Micro-Scale Properties of Different Bora Types

Abstract

Abstract:In this paper we use 20 Hz wind measurements on three levels (2, 5, and 10 m) to investigate the differences in micro-scale properties of different bora types, i.e., deep and shallow bora with further subdivision to cyclonic and anticyclonic bora cases. Using Fourier spectral analysis, we investigate a suitable turbulence averaging scale and bora gust pulsations. The obtained data set is further used to test the Monin-Obukhov similarity theory, the surface layer stratification, the behavior of the terms in the prognostic turbulence kinetic energy equation, and the wind profiles. One of our main goals is to identify possible micro-scale differences between shallow and deep bora types because of the possible different mountain wave dynamics in those flows. We found that a turbulence averaging scale of 30 min is suitable for this location and is in agreement with previous bora studies. The wind speed power spectral densities of all selected bora episodes showed pulsations with periods of 2-8 min. This suggests that mountain wave breaking was present in all cases, regardless of flow depth and synoptic type. The stability parameter analysis confirmed the near-neutral thermal stratification of bora; a consequence of intensive mechanical mixing. No significant differences related to bora type were observed in other micro-scale parameters.

Near-Ground Profile of Bora Wind Speed at Razdrto, Slovenia

“…That is primarily due to the lack of high temporal and spatial resolution measurements, needed to assess the turbulence properties of the wind. Recently, microscale properties of Bora have been investigated for different Bora types [20], confirming most of previous results related to Bora periodicity and turbulence properties [21,22]. Bora turbulence properties were studied in [23,24] as well, with a particular focus on turbulence intensity, Reynolds shear stress, and turbulence length scales.…”

“…Bora turbulence properties were studied in [23,24] as well, with a particular focus on turbulence intensity, Reynolds shear stress, and turbulence length scales. Research at specific regions in Croatia (Pometeno brdo near Split [22] and Maslenica [20]), revealed that the near-ground wind speed profile of Bora is in good agreement with the logarithmic law and power law description, which are commonly used in wind engineering. Thermal stratification during Bora episodes at these sites was found to be near-neutral, due to intensive mechanical mixing.…”

“…This assumption should not introduce significant errors, as the difference between the highest and lowest measurement point was only 21.7 m, which is in Bora-like wind conditions expected to be smaller then characteristic length scale [25,26]. There is no standard way to identify Bora episodes, and various criteria have been applied by different authors so far [20,22]. In our case, we used the following criteria.…”

Investigation of Aerosol Properties and Structures in Two Representative Meteorological Situations over the Vipava Valley Using Polarization Raman LiDAR

“…Bora flow was accompanied by the presence of Kelvin-Helmholtz instability above the valley at the height of the barrier, as suggested by different ground (NE) and radiosonde (SW) wind directions above the barrier as well as by the periodicity of atmospheric structures retrieved by LiDAR (Figure 2). Due to differences in airmass density, wind speed and wind direction above and below the barrier, waves formed at the interface between the layers, pushing the denser air above the less dense air, leading to convective instability, wave breakdown and turbulence production [2]. The investigation of aerosol structures and properties above the valley is based on LiDAR data between 10:30 to 14:00 CET.…”

“…As a part of the Mediterranean region, the area is also frequently affected by long-range transport of Saharan dust from North Africa across the Mediterranean and the Adriatic Sea, as well as sea salt. The most important phenomenon in the region that ventilates the valleys are episodes of Bora [2], katabatic wind capable of inducing mixing particulate matter and trace gases from the surface into the free troposphere [3,4]. To monitor spatial loading distribution of different aerosol types and to understand the effects of various aerosol sources, experimental data with sufficiently high temporal and spatial resolution are needed.…”