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Recent observational data (turbulence variables by sonic anemometers and three-dimensional flow pattern by Doppler lidar), obtained during the Cooperative Atmosphere Surface Exchange Study field campaign in October 1999 (CASES-99), show evidence of a layered structure of the near-neutral surface layer: (i) the eddy surface layer (ESL), which is the lower sublayer where blocking of impinging eddies is the dominating mechanism; and (ii) the shear surface layer (SSL), which is an intermediate sublayer, where shear affects the isotropy of turbulence. The origin of the eddies impinging from aloft (probably from the SSL) down to the ESL is preliminarily addressed in this study, since the Doppler lidar data show evidence of linearly organized eddies embedded in the surface layer (i.e., about 100-m vertical extent) and horizontally spaced by about 300 m. This is consistent with theories predicting that the primary mechanism of eddy motion in high Reynolds number wall layers is "top-down." The layered structure of the surface layer also has a visible effect on vertical profiles of vertical velocity variance (&w²macr;) and momentum transport. In the ESL, &w²macr; scales as z2/3 while it is constant or slightly decreases within the SSL. Concerning momentum transport, ejections contribute identically to the momentum flux as do sweeps in the ESL, whereas in the SSL, ejections give about 50% higher relative contribution. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]