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Turbulence statistics are analysed for Bora episodes that occurred simultaneously in the town of Senj and at the Vratnik pass at the northern Croatian Adriatic coast. The wind record spans over 9 months. The results are compared to address the difference in characteristics of near-ground turbulence in the town of Senj, located at the shoreline leeward of the main mountain peaks, and at the Vratnik pass, located at the mountain ridgeline 7.5 km upstream of Senj. The analysis reveals some important findings. In particular, turbulence kinetic energy (TKE) is systematically larger at Senj, approximately twice the value at Vratnik, yet the mean wind speed is higher at Vratnik. Furthermore, the TKE dissipation rate ε is nearly identical at the two locations. This is further explored by comparing how the turbulence at the two locations adheres to a simple shear- dissipation model where the standard deviation of the longitudinal velocity component is proportional to the mean longitudinal wind velocity. Vratnik relates to this model better, while the TKE balance in Senj is more complex. The difference in statistics and TKE balance can, in part, be attributed to the relative proximity of the anemometers to the rough surface. Turbulence length scale is substantially larger in Senj. Turbulence intensities are in correspondence with values recommended in engineering codes.

Unique data from a 100-m meteorological mast located on the windward side of the Dinaric Alps, Croatia, are compared to high-resolution Weather Research and Forecasting (WRF) model simulations. This was performed for an especially strong and long-lasting (more than 20 days) wintertime bora event. The agreement between the measurements on the mast and the respectiveWRF simulation was generally very good, even with respect to the time series of the turbulence kinetic energy. Based on this finding, which validates the WRF model suitability for numerical simulations of transient winds in windward areas, this approach can be used in future studies to explore the severe bora upwind of the coastal mountains, which has been studied inadequately thus far. In this context, some of the preliminary results are outlined here.

Bora is a well-known downslope windstorm that blows at the NE Adriatic coast from inland towards the sea, mainly from the NE direction. One of the striking bora characteristics are the quasi-periodic gust pulsations. In this work, their characteristics during severe bora in the lee of the Dinaric Alps, near the town of Senj, are explored using rotational spectral analysis and nine months of continuous high-frequency sonic anemometer measurements. Analysis shows that the pulsations' propagation direction does not correspond to the measured near-ground wind direction. Additionally, these two directions seem to be negatively correlated, and their relative angle is larger when bora is forced by an anticyclonic pressure gradient, as opposed to when it is forced by a cyclonic one. Although the pulsations are mostly rectilinear (majority of the energy contained parallel to their propagation direction), positive rotational component is larger than the negative in all but one bora episode, and the motions can become briefly elliptical. Several more notable bora episodes exhibiting most of the aforementioned features are highlighted. Possible reasons for these observations are discussed, such as large directional shear near the top of the lee-side jet (shooting flow) and the presence of the potential vorticity banners. High-resolution simulations are planned to elucidate these observations.

Leaf removal in the cluster zone is one of the commonly applied viticultural practices used to increase light penetration and to decrease humidity in dense foliage. A two- year study (2018/2019) was carried out on the Merlot variety grown in the vineyard on the Croatian hillside with continental climatic conditions, to study the effects of basal leaf removal on canopy microclimate, grape chemical composition and vine production. Leaf and lateral shoot removal was performed at berry set. Meteorological data were recorded together with microclimatic measurements: air temperature, UV radiation and relative humidity in the fruiting zone. Leaf removal altered microclimate by increasing the UV radiation within the cluster zone. Sugar concentration, bud fruitfulness and yield performance was not affect by the treatment, while leaf removal significantly reduced the concentration of titratable acidity in berry samples and increased the concentration of phenols, anthocyanins, flavonols and flavan-3-ols in berry skin. Leaf removal had a higher influence on the grape anthocyanin and flavonol composition in a season with the cooler berry ripening conditions. These results revealed important difference in leaf removal efficiency on phenolic composition of the grapes in different seasonal climatic conditions during ripening.

