Atmospheric disturbances that generate intermittent turbulence in nocturnal boundary layers

Using the unprecedented observational facilities deployed during the 1999 Cooperative Atmosphere-Surface Exchange Study (CASES-99), we found three distinct turbulent events on the night of 18 October 1999. These events resulted from a density current, solitary wave, and internal gravity wave, respectively. Our study focuses on the turbulence intermittency generated by the solitary wave and internal gravity wave, and intermittent turbulence episodes associated with pressure change and wind direction shifts adjacent to the ground. Both the solitary and internal gravity waves propagated horizontally and downward. During the passage of both the solitary and internal gravity waves, local thermal and shear instabilities were generated as cold air was pushed above warm air and wind gusts reached to the ground. These thermal and shear instabilities triggered turbulent mixing events. In addition, strong vertical acceleration associated with the solitary wave led to large non-hydrostatic pressure perturbations that were positively correlated with temperature. The directional difference between the propagation of the internal gravity wave and the ambient flow led to lateral rolls. These episodic studies demonstrate that non-local disturbances are responsible for local thermal and shear instabilities, leading to intermittent turbulence in nocturnal boundary layers. The origin of these non-local disturbances needs to be understood to improve mesoscale numerical model performance.