4.4 TURBULENCE AND MIXING IN THE NOCTURNAL BOUNDARY LAYER OVER A SLOPE – VTMX FIELD PROGRAM RESULTS Marko Princevac *1 , P. Monti 2 , H.J.S. Fernando 1 , T.A. Kowalewski 3 , and E.R. Pardyjak 4 1 Arizona State University, 2 Università degli Studi di Roma “La Sapienza”, 3 Polish Academy of Sciences, 4 University of Utah 1. Introduction ∗ The Vertical Transport and Mixing Experiment (VTMX) took place in Salt Lake City, Utah, in October 2000. This field campaign was carried out under the sponsorship of the Atmospheric Sciences Program of the Department of Energy to study the transport and mixing processes in complex terrain under stable conditions. Details of the filed campaign are given in J.C. Doran, J.D. Fast and J. Horel (‘The VTMX 2000 Campaign’, submitted to Bull. Am. Meteorol. Soc.) Measurements presented in this communication were conducted on an northeastern slope of the Salt Lake Basin with particular interest in nocturnal boundary layer (katabatic flows) in the absence of significant synoptic influence. Extensive measurements of mean flow, turbulence, temperature and solar radiation were made, from which circulation patterns on the slope and nature of stratified turbulence in katabatic winds were inferred. 2. Site and Instrumentation The observation site was located in the northeastern side of the valley, in a grassy open area (aerodynamic roughness length ~0.1 m), adjoining the Mount Olivet Cemetery, having a gentle slope (~0.07, i.e. 4°). Because the measurements were made away from buildings and trees, the data can be considered as free from the immediate effects of obstacle wakes. Deployed instruments consisted of a 14 m mast equipped with two 3-cup anemometers, two thermistors, upward facing spectral pyranometer, and a downward facing pyrgeometer. A Data Logger provided computation and storage of 5 min-averaged air temperature, wind speed and radiation. Two ultrasonic fast-response anemometers-thermometers were used to measure three velocity components and air temperature with a sampling rate of 10 Hz. In order to analyze the vertical structure of the lower atmosphere, two tethered balloons were used to measure the air temperature, relative humidity, pressure, wind speed and wind direction. Simultaneous measurements of ground level aerosols and vertical aerosols profiles were also made. 3. Data Analysis Sixty-second averages of ultrasonic data were used to evaluate eddy diffusivity of momentum ∗ corresponding author address: Marko Princevac, Arizona State Univ., Dept. of Mechanical and Aerospace Engineering, Tempe, AZ 82587-6106       ∂ ∂ ′ ′ − = z U w u K M (1) and heat dz d w K H θ θ ′ ′ − = , (2) where z is the vertical coordinate, U mean slope velocity, θ mean potential temperature, ' ' θ w vertical heat flux, and ' ' w u momentum flux. Figure 1 shows the dimensional form of KM and KH as a function of the Gradient Richardson number defined as 2 2 2 2 2 ~       ∂ ∂ +       ∂ ∂ = ∂ ∂ = z V z U N z V N Ri g , (3) where N is the buoyancy frequency and V ~ the mean velocity vector. Figure 1