Strong winds crossing elevated terrain and descending to its lee occur over mountainous areas worldwide. Winds fulfilling these two criteria are called foehn in this paper although different names exist depending on the region, the sign of the temperature change at onset, and the depth of the overflowing layer. These winds affect the local weather and climate and impact society. Classification is difficult because other wind systems might be superimposed on them or share some characteristics. Additionally, no unanimously agreed-upon name, definition, nor indications for such winds exist. The most trusted classifications have been performed by human experts. A classification experiment for different foehn locations in the Alps and different classifier groups addressed hitherto unanswered questions about the uncertainty of these classifications, their reproducibility, and dependence on the level of expertise. One group consisted of mountain meteorology experts, the other two of master’s degree students who had taken mountain meteorology courses, and a further two of objective algorithms. Sixty periods of 48 h were classified for foehn–no foehn conditions at five Alpine foehn locations. The intra-human-classifier detection varies by about 10 percentage points (interquartile range). Experts and students are nearly indistinguishable. The algorithms are in the range of human classifications. One difficult case appeared twice in order to examine the reproducibility of classified foehn duration, which turned out to be 50% or less. The classification dataset can now serve as a test bed for automatic classification algorithms, which—if successful—eliminate the drawbacks of manual classifications: lack of scalability and reproducibility.

A correct estimation of turbulent variances and covariances in the atmospheric boundary layer relies on the determination of turbulent perturbations of wind speed components and scalar quantities, which requires the presence of a so- called spectral gap. The goal of this work is to determine the range of gap scales necessary to define turbulent perturbations in a daytime valley boundary layer. To accomplish this, we analyze data from a large number of propeller- vane and sonic anemometers using the fast Fourier transformation and the multiresolution flux decomposition. Daytime gap scales are found to range from 17 to 29 min and show large spatial variability across the valley floor and the adjacent slopes. Synoptically driven conditions that favor the occurrence of mesoscale phenomena, such as rotors and mountain waves, shift daytime gap scales toward longer periods. The low- frequency end of the gap is also affected by the presence of slope winds that are characterized by a periodicity ranging from 80 to 200 min. Finally, we present a conceptual model of the daytime valley spectral gap which summarizes the findings of this study.

A surface energy budget underclosure, equivalent to an undersampling of sensible and latent heat fluxes at the surface, is commonly observed in the daytime convective boundary layer over land. The two mechanisms currently held responsible for such underclosure are, on one hand, entrainment-induced reduction of heat fluxes due to low-frequency circulations spanning the depth of the convective boundary layer, and on the other hand, horizontal advection effects induced by surface horizontal heterogeneity. Given the involvement of both heat fluxes in the budget equation, it is expected that the turbulent perturbations of variables involved in these fluxes (potential temperature θ ′ and specific humidity q ′ ) contain the signature of both mechanisms. Here, we assess to what extent non-local entrainment effects contribute to the budget underclosure in complex terrain using a quadrant analysis and multiresolution flux decomposition approach. We use data collected in the semi-arid Owens Valley, CA, USA, during the Terrain-Induced Rotor Experiment (T-REX) campaign. The budget components are sampled near the surface of the valley floor and the adjacent slope, with turbulence flux measurements, pyranometers, pyrgeometers, as well as ground heat flux measurements. We found that the budget underclosure was within the range of underclosures reported in past studies, with a systematically poorer closure over the valley slope. Although quadrant analysis revealed a dominance of scalar-similar events, associated with convective boundary layer overturning, a high degree of disagreement was found between the scalar-dissimilar quadrants which represent the counter-gradient motions. Specifically, those turbulent motions associated with entrainment of free tropospheric air to the surface, were found to be of minor importance in degrading scalar similarity. Instead, we postulate that local horizontal advection and hectometre-scale land-use heterogeneity in the vicinity of both the valley floor and slope sites, are the major factors in controlling the relationship between the budget underclosure and scalar similarity.

The bora flow is typical for the lee side of Dinaric Alps, where many international airports are situated. If the wind direction is perpendicular to the runway, as it is the case with bora at the Dubrovnik airport, it can have a significant impact on aviation operations. The impact of the crosswind and wind shear is assesSed within SESAR 2020 programme, in the solution PJ.04-02 - Enhanced Collaborative Airport Performance Management. Lidar measurements were performed during winter 2017/2018. It was the first 3D lidar measurement of bora wind in Croatia. Due to complex terrain and possible different profiles of wind and stability, varieties of flow were expected and measured. Lidar was situated at the slope of the nearby mountain in order to examine and assess the bora flow structure around the airport. During standard bora episodes, horizontal profiles have shown very turbulent flow with sometimes smaller bands parallel to the stream. Vertical profiles confirmed marked jet with heights 0.5- 1.5 km, depending on the range and aerosols concentrations. In case of deep bora layer, a broad area of north-east wind flow is measured, but with small moving patches of high wind speed with strong turbulence. In the most extreme case found, the maximum horizontal shear of 25 ms-1 radial velocity in just 1km was in case of hydraulic jumps and associated rotor. Lidar profiles in the third bora type, nocturnal gap flow, showed very shallow homogeneous flow which is in accordance with a usual steady measured wind without gusts. From all available profiles, it can be concluded that one of the most turbulent areas around the airport is just in front of the runway R12. Although, to get complete 3D structure more lidar and other observations are needed, for the first time small mesoscale and some turbulent features are confirmed with the measurements during bora.

