intense, intermittent regime during which turbulent intensity levels can approach nighttime turbulent intensities near the surface.