Waves and turbulence in katabatic winds

The measurements taken during the Vertical Transport and Mixing Experiment (VTMX, October, 2000) on a northeastern slope of Salt Lake Valley, Utah, were used to calculate the statistics of velocity fluctuations in a katabatic gravity current in the absence of synoptic forcing. The data from ultrasonic anemometer-thermometers placed at elevations 4.5 and 13.9 m were used. The contributions of small-scale turbulence and waves were isolated by applying a high-pass digital (Elliptical) filter, whereupon the filtered quantities were identified as small-scale turbulence and the rest as internal gravity waves. Internal waves were found to play a role not only at canonical large gradient Richardson numbers $$(\overline{\hbox {Ri}_\mathrm{g} } >1)$$ , but sometimes at smaller values $$(0.1 < \overline{\hbox {Ri}_\mathrm{g}}1$$ , the use of both scaling types is questionable, although filtered data showed a tendency to follow local scaling. A relationship between z/L and $$\overline{\hbox {Ri}_\mathrm{g} }$$ was identified. Eddy diffusivities of momentum $$\hbox {K}_\mathrm{M}$$ and heat $$\hbox {K}_\mathrm{H}$$ were dependent on wave activities, notably when $$\overline{\hbox {Ri}_\mathrm{g} } > 1$$ . The ratio $$\hbox {K}_{\mathrm{H}}/\hbox {K}_{\mathrm{M}}$$ dropped well below unity at high $$\overline{\hbox {Ri}_\mathrm{g} }$$ , in consonance with previous laboratory stratified shear layer measurements as well as other field observations.