We analysed 10 years (2008&ndash, 2017) of continuous eddy covariance (EC) CO2 flux measurements of net ecosystem exchange (NEE) in a young pedunculate oak forest in Croatia. Measured NEE was gap-filled and partitioned into gross primary productivity (GPP) and ecosystem reparation (RECO) using the online tool by Max Planck Institute for Biogeochemistry in Jena, Germany. Annual NEE, GPP, and RECO were correlated with main environmental drivers. Net primary productivity was estimated from EC (NPPEC), as a sum of &minus, NEE and Rh obtained using a constant Rh:RECO ratio, and from independent periodic biometric measurements (NPPBM). For comparing the NPP at the seasonal level, we propose a simple model that aimed at accounting for late-summer and autumn carbon storage in the non-structural carbohydrate pool. Over the study period, Jastrebarsko forest acted as a carbon sink, with an average (&plusmn, std. dev.) annual NEE of &minus, 319 (&plusmn, 94) gC m&minus, 2 year&minus, 1, GPP of 1594 (&plusmn, 109) gC m&minus, 1, and RECO of 1275 (&plusmn, 1. Annual NEE showed high inter-annual variability and poor correlation with annual average global radiation, air temperature, and total precipitation, but significant (R2 = 0.501, p = 0.02) correlation with the change in soil water content between May and September. Comparison of annual NPPEC and NPPBM showed a good overall agreement (R2 = 0.463, p = 0.03), although in all years NPPBM was lower than NPPEC, with averages of 680 (&plusmn, 88) gC m&minus, 1 and 819 (&plusmn, 89) gC m&minus, 1, respectively. Lower values of NPPBM indicate that fine roots and grasses contributions to NPP, which were not measured in the study period, could have an important contribution to the overall ecosystem NPP. At a seasonal level, two NPP estimates showed differences in their dynamic, but the application of the proposed model greatly improved the agreement in the second part of the growing season. Further research is needed on the respiration partitioning and mechanisms of carbon allocation.

Various criteria have been developed to remove non- stationarity in turbulence time series, though it remains unclear how the choice of the stationarity criterion affects similarity functions in the framework of the Monin–Obukhov similarity theory. To investigate this, we use stationary datasets that result from applying five common criteria to remove non-stationarity in turbulence time series from the Terrain- Induced Rotor EXperiment conducted in Owens Valley, California. We determine the form of the flux–variance similarity functions and the scatter around these similarity functions for all five stationary datasets. Data were collected at two valley locations and one slope location using 34-m flux towers with six levels of turbulence measurements. Our results show (i) systematic differences from previously found near-neutral values of the parameters in the flux–variance similarity functions over flat terrain, indicating a larger anisotropy of the flow over complex than over flat terrain, (ii) a reduction of this anisotropy when stationary data are used, with the amount of reduction depending on the stationarity criterion, (iii) a general reduction in scatter around the similarity functions when using stationary data but more so for stable than for unstable stratification, and for valley locations than for the slope location, and (iv) a weak variation with height of near-neutral values of parameters in the flux–variance similarity functions.

