Extended Abstract (120K)10.3
Intermittent turbulence and oscillations in the stable boundary layer: a system dynamics approach
B. J. H. Van de Wiel, Wageningen University, Wageningen, Netherlands; and A. F. Moene, O. K. Hartogensis, R. J. Ronda, H. A. R. De Bruin, and A. A. M. Holtslag
The stable boundary layer (SBL) is often characterised by turbulence which is not continuous in space and time. This socalled intermittent turbulence may affect the whole depth of the SBL. In this study intermittent turbulence is studied from both theoretical and experimental point of view. The study is restricted to the type of intermittency which is caused by the so-called Businger-Blackader mechanism. According to this mechanism the following picture emerges:
During clear and stable nights often stability develops faster than shear due to the strong surface radiation. This causes the Richardson number to increase, leading to cessation of turbulence. 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 strong mixing shear is rapidly reduced and stability takes over. Now the situation has returned to its begin and the mechanism starts over again, causing intermittent bursts of turbulence.
In this study we seek for a theoretical foundation and an experimental validation of this mechanism. In the theoretical part of the study, the mechanism described above is simplified to its physical essence. For a certain parameter range the outcome from the numerical runs shows intermittent behaviour. Furthermore this model is studied analytically from a system-dynamics point of view. By doing so a dimensionless parameter is found which determines the equilibrium behaviour of the model (e.g. intermittent or non-intermittent behaviour). This critical parameter is merely a function of external ‘forcings’ such as pressure gradient, cloud cover and soil roughness.
The experimental part of our study on intermittent turbulence was tackled during an extensive cooperative nocturnal boundary layer experiment( CASES99) in Kansas, USA. Apart from conventional eddy correlation systems, two types of scintillometers were used; a large aperture and a split-beam laser scintillometer. These instruments provide directly an areally averaged flux which has the main advantage of interchanging of space over time averaging. This allows shorter averaging times of fluxes, which is a major advantage in the non-stationary conditions encountered of the SBL.
Our results indicate that the intermittency mechanism described above is indeed a likely candidate to explain intermittent turbulence in the stable boundary layer over land.
Session 10, Stable BLs - I
Thursday, 18 July 2002, 8:30 AM-9:59 AM
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