Structure of Turbulence in Katabatic Flows Below and Above the Wind-Speed Maximum

Abstract

Measurements of small-scale turbulence made in the atmospheric boundary layer over complex terrain during the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program are used to describe the structure of turbulence in katabatic flows. Turbulent and mean meteorological data were continuously measured on four towers deployed along the east lower slope (2-4 • ) of Granite Mountain near Salt Lake City in Utah, USA. The multi-level (up to seven) observations made during a 30-day long MATER-HORN field campaign in September-October 2012 allowed the study of temporal and spatial structure of katabatic flows in detail, and herein we report turbulence statistics (e.g., fluxes, variances, spectra, and cospectra) and their variations in katabatic flow. Observed vertical profiles show steep gradients near the surface, but in the layer above the slope jet the vertical variability is smaller. It is found that the vertical (normal to the slope) momentum flux and horizontal (along-slope) heat flux in a slope-following coordinate system change their sign below and above the wind maximum of a katabatic flow. The momentum flux is directed downward (upward) whereas the along-slope heat flux is downslope (upslope) below (above) the wind maximum. This suggests that the position of the jet-speed maximum can be obtained by linear interpolation between positive and negative values of the momentum flux (or the along-slope heat flux) to derive the height where the flux becomes zero. It is shown that the standard deviations of all wind-speed components (and therefore of the turbulent kinetic energy) and the dissipation rate of turbulent kinetic energy have a local minimum, whereas the standard deviation of air temperature has an absolute maximum at the height of wind- A. A. Grachev et al. speed maximum. We report several cases when the destructive effect of vertical heat flux is completely cancelled by the generation of turbulence due to the along-slope heat flux. Turbulence above the wind-speed maximum is decoupled from the surface, and follows the classical local z-less predictions for the stably stratified boundary layer.

Exchange Processes in the Atmospheric Boundary Layer Over Mountainous Terrain

“…Despite being essentially turbulent, katabatic flows or other flows with a low-level wind maximum do not match the standard ABL structure [185,190,191]. Therefore, the applicability of MOST or of local scaling approaches for katabatic flow has been called into question [192] and is a matter of on-going debate [191].…”

Turbulent Mechanical Energy Budget in Stably Stratified Baroclinic Flows over Sloping Terrain

“…One of the important aspects of this research is the turbulent exchange and turbulence structure close to the surface. A recent analysis of observational data by Oldroyd et al (2016b) has shown the important role of the along-slope heat flux in the buoyant generation or suppression of the turbulent kinetic energy (TKE), thus supplementing earlier work (e.g., Horst and Doran 1988;Denby 1999;Grachev et al 2016 The buoyancy term in the TKE budget contains the vertical component of the turbulent heat flux, and may simply be shown to describe the work done by or against the buoyancy force during vertical displacements of particles involved in turbulent motions. In a horizontally homogeneous atmospheric boundary layer (hereafter, the abbreviation ABL will be used, and it will also imply the horizontally homogeneous case), horizontal components of the turbulent heat flux play no role in this energy transformation process.…”

“…This schematic explanation may apply to different situations identified in slope flows, such as those discussed by, e.g., Horst and Doran (1988), Grachev et al (2016), Oldroyd et al (2016b). In a horizontal planar coordinate frame, the term containing θ 2 in Eq.…”

“…Recent experiments, such as the MATERHORN-X (Fernando et al 2015) and SELF-2010 (Nadeau et al 2013) field campaigns provided new information that encouraged many subsequent studies (e.g., Oldroyd et al 2014Oldroyd et al , 2016aGrachev et al 2016). One of the important aspects of this research is the turbulent exchange and turbulence structure close to the surface.…”

A study of turbulent fluxes and their measurement errors for different wind regimes over the tropical Zongo Glacier (16° S) during the dry season

“…In that case, the formulation of the Obukhov length in Eq. (9) may have to be modified to include scalar fluxes tilted to the gravity vector (Horst et al, 1988;Grachev et al, 2015). We did not attempt to derive such a formulation in the present study.…”