Bora Flow Characteristics in a Complex Valley Environment

This paper complements the existing studies of Bora flow properties in the Vipava valley with the study of Bora turbulence in a lower region of the troposphere. The turbulence characteristics of Bora flow were derived from high resolution Doppler wind lidar measurements during eight Bora wind episodes that occurred in November and December 2019. Based on the vertical profiles of wind velocity, from 80 to 180 m above the valley floor, the turbulence intensity related to all three spatial directions and the along-wind integral length scales related to three velocity components were evaluated and compared to the approximations given in international standards. The resulting turbulence characteristics of Bora flow in a deep mountain valley exhibited interesting behaviour, differing from the one expected and suggested by standards. The intensity of turbulence during Bora episodes was found to be quite strong, especially regarding the expected values for that particular category of terrain. The specific relationship between along-wind, lateral and vertical intensity was evaluated as well. The scales of turbulence in the along-wind direction were found to vary widely between different Bora episodes and were rather different from the approximations given by standards, with the most significant deviations observed for the along-wind length scale of the vertical velocity component. Finally, the periodicity of flow structures above the valley was assessed, yielding a wide range of possible periods between 1 and 10 min, thus confirming some of the previous observations from the studies of Bora in the Vipava valley.

Spatio-temporal variations of cave-air CO2 concentrations in two Croatian show caves: Natural vs. anthropogenic controls

Carbon dioxide (CO2) concentration (CDC) plays an important role in karst processes, governing both carbonate deposition and dissolution, affecting not only natural processes, but also human activities in caves adapted for tourism. Its variations due to various controlling parameters was observed from 2017 to 2021 in two Croatian show caves (Manita peć and Modrič) where we examined inter- and within-cave correlation of internal aerology regarding the sources, sinks and transport mechanism of CDC in a karst conduit setting. In both caves, the main sources of CO2 are: i) plant and microbial activity i.e. root respiration and organic matter decay within soil horizons and fractured epikarst, and ii) degassing from CO2-rich percolation water. The main sink of CO2 is dilution with outside air due to cave ventilation. Chimney-effect driven ventilation controlled by seasonal differences between surface and cave air temperatures shows winter (Tout<Tcave) and summer (Tout>Tcave ) ventilation regime, which are modulated by the geometry of cave passages, the transmissivity of the overlying epikarst, and occasionally by the external winds, especially the gusty north-eastern bora wind. In these terms, the Modrič Cave appears to be more confined and less ventilated, with a substantial CDC difference between the left (550-7200 ppm) and right (1475- >10,000 ppm) passages. The Manita peć Cave is, in contrast, ventilated almost year-round, having 7 months of CDC equilibrated with the outside atmosphere and the highest summer CDC values of ~1410 ppm. In both caves, at the current level of tourist use, anthropogenic CO2 flux is not a matter of concern for cave conservation. In turn, in the innermost part of the right Modrič Cave passage visitors’ health might be compromised, but the tourists are allowed only in the left passage. Speleothem growth rate, recognized as a useful palaeoenvironmental proxy for speleothem-based palaeoclimate studies, strongly depends on CDC variations, so the high CDCs recorded in the Modrič Cave indicate the potential periods with no speleothem deposition due to the hampered degassing of CO2 from the dripping groundwater. The opposite effect i.e. enhanced ventilation (that supports calcite precipitation) during the windy glacials/stadials, as well as substantial vegetational changes must also be taken into consideration when interpreting environmental records from spelean calcite.

Simulation of pollutants spreading from a sewage outfall in the Rijeka Bay

Results from a 3D numerical simulation of wind-induced currents and pollution spreading from a sewage outfall are described.The goal was to predict seawater sanitary quality in the Rijeka Bay, Adriatic Sea. A sea motion model was coupled with a modelof transport and chemical reactions of fecal coliforms (FC), fecal streptococci (FS) and dissolved oxygen. The selected simulationperiod of 36 hours following wastewater discharge was found to be sufficient for a significant extinction of bacteria. The simulationwas carried out for eight wind directions and two intensities (moderate and high). Mesh convergence was obtained. Twomesh sizes were coupled: coarse, for the whole Bay, and fine, for the northern part of the Bay, close to the sewage discharge ofthe Rijeka city.For all considered wind directions, the pollution plume with a concentration higher than 100 FC and 100 FS per 100 ml of seawateris conveyed mostly parallel to the coast in either north-west or south-east directions. The plume does not rise to the surfacebut stays at the depth of 10 to 20 meters. This is a consequence of the hydrodynamics of the Rijeka Bay: the bora wind carriesthe surface water layer out of the Bay through the Middle and Great Gates, while cold water enters the Bay from the layer belowthe thermocline. During the southern wind (jugo), the situation is reversed: warmer surface water enters the Bay through eitherthe Middle Gate or the Great Gate, while cold seawater exits through the bottom layer, accumulating warmer seawater in the Bay.The conclusion is that the Rijeka city sewage discharge Delta is well-designed, and the microorganism concentration is wellwithin the suggested regulatory range. The discharge site is far enough from the coast, where local streamlines are mostly parallelto the coast, hence the elevated pollution concentration does not come close to swimming and recreational areas. Even if thedischarge increases by 50%, which is unlikely in the near future, the pollution at beaches will stay within regulatory boundaries.

