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Turbulence measurements in the surface layer over the residential part of Zagreb

We investigated characteristics of the three-dimensional turbulent flow within the surface layer over a residential part of Zagreb, Croatia based on local wind measurements. WindMaster ultrasonic anemometer (Gill Instruments) was placed at the top of the Geophysical Institute building, approximately 17 m above the ground. Measuring location is in the northern residential part of Zagreb, at south-facing slopes of Medvednica mountain at 184.30 m ASL. Terrain is moderately hilly, with some interspersed, though rather high trees (~ 10 m). Measurements were performed during the 12 May 2004 - 20 July 2004 period with a 1- s sampling interval. Collected 3D wind database did not reveal any preferred scale of turbulent eddies at the selected location. Turbulent kinetic energy, turbulence intensity due to perturbations of vertical wind component, kinematic surface layer eddy fluxes of momentum, friction velocity and vertical eddy diffusion coefficient for momentum based on gradient transport theory were calculated for smoothing intervals of 1, 10 and 60 minutes, where extension of smoothing interval resulted with increase of the values of calculated turbulent quantities. Analyses of above turbulent quantities were performed with respect to Pasquill stability categories which were determined from the horizontal wind speed and the vertical temperature gradient. Obtained empirical values of friction velocity and vertical eddy diffusion coefficient may be used in parameterization of turbulence over suburban surface layer.

turbulence; ultrasonic anemometer; measurements; friction velocity; vertical eddy diffusion coefficient

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