engleski

Evaluation of vertical eddy fluxes divergence for multiple bora events

The eastern Adriatic coast is well known for the downslope windstorm called bora. This windstorm often causes difficulties in traffic, agriculture and generally in everyday life. Hence, it is a subject of intensive and continuous scientific research. In the past few decades various statistical, synoptic and mesoscale characteristics of the bora flow have been investigated intensively ; however, certain aspects of bora dynamics still remain unexplored, especially its turbulence. The main reason for such a situation was the lack of appropriate measurements needed to address bora turbulence. This has been especially the case for the middle and southern Adriatic coast where, until recently, no systematic turbulence measurements were conducted. From April 2010 to June 2011, a three- level tower (10, 20 and 40 m) was equipped with WindMaster Pro ultrasonic anemometers (Gill Instruments), measuring 3D wind speed and sonic temperature at 5 Hz sampling rate. The tower was mounted at the top of Pometeno brdo (≈ 600 m ASL) in Dugopolje, bora upwind site from the city of Split. During that period, the anemometers captured multiple bora events in all annual seasons. This work concentrates on 17 registered bora events during a three month winter period, ranging from 10 up to 120 hours in bora duration. Upon determining the appropriate Reynolds averaging scale in order to define various turbulent perturbations, a thorough analysis of all kinematic vertical eddy fluxes (both momentum and heat) has been made. In particular, divergence of the aforementioned fluxes was also estimated, in order to determine their role in the very simplified equations of motion, as well as to determine the justification of assumptions that are used, such as horizontal homogeneity. Considering the measuring site and the nearby orography, interesting results were obtained. It is also worth noting that vertical eddy fluxes and the corresponding divergences, like those assessed here, have never been provided for a bora case.

Bora; Pometeno brdo; Turbulent fluxes

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