Within the collaboration of VITiculture and CLImate Change in Croatia (VITCLIC) project and Croatian-Hungarian bilateral scientific program, micrometeorological measurements are performed at the Faculty of Agriculture experimental vineyard near Zagreb in the hilly experimental field during the vegetation periods of 2017/2018 and 2018/2019. The microclimate of two places of cordon cultivated grape has been studied for the investigation of the effect of cultivation method. In one row, the grapes were left to be naturally covered by leaves, while in the other row the leaves were being thinned corresponding to the cultivation method. For characterizing the microclimate, the relative humidity and air temperature, wind speed and direction, UV radiation, leaf wetness and leaf temperature were measured inside the cordon rows among the leaves. Air temperature, relative humidity and wind speed gradient have also been measured above the plants. Radiation budget components were detected with CNR1 net radiometer. Heat flux into the soil and the soil temperature and moisture profiles from the surface to a depth of 1 m were also determined. Two soil heat flux plates were set at 8 cm deep. Measurement frequency was 5 sec and the average time was 1 min using Campbell data collecting systems. Our goal, besides the agroclimatological investigations, is the estimation of soil and surface energy budget components (using Bowen-ratio and gradient methods) and the determination of the optimum roughness length and displacement height as a function of the wind velocity. Daily variation of meteorological elements and energy budget components are demonstrated with case studies.

Deep moist convection often yields hail, which is a subject of this work, as a significant precipitation form in many parts of the world. Located in the mid-latitudes of the northern hemisphere, Croatia is exposed to frequent occurrence of thunderstorms and hail, especially in its continental part. In the second half of twentieth century, in order to reduce the risk of damage in agricultural production, a hail suppression system (HSS) was established in the area. Total number of hail cases recorded on HSS stations in time period 1981-2016 is 13995. With the goal to obtain objective and precise hailstone measurements for research studies, hailpads were installed in 2001 on all hail suppression and main meteorological stations. In addition, in 2002 a dense network of hailpads (polygon) was built in the northwest of the continental part of Croatia. In this study, hail suppression station data, radar data, lightning data and hailpad polygon data are used in order to test the hypothesis about the quantitative influence of orography on the spatio-temporal distribution of hail (2002-2016). Comparison of average hailfall duration per season on all hail suppression stations in Croatia and on the polygon shows that the duration on polygon is by 50 % longer than in the whole continental part of Croatia. While hail suppression stations data, radar data and lightning data indicated the orography influence in rather descriptive way, we also use a more objective approach for testing the hypothesis, namely, the Empirical Orthogonal Function (EOF) analysis on the polygon data. EOF analysis is a method that is used for identification of variation patterns that are common to a certain group of data The EOF approach is for the first time applied to such kind of data and the results have confirmed the influence of orography on the distribution of hail. Since elevations of our mountains are below 1500 m, and since they are almost parallel and relatively close to each other, our results suggest that the hailstone distribution over our polygon may be caused by the channelling of the flow between mountain ranges with stagnation and growth of Cb-cells at front of the mountains superimposed.

In the original article, there were mistakes in Figures 2, 4, 6 as published. The interaction term [last right-hand side term in Equation (3) of our published paper] was erroneously multiplied by the average heat conductivity constant. The corrected Figures 2A–C, Figures 4A–C, and Figure 6 appear in the correction article.

Stationarity is a fundamental assumption in the statistical investigation of turbulence and the development of similarity functions that are widely used in surface-layer parametrization schemes. Non-stationary time series should therefore be removed from the analysis before assessing turbulence statistics used in similarity functions. Many approaches have been developed over the years to determine non-stationarity of means and (co-)variances, but there has been no systematic investigation of the differences and similarities between the approaches. In this article, we contrast several frequently used approaches, including two statistical tests to determine trends, the determination of a non-stationarity ratio, and the determination of differences in turbulence statistics calculated for different averaging times. We apply these approaches to near-surface time series of wind and temperature collected during the Terrain-induced Rotor Experiment (T-REX). Our results show that the degree of non-stationarity varies considerably with the approach used. Investigation of the time series that are declared stationary by all the above approaches simultaneously reveals that in many cases such time series still show a behaviour indicative of intermittent turbulence, both for stable and unstable conditions. When these approaches are combined with an additional condition for the intermittency level, a rigorous approach for the detection of non-stationarity is developed.

