Abstract : 3N.11
Estimation of turbulence kinetic energy dissipation rate in a Bora event

Zeljko Vecenaj, Danijel Belusić, Branko Grisogono
Department of Geophysics, Faculty of Science, Universety of Zagreb

Bora is a downslope windstorm that blows at the Eastern Adriatic coast from the northeast quadrant, most often during winter season. It possesses a wide spectrum of average wind speeds, and due to gustiness the speed maxima may surpass 60 m/s. During a bora event, the turbulence is strongly developed in the lee of the mountain.

Horizontal and vertical wind is measured at Senj (44.99°N, 14.90°E, 2 m above MSL) at a height of 13 m above the ground with the WindMaster ultrasonic anemometer (Gill Instruments). The anemometer records the data with a sampling frequency of 4 Hz. The data for the bora episode extending from 7 to 10 January 2006 is used to evaluate the TKE dissipation rate (DR). The inertial dissipation technique, provided by Kolmogorov's 1941 hypotheses, is employed. To translate the time domain into the space domain the Taylor's hypothesis of frozen turbulence is used. The estimated value of DR for this event with this technique amounts to 1.22 m2s-3 and is taken as the reliable one. An alternative method, applicable only to the inertial subrange in a strict sense (i.e. requires isotropy), is based on the third-order streamwise velocity structure function, i.e. the Kolmogorov’s four-fifths law. Using this approach a significantly lower value of DR is obtained, not surpassing 0.70 m2s-3. Inspecting the stronger isotropy condition for the inertial subrange, i.e. the 4:3 ratio of vertical and transverse to streamwise velocity spectra, it is found that the present anemometer’s sampling rate only approaches the inertial subrange. This partly explains the lower value of the DR obtained using the second method. Thus, the anemometer of a much higher sampling frequency is needed for further studies of the bora-related turbulence.