A Numerical and Analytical Study of Intermittency in the Stable Boundary Layer

In this study we investigate the intermittent behaviour of the stable boundary layer, i.e. the phenomenon where the atmosphere switches, both locally and temporally, between states of appreciable turbulent intensity and vanishing turbulent intensity. The mechanism behind this phenomenon is currently not well understood.

We model the boundary layer as a simple plane channel geometry and propose a surface model in which the surface-atmosphere interactions are modelled based on the surface energy equation. The time-dependent boundary condition for temperature is determined by this model. We study the behaviour of the coupled land-atmosphere model by two different numerical approaches: 1) with a 1D-model, in which the turbulence is parameterised using a standard Richardson-closure and 2) with a 3D Direct Numerical Simulation (DNS) model in which the turbulence is fully resolved but at a low Reynolds number.

In the 1D model intermittent behaviour is obtained for a large range of parameters. Hence, the coupling with the surface model provides the required complexity to allow intermittent dynamics in the boundary layer. The influence of the surface is clear from the surface fluxes.

With the DNS model, intermittency was not obtained. Once collapsed, the turbulent state does not recover within a reasonable time frame. Applying linear stability analysis on the obtained DNS profiles of velocity and temperature, we find that the Reynolds number plays a decisive role: in the DNS the Reynolds number is too low to allow for (linear) instabilities at the Richardson numbers considered. However, the analytical method paves the way to finding those conditions for which instabilities, ultimately leading to intermittent behaviour, take place in the DNS.