engleski

Bora-induced near-surface layer turbulence scales

The town of Senj settled at the north-eastern Adriatic coast (44.99°N, 14.90°E, 2 m above MSL) is famous for frequent occurrence of bora wind, which makes it to be perfect natural laboratory for investigation of bora. While large- and mesoscale features of bora are intensively investigated in the past few decades and nowadays well understood, there is still insignificant amount of knowledge about its turbulence characteristics. Thus, from March 2004 to June 2006, single point 3D high frequency wind measurements were conducted in Senj using WindMaster ultrasonic anemometers (Gill Instruments) mounted 13 m above the ground. This instrument recorded the data with a sampling frequency of 4 Hz. It was continuously operational for more than two years recording all kinds of airflows. Using very basic criteria to define a bora event (wind of azimuth between 30° and 90° blowing at least for 3 h), we sorted out 294 bora events from these dataset with cumulative duration of almost 7000 h. Usually, one of the first steps in investigation of turbulence is to obtain spatial/temporal scale(s) within which the turbulence will be defined in an observed flow. Regarding the scales of turbulence, one can find variety of different definitions in the literature. Most frequently used are the spectral gap scale at mesoscale derived from Fourier spectrum, the integral scale derived from autocorrelation function and the scale at which normalized Fourier spectrum achieves its maximum value. In this work we estimate these scales and show that they significantly differ for bora flows in Senj. While the spectral gap scale seems to converge to 30 min, integral and maximum spectrum scales show pronounced dependence on mean bora wind speed. Among other reasons, estimations of these scales are important for numerical model evaluations, turbulence parameterization scheme improvements and various engineering applications.

Bora wind; Integral turbulence scales

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