Turbulence in very stable boundary layers often exhibits singular behavior that is not observed in less stable boundary layers. The foremost characteristic of this behavior is intermittency. That is, measurements of turbulence in very stable boundary layers show long periods of quiescence punctuated by short periods of turbulent activity. How this intermittency comes about is not known. To investigate the mechanisms that cause intermittency, we have performed a Direct Numerical Simulation (DNS) of the very stable boundary layer (in particular, an Ekman layer). The DNS method resolves all the scales of the flow, and does not require a sub-grid-scale model to accounts for the smaller scales of turbulence.

Starting from a fully developed, neutrally stratified flow, we cool the lower surface of the DNS with a heat flux of sufficient magnitude to make the turbulence intermittent. An analysis of the mechanisms leading to the observed intermittency is then possible. It was found that the intermittency originates from a vigorous, inflectional instability that lifts colder air over warmer air, thus leading to a localized convective instability and a small patch of intense turbulence. This turbulence is short-lived because the lifting motion of the instability is destroyed after the turbulence is generated. In this process, an inertial oscillation plays a key role in transferring energy to the instability so that cold air can be lifted above the warm air.