Intermittent Turbulence in Stable Boundary Layers

We investigated the intermittent turbulence during the Cooperative Atmosphere-Surface Exchange Study in 1999 (CASES-99). We found two turbulence regimes at each level: one with weak turbulence associated with weak winds, and the other one with more significant turbulence where the turbulence velocity scale increases linearly with wind speed. The two regimes are separated by a threshold wind speed, which increases with height. We found that the intermittent turbulence close to the ground is mainly due to the wind speed variation between the two regimes. However, close to the top of the 60-m tower, the wind speed exceeds the threshold value less of ten compared to the lower levels and thermal instability generated by large shear instability contributes significantly to the observed intermittent turbulence. In addition, strong winds above, such as nocturnal low-level jets, provides the ideal condition for large wind oscillations close to the ground. With weak winds aloft, large temperature oscillations, but not wind speed oscillations, were observed.