The Prandtl model couples, probably in the most succinct way, basic boundary-layer dynamics andthermodynamics for pure anabaticand katabaticflowsoverinclined surfaces by assuming a one-dimensional steady-state balance between buoyancy and turbulent friction. Although the classic Prandtl model is linear, having an a priori assigned vertically constanteddydiffusivityandheatconductivity,K,inthisanalyticworkwepartlyrelaxboth of these restrictions. The first restriction is loosened by using a weakly nonlinear approach where a small parameter, e, controls feeding of the flow-induced potential temperature gradient back to the environmental potential temperature gradient, because the former, belowthekatabaticjet,canbe20‐50timesstrongerthanthelatter,backgroundorfree-flow gradient. An appropriate range of values for e, controlling the weak nonlinearity for pure katabatic flow, is provided. In this way, the near-surface potential temperature gradient becomes stronger and the corresponding katabatic jet somewhat weaker (at a slightly lower height) than that in the classic Prandtl solution. The second restriction is partly relaxed by usingaprescribed,graduallyvaryingK withdistancefromtheunderlyingsurface,allwithin the usual validity of the zero-order Wentzel‐Kramers‐Brillouin approximation to solve the coupled differential equations. The new model is compared with the glacier wind data from the Pasterze experiment (PASTEX-94), Austria. Further discussion includes gradient Richardson number consideration and an application to simple anabatic flows. The model may be applied for estimation and interpretation of the wind affecting glacier mass balance and air pollution.

Winds through the Vratnik Pass, a mountain gap in the Dinaric Alps, Croatia, are polarized along the gap axis that extends in the northeast–southwest direction. Although stronger northeasterly wind at the Vratnik Pass is frequently related to the Adriatic bora wind, especially at the downstream town of Senj, there are many cases in which the wind at Senj is directionally decoupled from the wind at the Vratnik Pass. A cluster analysis reveals that this decoupling is sometimes related to lower wind speeds or a shallow southeasterly sirocco wind along the Adriatic, but in many cases the bora blows over a wider region, while only Senj has a different wind direction. Several mechanisms can be responsible for the latter phenomenon, including the formation of a lee wave rotor. A numerical model simulation corroborates the decoupling caused by a rotor for a single case. The prevalence of northeasterly winds at the Vratnik Pass during southeasterly sirocco episodes is another result that challenges the current understanding. It is shown that, at least in one of these episodes, this phenomenon is related to a secondary mesoscale low pressure center in the eastern lee of the Apennines that forms as a subsystem of a broader Genoa cyclone. Less frequent southwesterly winds through the gap are predominantly related to the thermal sea breeze and anabatic circulations that are sometimes superimposed on the geostrophic wind.

International audience; This study investigates the sensitivity of a high-resolution mesoscale atmospheric model in the model reproduction of thermally induced local wind (i.e., sea breezes, SB) on the development of deep convection (Cb). The three chosen cases are simulated by the Weather and Research Forecasting (WRF-ARW) model at three (nested) model domains, whereas the area of the interest is Istria (peninsula in the northeastern Adriatic). The sensitivity tests are accomplished by modifying (1) the model setup, (2) the model topography and (3) the sea surface temperature (SST) distribution. The first set of simulations (over the three 1.5-day periods during summer) is conducted by modifying the model setup, i.e., microphysics and the boundary layer parameterizations. The same events are simulated with the modified topography where the mountain heights in Istria are reduced to 30% of their initial height. The SST distribution has two representations in the model: a constant SST field from the ECMWF skin temperature analysis and a varying SST field, which is provided by hourly geostationary satellite data. A comprehensive set of numerical experiments is statistically analyzed through several different approaches (i.e., the standard statistical measures, the spectral method and the image moment analysis). The overall model evaluation of each model setup revealed certain advantages of one model setup over the others. The numerical tests with the modified topography showed the influence of reducing the mountains heights on the pre-thunderstorm characteristics due to: (1) decrease of sensible heat flux and mid-tropospheric moisture and (2) change of slope-SB wind system. They consequently affect the evolution and dimensions of SBs and the features of the thunderstorm itself: timing, location and intensity (weaker storm). The implementation of the varying SST field in the model have an impact on the characteristics and dynamics of the SB and finally on the accuracy of Cb evolution, duration and the intensity. SST variations emphasized the importance of the phase matching in both daytime cycles of SB and Cb due to their extremely strong nonlinear relationship.

