Intermittent Turbulence in Stable Boundary Layers and Its Influence on Turbulence Parameterization

We have examined all the possible mechanisms that led to intermittent turbulence during CASES-99. We categorize them as 1) decoupling due to ground radiative cooling, 2) internal gravity waves, 3) drainage flow, 4) downward transport of LLJ, and 5) frontal passage. We found that all these intermittent events occurred when the 60-m tower wind was less than 4-6 ms-1. Among the listed mechanism, internal gravity waves (IGW) led to about two thirds of the observed intermittent turbulent events. Due to vertical variations of Richardson number, turbulence triggered by IGW can come from all directions and appear in the middle of the observation layer, or close to the ground, or at the bottom and the top of the observation layer with no turbulence in the middle. Under intermittent turbulent conditions, flux-gradient relationship highly depends on averaging strategies. We will present our latest results on flux-gradient relationships under intermittent turbulence conditions.