2.  Esau, I.: Large scale turbulence structures in the Ekman boundary layer.

The Ekman boundary layer (EBL) is a non-stratified turbulent layer of fluid in a rotated frame of reference. The EBL is frequently observed in the atmosphere and the ocean. The EBL comprises two sub-layers, namely, the surface sub-layer, where small scale well-developed turbulence dominates, and the core sub-layer, where large scale self-organized turbulence dominates. This study reports self-organization of large scale turbulence in the EBL as simulated with the large-eddy simulation (LES) model LESNIC. The simulations were conducted in a large domain (144 km in the cross-flow direction, which is an equivalent to about 50 EBL depths) to resolve statistically significant number of the largest self-organized eddies. Analysis revealed that the latitude of the LES domain and, unexpectedly, the direction of the geostrophic wind forcing control the self-organization, turbulence scales, evolution and the quasi steady-state averaged vertical profiles in the EBL. The LES demonstrated destabilization of the EBL turbulence and its mean structure by the horizontal component of the Coriolis force. Visualisations of the EBL disclosed existence of quasi-regular large scale turbulent structures composed of counter-rotating vortices when the geostrophic flow was set from East to West. The corresponding structures are absent in the EBL when the geostrophic flow was set in the opposite (i.e. West to East) direction. These results finally resolve the long-standing controversy between the Leibovich-Lele and the Lilly-Brown instability mechanisms acting in the EBL. The LES demonstrated that the Lilly-Brown mechanism, which involves the vertical component of the Coriolis force, is working in the polar EBL where its impact is nevertheless rather small. The Leibovich-Lele mechanism, which involves the horizontal component of the Coriolis force, acts in low latitudes where it completely alters the turbulent structure of the EBL.

Keywords: atmospheric boundary layer, large-eddy simulations, Ekman boundary layer, turbulence self-organization


3.  Babić, K., Z., Bencetić Klaić and Ž. Večenaj: Determining a turbulence averaging time scale by Fourier analysis for the nocturnal boundary layer.

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 defines an integral time scale for turbulent fluctuations. 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 first 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 flux and momentum flux cospectra. The ogive method gives time scales that define the high-frequency end of the spectral gap.

Keywords: stable boundary layer, integral time scale, spectral gap, ogive function


4.  Bityukova, V. R. and N. S. Kasimov: Atmospheric pollution of Russia-s cities: Assessment of emissions and immissions based on statistical data.

In Russia, 60 million people live in the cities with high levels of air pollution. Comparative statistical analysis of pollutant emission and immission processes in 1099 cities in the country revealed the role of climate and other environmental factors, fuel mix, and the impact of agglomeration effect on the distribution of pollutants in the cities’ atmosphere. In 80% of Russia’s cities, air pollution is connected to the levels of anthropogenic emissions; in 5% of the cities, urban pollution levels (pollutants concentration levels according to monitoring (measuring) data) are lower than emissions; and in 15% of the cities, natural conditions amplify the anthropogenic impact. The level of anthropogenic impact in Russia’s cities is largely determined by a combination of low efficiency and high power intensity, outdated industrial specialization and inherited transport networks that cannot adequately accommodate current traffic flows. The system of proposed indicators of ecological conditions of the urban environment can be used in assessment of the environmental component of quality of life and its modern processes providing the basis for further ecological and geochemical studies of urban areas.

Keywords: city ecology, anthropogenic impact, indicators, pollution sources, environmental risk, emission, immission, statistical analysis of pollutant


5.  Bencetić Klaić Z., K. Ružman, I. Smiljanić, M. Stojnić and P. Erbe: The influence of meteorological conditions and weak to moderate traffic density on PM1 levels in a residential area of Zagreb, Croatia.

The levels of 1-min mean ambient particles with an aerodynamic diameter up to 1 µm (PM1) recorded at two nearby measuring sites in a residential area of Zagreb, Croatia, were analyzed with respect to the meteorological conditions (air pressure, horizontal and vertical wind speed, wind direction, global radiation, air temperature and relative humidity) and local traffic. PM1 mass concentrations were measured using a Model 8520 DUSTTRAKTM Aerosol Monitor (TSI Inc., Shoreview, MN, USA). One measuring site was placed next to a road with weak to moderate traffic, and the other was on a roof terrace approximately 100 m east of that road; the heights of the aerosol monitor inlet were at 1.7 m (road) and 15.8 m (roof) above the ground level (AGL). Both road and the roof experiments were performed by the same instrument, and thus, they correspond to different time intervals. The roof results show that nearby roads do not affect the PM1 concentrations at the site 100 m from the road, while, according to preliminary road results, the effects of the local traffic appear only if more than 22 vehicles per minute pass by the instrument. However, PM1 levels seem to be strongly affected by traffic occurring in the more distant areas of Zagreb. Additionally, the results for certain wind directions also exhibit the influence of the regional and/or long-range transport of pollution on PM1 levels. Finally, it is shown that 1-min mean PM1 mass concentrations depend on the 1-min mean meteorological variables investigated.

Keywords: advection of particles, ambient concentrations, DUSTTRAKTM Aerosol Monitor, traffic density, 1-min mean


6.  Babić, K., P. Mikuš and M. Telišman Prtenjak: The relationship between shallow thermal circulation regimes and cumulonimbus clouds along the northeastern Adriatic coast.

The aim of this study is to determine the relationship between the occurrence of a sea breeze (SB) and cumulonimbus (Cb) clouds over the Istrian Peninsula. For this purpose, available standard surface measurements, i.e., the near-surface wind, the air and sea surface temperatures and cloudiness at two stations (Pula-Airport and Pazin), and satellite images were analyzed. The study was performed during the summer months (from June to September) for the years 1997-2006. The analysis showed that the Cb development was typically associated with certain meteorological conditions. These conditions were as follows: (i) SB speed at Pula-Airport in the range of 3-5 ms-1, (ii) maximum temperature difference between sea and land near Pula-Airport around 4 °C, and (iii) maximum air temperature in Pazin in the range of 25 to 31 °C. On average, during the days with simultaneous development of SB and Cb clouds, the land breeze was weaker and the air temperature was higher than on other analyzed days. Diurnal cloud evolution showed that Cb clouds usually develop (above Pazin) between 13 and 14 h of Central European Time (CET). Cumulus clouds, as indicators of the SB inland penetration, preceded the Cb development. Satellite images for 2000-2006 obtained by the geostationary satellites Meteosat 7 and 8 were investigated, and a spatial distribution and a temporal development of 30 chosen Cb events were detected. Two characteristic regions of the Cb origin were noted: the northern and southeastern parts of the Istrian Peninsula. The Cb clouds usually formed between 11 and 13 CET, lasting in general from 3 to 5 hours and disintegrating between 15 and 17 CET.

Keywords: sea/land breeze, Istria, convergence zone, satellite images, sea breeze index