A case study of mountain-wave-induced turbulence observed during the Terrain-Induced Rotor Experiment (T-REX) in Owens Valley, California, is presented. During this case study, large spatial and temporal variability in aerosol backscatter associated with mountain-wave activity was observed in the valley atmosphere by an aerosol lidar. The corresponding along- and cross-valley turbulence structure was investigated using data collected by three 30-m flux towers equipped with six levels of ultrasonic anemometers. Time series of turbulent kinetic energy (TKE) show higher levels of TKE on the sloping western part of the valley when compared with the valley center. The magnitude of the TKE is highly dependent on the averaging time on the western slope, however, indicating that mesoscale transport associated with mountain-wave activity is important here. Analysis of the TKE budget shows that in the central parts of the valley mechanical production of turbulence dominates and is balanced by turbulent dissipation, whereas advective effects appear to play a dominant role over the western slope. In agreement with the aerosol backscatter observations, spatial variability of a turbulent-length-scale parameter suggests the presence of larger turbulent eddies over the western slope than along the valley center. The data and findings from this case study can be used to evaluate the performance of turbulence parameterization schemes in mountainous terrain.

This paper introduces two changes of the turbulence parameterization for the EMEP (European Monitoring and Evaluation Programme) Eulerian air pollution model: the replacement of the Blackadar in stable and O'Brien in unstable turbulence formulations with an analytical vertical diffusion profile (K(z)) called Grisogono, and a different mixing height determination, based on a bulk Richardson number formulation (RiB). The operational or standard (STD) and proposed new parameterization for eddy diffusivity have been validated in all stability conditions against the observed daily surface nitrogen dioxide (NO2), sulphur dioxide (SO2) and sulphate (SO42−) concentrations at different EMEP stations during the year 2001. A moderate improvement in the correlation coefficient and bias for NO2 and SO2 and a slight improvement for sulphate is found for the most of the analyzed stations with the Grisogono K(z) scheme, which is recommended for further application due to its scientific and technical advantages. The newly extended approach for the mechanical eddy diffusivity is applied to the Large Eddy Simulation data focusing at the bulk properties of the neutral and stable atmospheric boundary layer. A summary and extension of the previous work on the empirical coefficients in neutral and stable conditions is provided with the recommendations to the further model development. Special emphasis is given to the representation of the ABL in order to capture the vertical transport and dispersion of the atmospheric air pollution. Two different schemes for the ABL height determination are evaluated against the radiosounding data in January and July 2001, and against the data from the Cabauw tower, the Netherlands, for the same year. The validation of the ABL parameterizations has shown that the EMEP model is able to reproduce spatial and temporal mixing height variability. Improvements are identified especially in stable conditions with the new ABL height scheme based on the RiB number.

The Prandtl model succinctly combines the 1D stationary boundary-layer dynamics and thermodynamics of simple anabatic and katabatic flows over uniformly inclined surfaces. It assumes a balance between the along-the-slope buoyancy component and adiabatic warming/cooling, and the turbulent mixing of momentum and heat. In this study, energetics of the Prandtl model is addressed in terms of the total energy (TE) concept. Furthermore, since the authors recently developed a weakly nonlinear version of the Prandtl model, the TE approach is also exercised on this extended model version, which includes an additional nonlinear term in the thermodynamic equation. Hence, interplay among diffusion, dissipation, and temperature-wind interaction of the mean slope flow is further explored. The TE of the nonlinear Prandtl model is assessed in an ensemble of solutions where the Prandtl number, the slope angle and the nonlinearity parameter are perturbed. It is shown that nonlinear effects have the lowest impact on variability in the ensemble of solutions of the weakly nonlinear Prandtl model when compared to the other two governing parameters. The general behavior of the nonlinear solution is similar to the linear solution, except that the maximum of the along- the-slope wind speed in the nonlinear solution reduces for larger slopes. Also, the dominance of PE near the sloped surface, and the elevated maximum of KE in the linear and nonlinear energetics of the extended Prandtl model are found in the PASTEX-94 measurements. The corresponding level where KE>PE most likely marks the bottom of the sublayer subject to shear- driven instabilities. Finally, possible limitations of the weakly nonlinear solutions of the extended Prandtl model are raised. In linear solutions, the local storage of TE term is zero, reflecting the stationarity of solutions by definition. However, in nonlinear solutions, the diffusion, dissipation and interaction terms (where the height of the maximum interaction is proportional to the height of the low-level jet by the factor ≈4/9) do not balance and the local storage of TE attains nonzero values. In order to examine the issue of non- stationarity, the inclusion of velocity- pressure covariance in the momentum equation is suggested for future development of the extended Prandtl model.

