First observation of the bora-wind turbulence using hot-wire anemometer

During a bora event, i.e., a famous downslope windstorm at the eastern Adriatic coast, difficulties often emerge in traffic, industry and, generally, in everyday life. This is mainly due to bora persistency and severe gustiness (the wind-speed maxima may surpass even 70 m s-1), which makes bora being a subject of intensive scientific research. In the past few decades, statistical, synoptic and main mesoscale characteristics of the bora flow have been comprehensively studied, but some aspects of bora dynamics are still insufficiently investigated, especially its turbulence. The main reason for such situation has been the lack of suitable measurements which are needed to asses these aspects. In this work we address the near-surface bora turbulence.

We mounted a single point 1D hot-wire anemometer (HWA) (Dantec Dynamics Multichannel CTA Anemometer System) and 3D ultrasonic anemometer (USA) (Gill Instruments WindMaster) at the north-eastern Adriatic coast in Vratnik Pass (44.98°N, 14.98°E, 700 m above MSL) on the mast 2.8 m above the ground. We measured a moderate bora event that occurred on 25 March 2014 and lasted for several days. We gathered 220 min of data on 25 March (1303 – 1642 LST) and 365 min of data on 26 March (0809 – 1413 LST). HWA sampled streamwise wind speed component with sampling rate of 50 kHz while USA sampled all three wind speed components using 10 Hz sampling rate. We performed an in situ calibration of the HWA voltage data using the wind speed data from the collocated USA. In this work we focus on calculation of turbulence kinetic energy dissipation rate, ε. We calculate and compare ε using the direct dissipation technique and two indirect dissipation techniques: the inertial dissipation technique and the Kolmogorov's four-fifths law. The goal is to find out which of the two indirect techniques provides better results for bora-wind turbulence because the use of indirect techniques significantly reduces expenses needed for the reliable estimation of ε.