Development of an in-situ probe to observe fine-scale stable atmospheric boundary layer turbulence

Typical point measurements of Atmospheric Boundary Layer (ABL) turbulence are conducted using point measurement devices such as sonic anemometers or using remote sensors, however the finest resolution permissible by these instruments is ~ 10cm which does not permit the direct measurement of fine-scale parameters such as the turbulent kinetic energy (TKE) dissipation. As such, the TKE dissipation is estimated by fitting the 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 the low resolution of sonic measurements can be substantial. 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 the mean flow must be aligned with the probe. 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, where the hot-film was located on a gimbal within the sonic probe volume, which allowed 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 during 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 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 spectra 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.