Well known for its severity, the bora downslope windstorms have been extensively studied over the last several decades. This study focuses on the turbulence characteristics of bora at a topographically complex site near the eastern coast of the Adriatic Sea. For this purpose, a 3-month eddy-covariance dataset obtained at three levels (10, 22, 40 m) on a 60-m flux tower is used. After determining a suitable averaging time scale of 15 min using the fast Fourier transform and the ogive method, vertical fluxes of momentum and heat were calculated for 17 bora episodes. Up to a wind speed of $$12\,\mathrm {m\, s}^{-1}$$ , typical vertical profiles of momentum and heat were observed. However, for wind speeds > $$12\,\mathrm {m\, s}^{-1}$$ , several interesting observations arose. First, the nighttime heat flux at the 10-m level was often found to be positive rather negative. Second, vertical profiles of the momentum flux were larger at the 22-m level than at 10- and 40-m levels, mostly during nearly neutral to weakly stable thermal stratification. Third, these momentum flux profiles showed a large dependence on wind direction, with virtually no vertical transport of momentum for the largest observed wind speeds. For the first time, bora coherent structures have been analyzed using the so-called variable-interval time averaging (VITA) method. The method detected coherent structures in all three wind-speed components, with structure topologies similar to those observed over forest canopies. The momentum flux increase at the 22-m level, relative to the 10- and 40-m levels, is further supported by the VITA findings.

Bora is a strong, temporally and spatially variable downslope wind that blows along the Eastern Adriatic Coast and many other dynamically similar places around the world. Bora mean velocity rarely exceeds 20 m/s, while its strong gusts create significant problems for engineering structures, traffic and agriculture. Previous meteorological and geophysical studies laid foundations on Bora large- and mesoscale motions, but further research is necessary to fully understand and resolve Bora turbulence in a form usable for engineers. In this study, unique high-frequency measurements, carried out simultaneously in three heights on a meteorological tower in the hinterland of the city of Split, Croatia, are analyzed for a single summertime Bora episode. Time histories and vertical profiles are studied for turbulence intensity, Reynolds shear stress and turbulence length scales in comparison with values recommended in major international wind engineering standards. Turbulence intensity and Reynolds shear stress proved to be not that sensitive to meandering of the mean wind velocity, as the observed values remain within the same range during the time record. This trend applies on mean wind velocities larger than 5 m/s, as for smaller velocities the spread of values increases considerably. Turbulence length scales are observed to increase with increasing mean wind velocity and vice versa. With increasing height from the ground, turbulence intensity and absolute Reynolds shear stress decrease, turbulence length scales increase, all in agreement with the atmospheric physics. However, while turbulence intensity, Reynolds shear stress and longitudinal turbulence length scales generally agree well with the values recommended in international standards for the respective terrain type, turbulence length scales related to lateral and vertical velocity fluctuations are much larger than the standard values.

The along-coast, offshore turbulence structure of the Bora flow that occurred on 7 November 1999 during the Mesoscale Alpine Programme (MAP) Intensive Observation Period 15 is examined. In this analysis we employ the aircraft and dropsonde data obtained over the Adriatic Sea, where the turbulence structure is determined by estimating turbulent kinetic energy (TKE) and its dissipation rate along the flight legs. The turbulence characteristics of Bora in the lee of the Dinaric Alps is greatly influenced by the mesoscale Bora flow structure over the Adriatic Sea, which in the cross-wind direction features an interchange of jets and wakes related to mountain gaps and peaks. In order to establish the origin of turbulence, the Weather Research and Forecasting—Advanced Research WRF (WRF-ARW) numerical model is used and its results are compared to the measurements. All five TKE-prediction parametrization schemes available in the model show reasonable agreement with the measured values. Since these parametrization schemes do not have horizontal advection included, they suggest that the along-flight structure of the Bora turbulence is principally generated by the local vertical wind shear. Further evidence is needed to support this hypothesis.

