Fine-Scale Turbulent Bursts in Stable Atmospheric Boundary Layer in Complex Terrain

The Stable Atmospheric Boundary Layer (SBL) is one of the least understood of all boundary layers, due mainly to the complex set of phenomena it endures. Typical measurements of SBL have been conducted using point measurement devices such as sonic anemometers or using remote sensors, but the finest resolution permissible by these instruments is ~ 10cm. In the data analysis, smaller scale phenomena are neglected except the turbulent kinetic energy dissipation, which is estimated by fitting standard Kolmogorov spectral form to the inertial subrange. Given the typical small integral scales of stratified turbulence (~ 1m), it has long been argued that the missing flux contributions due to low resolution of sonic measurements can be substantial. The hot-film anemometry can be employed to capture the missing scales, which measures down to the Kolmogorov dissipation scales, but the field deployments of hot-films are cumbersome due to the need for frequent calibrations and probe alignment with the mean flow. To this end, during the field campaigns (September 25-October 31, 2012; May 1-30, 2013) of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program, a sonic and hot-film anemometer dyad (a combo probe) was employed, with the hot-film located on a gimbal within the sonic probe volume, allowing the hot-film to rotate and align with the mean flow based on a feedback signal provided by the mean velocity vector measured by the sonic.

The deployments over the extensive MATERHORN observations period provided an opportunity to observe turbulence over a vast range of background flow and stratification conditions. Of particular interest was a stable stratified period over which the flow slowly transitioned from an almost laminar flow regime to a fully developed turbulent flow regime. During this phase, the combo probe revealed an intriguing phenomenon - the occurrence of strong bursting events at finer scales that generates turbulence. In the absence of bursting, say in convective boundary layer periods or low stabilities, the spectra takes the form of classical Kolmogorov turbulence, while the presence of bursting leads to a spectral shape resembling that of the bottleneck effect, where a bump in the spectrum appears between inertial and dissipation subranges. A possible explanation is provided for these observations, and implication of results in SBL studies are described in this paper.