TKE budget and vertical structure of intermittent turbulent bursts: modelling and observations

Although the occurrence of intermittent turbulence is quite common under stable conditions, very little is known regarding their structure and the processes that create, destroy and transfer turbulent kinetic energy during intermittent bursts. One of the reasons for that is the difficulty in reproducing such intermittent behavior numerically. Typical models fail to do so because they are not able to recreate turbulence once it has decayed. In this study, we use a numerical model that is capable of simulating intermittent turbulence bursts to analyze their vertical structure and the TKE budget during their occurrence. The model is based on prognostic equations for the wind components, air and ground temperature and TKE. Two types of intermittent bursts are identified. The first one corresponds to weak events, originated at the surface. Such epysodes are initiated by local shear increase near the surface and are restricted to a shallow layer, never propagating to the upper stable boundary layer. The second type refers to strong events that are observed at the entire depth of the boundary layer. These events are originated at the boundary layer top, and propagate downward. In these cases, the upper boundary layer Richardson number is barely supercritical, so that a subtle temperature fluctuation is capable of starting the turbulence mechanical production. From that point, a series of interactions between the TKE budget terms is started, amplifying the event intensity as it propagates downward. Such interactions are discussed in detail in the present study. Finally, it is given observational evidence that both types of intermittent events simulated are commonly observed in stable atmospheric boundary layers.