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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.

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.

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.

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.

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.