Intermittent Turbulence and Oscillations in the stable boundary layer: a System Dynamics Approach

The stable boundary layer (SBL) is often characterised by turbulence which is not continuous in space and time. This so-called intermittent turbulence may affect the whole depth of the SBL. In the past this intermittency phenomenon has been studied mostly numerically. In this study we use both a numerical as an analytical approach.

We focus on clear and stable nights over land formed by cooling due to the strong surface radiation. In such cases often stability develops faster than shear, which causes the Richardson number to increase. As a consequence air becomes decoupled from the surface. Soon, however, air will be accelerated by the omnipresent pressure force until shear is strong enough to break down the stability causing a turbulence burst. Because of this strong mixing, shear is rapidly reduced and stability takes over. The situation has returned to its begin and the mechanism starts over again, causing intermittent bursts of turbulence.

We use a simple physical model to describe the above mentioned mechanism. The results are supported by the intermittent behaviour observed in the SBL at different locations (Cabauw, WINTEX, CASES).

Subsequently, the model was studied analytically from a system-dynamics point of view. By doing so, a dimensionless parameter was found which appears to determine the equilibrium behaviour of the model (e.g. intermittent behaviour or 'stable' behaviour). This critical parameter was found to be merely a function of external 'driving agencies' such as pressure gradient, cloud cover and soil roughness. As such this new parameter may provide a new framework for future studies on SBL behaviour.