Određivanje turbulentne skale usrednjavanja u noćnom graničnom sloju na temelju Fourierove analize

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

ENERGY BUDGET AT THE EXPERIMENTAL VINEYARD IN ZAGREB

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

The Community Foehn Classification Experiment

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 region, sign of temperature change at onset, and depth of overflowing layer. They affect 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 Masters degree students who had taken mountain meteorology courses, and a further two of objective algorithms. Sixty periods of 48 hours were classified for foehn/no foehn 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 reproducibility of classified foehn duration, which turned out to be 50% or less. The classification dataset can now serve as a testbed for automatic classification algorithms, which - if successful - eliminate the drawbacks of manual classifications: lack of scalability and reproducibility

Exchange Processes in the Atmospheric Boundary Layer Over Mountainous Terrain

The exchange of heat, momentum, and mass in the atmosphere over mountainous terrain is controlled by synoptic-scale dynamics, thermally driven mesoscale circulations, and turbulence. This article reviews the key challenges relevant to the understanding of exchange processes in the mountain boundary layer and outlines possible research priorities for the future. The review describes the limitations of the experimental study of turbulent exchange over complex terrain, the impact of slope and valley breezes on the structure of the convective boundary layer, and the role of intermittent mixing and wave–turbulence interaction in the stable boundary layer. The interplay between exchange processes at different spatial scales is discussed in depth, emphasizing the role of elevated and ground-based stable layers in controlling multi-scale interactions in the atmosphere over and near mountains. Implications of the current understanding of exchange processes over mountains towards the improvement of numerical weather prediction and climate models are discussed, considering in particular the representation of surface boundary conditions, the parameterization of sub-grid-scal

Velocity profile of the gusty Bora wind

Abstract Vertical velocity profile of the gusty Bora wind blowing on the eastern Adriatic coast is analysed based on the meteorological-tower measurements carried out during summer. Those high-frequency field measurements are taken at three heights up to 40 m. The observed experimental data in this near-ground layer are generally in good agreement with the power-law and the logarithmic-law approximations. However, the most fascinating finding is an increase in the power-law exponent, friction velocity and aerodynamic surface length with decreasing Bora wind velocity that indicates an urban-like velocity profile for smaller wind velocities and a rural-like velocity profile for larger wind velocities