Intermittent mixing observed in the nocturnal surface layer

How often does mixing occur in the stable surface layer? Stable stratification indicates that turbulent energy for mixing is found away from the surface. Nappo (2002) argued that the energy extracted from some 'base' flow, is a consequence of breaking gravity waves. McNider (1995) noted that the equations describing the stable boundary layer used in mesoscale models allow for intermittent mixing, showing that idealized equations for stable boundary layer flows allow for two solutions. One is cooler, with limited mixing; the second the warmer, characterized by enhanced mixing. Do these theoretical constructs reflect the 'weakly stable' and 'strongly stable' surface layer states identified by Mahrt (1998)? Alternatively, these states may refer more properly to area-averaged properties, not what one might measure at an individual site. Acevedo and Fitzjarrald (2003) presented observations demonstrating that mixing occurs primarily at "active surface sites", usually at exposed locations at higher elevation. At sheltered locations few if any mixing events were observed.

We examine observations made during the Hudson Valley Ambient Meteorology Study (HVAMS). The HVAMS surface network consisted of nine flux-measuring weather stations (NCAR ISSF), 5 standard weather stations as well as a number of operational weather stations. At six stations along the valley microbarometers were operated. At three sites, additional ozone and CO2 sensors were installed. These interactions that produced these events may have little to do with the local stability at a given point. We identify nocturnal mixing episodes at each station during the six weeks of the intensive field program and examine what differences among results are a consequence of local site characteristics. By reference to the microbarometer sensor array, we seek evidence that breaking gravity waves might have provoked the mixing.