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

Southwest Slovenia is a region well-known for frequent episodes of strong and gusty Bora wind, which may damage structures, affect traffic, and poses threats to human safety in general. With the increased availability of computational power, the interest in high resolution modeling of Bora on local scales is growing. To model it adequately, the flow characteristics of Bora should be experimentally investigated and parameterized. This study presents the analysis of wind speed vertical profiles at Razdrto, Slovenia, a location strongly exposed to Bora during six Bora episodes of different duration, appearing between April 2010 and May 2011. The empirical power law and the logarithmic law for Bora wind, commonly used for the description of neutrally stratified atmosphere, were evaluated for 10-min averaged wind speed data measured at four different heights. Power law and logarithmic law wind speed profiles, which are commonly used in high resolution computational models, were found to approximate well the measured data. The obtained power law coefficient and logarithmic law parameters, which are for modeling purposes commonly taken to be constant for a specific site, were found to vary significantly between different Bora episodes, most notably due to different wind direction over complex terrain. To increase modeling precision, the effects of local topography on wind profile parameters needs to be experimentally assessed and implemented.

Seawater Icicles of the Adriatic Sea

The marine icicles that form on coastal constructions (e.g., piers and railings) are very common in polar and subpolar areas. The occasional formation of icicles in the coastal zones of the Mediterranean Sea occurs in relation to cold-air outbreaks from the polar region, such as the one presented in this paper in February 2018. The air temperatures over the northern Adriatic Sea, the northernmost part of the Mediterranean, dropped below –2.1°C, a necessary condition for seawater to freeze, with salinity between 38.0 and 38.5 PSU. The formation of icicles on the coastal structures was further enabled by the bora wind and related high seas along the coast. Measurements presented in this paper confirm that the icicles in the Bay of Piran in the Gulf of Trieste (45.55°N) were formed from the seawater. The measured salinity level of the melted icicles, around 9 PSU, is a typical value reported for marine icicles in polar regions.

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

Vipava valley in Slovenia is a representative hot-spot for complex mixtures of different aerosol types of both anthropogenic and natural origin. Aerosol loading distributions and optical properties were investigated using a two-wavelength polarization Raman LiDAR, which provided extinction coefficient, backscatter coefficient, depolarization ratio, backscatter Ångström exponent and LiDAR ratio profiles. Two different representative meteorological situations were investigated to explore the possibility of identifying aerosol types present in the valley. In the first case, we investigated the effect of strong downslope (Bora) wind on aerosol structures and characteristics. In addition to observing Kelvin–Helmholtz instability above the valley, at the height of the adjacent mountain ridge, we found new evidence for Bora-induced processes which inject soil dust aerosols into the free troposphere up to twice the height of the planetary boundary layer (PBL). In the second case, we investigated aerosol properties and distributions in stable weather conditions. From the observed stratified vertical aerosol structure and specific optical properties of different layers we identified predominant aerosol types in these layers.

A multiplatform investigation of Istrian Front dynamics (north Adriatic Sea) in winter 2015

In the northeastern Adriatic Sea, southwest of the Istrian Peninsula, a persistent thermohaline front is formed, called here the Istrian Front (IF). A Slocum glider was operated across the IF near the entrance to the Kvarner Bay between 24 and 27 February 2015. Three Acoustic Doppler Current Profilers (ADCPs) were also deployed at the entrance of the Kvarner Bay during the same period. The glider crossed twice the IF, which was characterized by a fast response to the local wind condition, detecting strong salinity, temperature and density gradients. During the first crossing a strong northeasterly Bora wind was blowing. This resulted in a very sharp and strong thermohaline front, extended vertically in the entire water column, between saltier and warmer water to the south, and the fresher and colder water to the north. Across the front the SST changed ~ 1.2 °C within a distance of 2.4 km. On the contrary, during the second crossing, about 2 days later, under weaker wind conditions, the IF appeared to be much smoother, inclined and wider while the SST changed ~ 1.2 °C within a distance of 8 km. A strong density gradient was also reported, coincident with the thermohaline IF. From previous observations, mainly experiments in 2003, the IF was known only as a thermohaline front compensated in density. In winter 2015, the density front was strong and well defined, demonstrating a density difference of about 0.36 kg/m3 within a distance of 2.4 km. The ADCP measurements and the numerical model simulations demonstrated a circulation of cold waters exiting from the Kvarner Bay in the southern part of the entrance, while during a Bora event this outflow was taking place in the northern part.