Bora is a gusty downslope wind blowing from the northeast in the lee of the Dinaric Alps and other dynamically similar parts of the world [1
]. It is dynamically generated by the interaction of airflow and orography [2
]. Bora macro-scale characteristics have been investigated since the middle of the 20th century [3
]. Early mesoscale research focused on the katabatic model of bora [4
], which was later shown to be deficient in explaining stronger bora events. A major breakthrough in bora mesoscale research started with the ALPEX (Alpine Experiment) project in 1981 [5
] and the subsequent findings by Smith [2
], which showed that bora is essentially a dynamically generated wind, best explained by the hydraulic and wave breaking theory [6
]. In recent years, bora’s mesoscale characteristics have been extensively studied during the Mesoscale Alpine Programme (MAP) project [8
]. A recent review of bora mesoscale properties at the northeastern Adriatic can be found in [11
With respect to the synoptic setup, three types of bora have been identified in the past: cyclonic, anticyclonic [3
], and frontal [12
]. The typical setup for cyclonic bora is when a mid-latitude cyclone moves to the southern Adriatic, pulling colder air from the continent to the eastern Adriatic coast. Anticyclonic bora blows when there is an anticyclone situated over Central Europe, extending over the Dinaric Alps [3
]. Frontal bora is characterized by a sudden increase in wind speed and short duration, following the passage of a cold front [12
Both cyclonic and anticyclonic bora can be either deep or shallow, depending on the depth of synoptic flow over the mountains (e.g., [13
]). For example, a common feature above a mature mid-latitude cyclone is the southwesterly flow in the divergent (eastern) flank of the upper-level trough (i.e., the flow from positive vorticity maximum to negative vorticity maximum at 300 hPa). This is why most cyclonic boras are shallow [2
]. However, in the case of an occluded cyclone, there is usually a cut-off low in the upper levels, sometimes favorably aligned with the surface cyclone, thus providing deep NE (northeasterly) to N (northerly) flow throughout the troposphere.
Anticyclonic deep bora occurs when a deep positively tilted trough passes and the upper N or NE flow in its western flank is above the southeastern quadrant of a surface anticyclone. Alternatively, the northwestern quadrant of a cut-off low can also provide deep NE flow above the surface anticyclonic bora. Anticyclonic deep bora seems to be the most frequent among deep bora types [16
] because its synoptic setup is more common than that of cyclonic deep bora. Since strong anticyclones can persist for days, it is not uncommon for this type of bora to last up to a week.
Deep bora is associated with vertically propagating mountain waves [2
], while shallow bora is associated with wave breaking and violent downslope windstorms [7
]. Shallow bora does not allow significant vertical propagation of wave energy, thus generating strong downslope windstorms in the lee. Vertical wind shear plays a significant role in the vertical propagation of waves in deep bora. In the case of positive wind shear (wind speed increasing with height), wave breaking does not occur at least until tropopause, because of the linearizing effect of the increasing wind speed [11
]. In the case of weak vertical shear or wind speed decrease with height, wave breaking is likely to happen in the lower or middle troposphere, again reflecting mountain wave energy to lower levels and generating violent downslope windstorm.
Regardless of more than a century of intensive research of bora climatology (e.g., [17
]) and bora macro- and mesoscale properties (e.g., [11
]), some important details about bora micro-scale properties are not yet known. One of them includes detailed characteristics of severe bora episodes. The most severe bora episodes (downslope windstorms), with gusts reaching up to 70 m·s−1
, are caused by wave breaking when there is a critical level above the mountaintop [7
]. The critical level is usually marked by strong inversion [23
] and a decrease in wind speed or change in wind direction by height, thus acting as an efficient reflector of wave energy. The critical level can be imposed by synoptic scale or generated by wave breaking itself—caused by wave amplitude increasing with height [24
Severe bora typically induces shooting flow in the lee of coastal mountains [24
] that may extend out over the sea in the form of multiple low-level jets behind mountain passes, while lee wakes (weaker flow regions) occur behind mountaintops [9
]. Sea surface SAR (Synthetic Aperture Radar) data analysis by Kuzmić et al. [13
] revealed the existence of secondary bora jets—caused by smaller mountain and island features (gaps and flanks)—that are only a few kilometers apart and several kilometers long. Moreover, they documented fine-scale convective cells pertaining to cold bora outbreak over relatively warm sea.
Bora pulsations with periods of 3–4 min were first mentioned in the work of Watanabe [25
], based on the experience of local fishermen. The first confirmation of those observations in the measured data was in the work of Petkovšek [26
], who found pulsations with periods between 3 and 11 min. Although the existence of pulsations has been known for a long time, the detailed physics behind the gustiness and pulsations of bora has been addressed only recently. Belušić et al. [28
] also found that the pulsations occur with periods between 3 and 11 min in the town of Senj, a location well known as a bora maximum site (Figure 1
). Furthermore, this was also confirmed by using fine-scale numerical modeling [29
], and measurements at the Pometeno Brdo (in a free translation, Pometeno Brdo means Swept Away Hill) [30
]—a bora site upwind of the city of Split (Figure 1
) that is about 200 km southeast from Senj. The former study found that the generation of gust pulsations was associated with mountain wave breaking and Kelvin-Helmholtz shear instability (KHI) above the bora shooting flow. This mechanism was first demonstrated by Peltier and Scinocca [31
] for mountain windstorms in Boulder, CO, USA. Measurements and numerical modeling studies [28
] also showed that the pulsations disappear in the presence of positive vertical wind shear above the mountaintop (e.g., the presence of an upper-tropospheric jet stream).
