Estimation of Bora Turbulence at the New Micrometeorological Research Facility

While bora macro-scale and even meso-scale features have been intensively investigated in the past few decades, the important details about bora related turbulence are yet not known. One of the reasons for this inadequacy is the lack of measurements suitable for bora micro-scale investigations. Those are high-frequency in situ measurements of wind speed and temperature in space (e.g., detailed aircraft measurements) and in time (single point ground based measurements on e.g., meteorological towers/masts). Therefore, Department of Geophysics, University of Zagreb, Croatia, conducted several projects in the past ten years within which high-frequency measurements of the wind speed and temperature along the eastern Adriatic coast have been performed. These are single point measurements at the town of Senj, Vratnik Pass and Pometeno Brdo (hinterland of the city of Split).

However, all of those measurements were performed using sonic anemometers mounted on the towers/masts 10 m and higher above the ground, leaving the turbulence structure of the bora wind below 10 m undisclosed. Here we present a new state-of-the-art micrometeorological tower measurements installed 200 m at the front of the new Maslenica Bridge (≈ 30 km NE of the city of Zadar, on the main motorway). The tower is 10 m tall, equipped with three levels of Gill WindMaster ultrasonic anemometers (2, 5 and 10 m) gathering the 3D wind speed and sonic temperature with the sampling rate of 20 Hz. Slow-response independent measurements of pressure, humidity and temperature are also obtained. Using Fourier spectral analysis, we investigate a suitable turbulence averaging scale and bora gust pulsations. Results are further used for testing the Monin-Obukhov similarity theory and the degree of closure of the prognostic TKE equation. These are expected to be considerably modified due to the complexity of the terrain and effects of vigorous air-sea-land interaction.