Atmospheric boundary layer motions are statistically non-stationary, and therefore, it is necessary to transform them into statistically stationary (statistical properties do not change in time) time series. This transformation is performed by the mean removal process, which defi nes an integral time scale for turbulent fl uctuations. The present study compares two methods for estimating physically relevant time scale in a stable, wintertime nocturnal boundary layer. This time scale is used to remove the unsteady mean component of the instantaneous time signal for any meteorological variable. In this way, turbulent fluxes that appear statistically stationary can be yield. The fi rst method employs the Fourier analysis in order to remove the contamination by mesoscale motions in calculation of eddy correlation flux. The second method used for determining a relevant turbulence avenging time scale represents a cumulative integral of the kinematic heat flux or momentum flux cospectrum: the ogive method. The time scale is evaluated using three-dimensional sonic anemometry data acquired at five levels of a meteorological mast (20, 32, 40, 55 and 62 m a.g.l.) located in the industrial part of Kutina, a town in Croatia. The results indicate the existence of the spectral gap scale in the spectrum of streamwise velocity component at all levels except at 32 m. This gap scale can be found for periods between 30 and 7 min for 20 m level and between 22 and 7 min at other levels. The time scale estimated by the second method is based on the location of the local maximum value of the ogive function of the kinematic heat fl ux and momentum fl ux cospectra. The ogive method gives time scales that defi ne the high-frequency end of the spectral gap., Gibanja u atmosferskom graničnom sloju su statistički nestacionarna te ih se stoganužno mora transformirati u statistički stacionarne vremenske nizove. (Statistička svojstva stacionarnih nizova se ne mijenjaju u vremenu.) Ova transformacija se izvodipostupkom uklanjanja srednjaka, čime se definira vremenska skala turbulentnih fluktuacija. U ovoj studiji uspoređujemo dvije metode pomoću kojih ocjenjujemo fizikalno relevantne turbulentne vremenske skale u stabilnom, zimskom noćnom graničnom sloju. Ovavremenska skala se koristi za uklanjanje nestacionarne srednje komponente u vremenskomnizu bilo koje od mjerenih meteoroloških varijabli. Prva metoda uključuje primjenu Fourierove analize, kako bi u računanju vertikalnih tokova turbulentnih kovarijanci utjecajmezoskalnih gibanja bio isključen. Druga metoda korištena za određivanje relevantne turbulentne vremenske skale usrednjavanja predstavlja kumulativni integral kospektrakinematičkog toka topline ili toka impulsa (eng. ogive metoda). Za ocjenu ove vremenskeskale koriste se trodimenzionalni podaci soničnih anemometara postavljenih na pet visinameteorološkog tornja (20, 32, 40, 55 i 62 m iznad tla), koji se nalazi u industrijskom dijelugrada Kutine. Rezultati upućuju na to da na svim visinama, izuzev 32 m, postoji procjep uspektru komponente vjetra usmjerene niz srednje strujanje. Spektralni procjep se uočavana periodima između 30 i 7 minuta na visini od 20 m, te između 22 i 7 minuta na ostalimvisinama. Vremenska skala turbulentnog usrednjavanja dobivena pomoću druge metodese temelji na pronalaženju frekvencije, pri kojoj se javlja lokalni maksimum u kumulativnom integralu kospektra kinematičkog toka topline i toka impulsa. Druga metoda dajevremenske skale koje definiraju visokofrekventni kraj spektralnog procjepa.

Settled in the mid latitudes of the Northern Hemisphere, Croatia is exposed to the frequent occurrence of severe thunderstorms and hail, especially in the continental part between Sava, Drava and Mura rivers (Pannonian basin). On the basis of the hail fall data collected in the continental part of Croatia which is protected from hail (26, 800 km^2) in period 1981-2006, it is found that, two different areas can be identified: western (hilly) part with the greatest number of days with hail, and eastern generally flat part which has mostly minimum days with hail. Spatial distribution of hailfall duration shows that majority of quadrants with longer average duration of hail fall are settled in the front of the mountains. Concerning the analysis of the hail pads data collected in period 2001-2006, it is also found that the areas with grater diameter of hail can be found in front of the mountains.

During a bora event, the turbulence is strongly developed in the lee of the Dinaric Alps at the eastern Adriatic coast. In order to study its properties, a 3-D ultrasonic anemometer operating at 4 Hz sampling frequency was placed in the town of Senj at 13 m above ground. The strong bora case that occurred on 7 January and lasted till 11 January 2006 is analyzed here. This data set is used for evaluation of the turbulent kinetic energy, TKE, and its dissipation rate, ε. The computation of ε is performed using the inertial dissipation method. The empirical length scale parameter for this event is estimated with respect to ε and TKE. Some considerations about defining turbulent perturbations of the bora wind velocity are also pointed out.