Micro-scale characteristics of severe turbulence in the wave breaking region are the focal point of current bora research. In order to improve turbulence parameterization schemes in numerical models, Večenaj et al. [33
] evaluated turbulence kinetic energy (TKE) and its dissipation rate for a bora event in the town of Senj. Večenaj et al. [34
] estimated the turbulence dissipation rate along the Adriatic sea coast, using 4 Hz aircraft and dropsonde data obtained during the MAP project. For the Pometeno Brdo site, Lepri et al. [35
] analyzed bora wind speed profiles from 5 Hz data and found that they agreed with commonly used empirical power-law and the logarithmic-law profiles. They also found that thermal stratification of the surface layer is near neutral due to strong mechanical mixing. Using the same data, Lepri et al. [1
] further investigated turbulence intensity, Reynolds shear stress and turbulence length scale profiles for the mentioned location.
Without such high-frequency in situ measurements of wind speed in space (e.g., aircraft measurements) and in time (single-point ground-based measurements on, e.g., meteorological towers/masts), the exploration of bora micro-scale properties would not be possible. For a more comprehensive insight into the nature of bora turbulence, even higher sampling frequency (e.g., >10 Hz) measurements are needed. This also hints at the goal of this study.
Bora has a major influence on all forms of transportation, engineering structures, electrical and telecommunication grids, agriculture, sea dynamics, air pollution, tourism, and firefighting. Engineering structures in areas prone to severe downslope windstorms must be strong enough to withstand these hurricane force winds. Agriculture in those areas must also be adapted to such harsh conditions. Transportation is the most vulnerable human activity, since severe bora episodes can completely shut down all road traffic to and from the coast. In some extreme cases, even the air traffic at the whole eastern Adriatic coast can be completely suspended.
The Maslenica Bridge is a very important transportation route, connecting the southern and central Croatian coast—the northeastern Adriatic coast—with inland parts of Croatia. The purpose of this study is to test whether some of the previous results obtained for different measuring sites apply to the Maslenica Bridge location. Namely, the turbulence averaging time scale, bora pulsations, thermal stratification, TKE budget, and wind speed profiles. Furthermore, we aim to identify possible differences in those micro-scale properties of different bora types. As this has not been attempted before, it could give new insights into the turbulence characteristics of bora wind. For this purpose, we classify bora episodes by the flow depth and the synoptic type. As already mentioned, the flow depth is important in defining the mountain wave dynamics. We think that the synoptic type (i.e., cyclonic, anticyclonic or frontal) can influence the micro-scale properties of bora mainly because of different wind speeds, but also with different vertical wind and temperature profiles (e.g., stronger inversions in anticyclones), which additionally define mountain wave dynamics. The maximum wind speeds depend on the synoptic type because anticyclones have horizontal pressure gradient limit (inertial instability) and thus can never have wind speeds as high as very deep cyclones. Finally, the flow depth itself is also dependent on synoptic situation. In the following sections, we will explain all the methods and data used, show and discuss the obtained results, summarize the main findings, and provide conclusions.
We carried out, for the first time, a detailed analysis of high-frequency (20 Hz, downsampled to 10 Hz) wind data for several bora episodes measured at the Maslenica Bridge site in Croatia during autumn and winter 2015/2016 on three vertical levels (2, 5, and 10 m). A total of 14 bora episodes were detected and classified by depth and synoptic type, of which three typical episodes were selected and presented in this study: B01 (shallow cyclonic), B09 (deep anticyclonic), and B13 (shallow anticyclonic).
Our results confirm the majority of the previous results [28
]. The minimum energy (energy “gap”) in the frequency-weighted power spectral density graphs for the majority of the episodes is located at 30 min. We could not find clear evidence that it depends only on the bora type. Furthermore, power spectral densities disclose energy peaks at periods between 2 and 8 min for all three episodes, which are most likely related to bora pulsations. This further implies that mountain wave breaking occurred in all analyzed episodes. The thermal stratification during a bora episode is near-neutral due to intensive mechanical mixing, independent of the type of the episode. Deviations from this can be seen at the 10 m height in the nighttime, when the most statically stable bora cases occur. However, these never go beyond weak stratifications.
The use of similarity functions in the bora surface layer was also tested. We suggest adopting the similarity theory for bora episodes with caution, since they fail to give reliable results, especially above a certain height. This is probably due to the fact that the main assumptions of the similarity theory are violated (i.e., quasi-stationarity and horizontal homogeneity).
The vertical wind speed profiles—reconstructed with mean and median values—agree well with the logarithmic profile for the surface layer during all analyzed bora episodes.
In the TKE equation, the shear term dominates in all three episodes extracting the kinetic energy from the mean flow and transforming into the TKE. The shear term is mainly balanced by the pressure transport (residual) and dissipation term.
In the small set of typical bora types we analyzed (SC, DA, and SA), we found no evidence that possible differences in micro-scale properties are related to different bora types—which was one of the main goals of the study. The inspected elements that explicitly depend on the wind speed (i.e., friction velocity, TKE, and vertical wind profiles) are different, but that is not necessarily a function of bora type. The friction velocity and TKE budget terms increase with the increase of the mean streamwise wind component.
In this paper, we present a novel approach to bora time series analysis, but we are aware of the possible limitations in finding micro-scale differences for different flow depth dynamics. The very sparse time series of sounding data (00 UTC and 12 UTC only) and the one-point measurements at Maslenica are a few of them. In order to further enhance this study (e.g., in the application of the similarity function), future work should aim for a more precise flow depth analysis, and perhaps more complex classification (e.g., by considering vertical wind shear and stability); but above all, more cases and measurements in a denser grid are needed to account for horizontal inhomogeneity. Furthermore, this study showed that the strongest bora episodes are mainly transitional in type (possible change in flow depth dynamics), so to investigate the micro-scale properties of such cases, episodes should be divided into parts according to flow depth and synoptic type, and then analyzed.