Ivana Tošić

Geostrophic Adjustment on Hexagonal Grids

A simple two-dimensional linearized model of the atmosphere is used to investigate behavior of the geostrophic adjustment process on five selected horizontal hexagonal grids. This study shows that hexagonal lattices have some advantages over commonly used square grids. Having better isotropy, they provide more accurate dispersion of gravity waves than square grids do, and therefore they can be more appropriate for simulation of smaller-scale divergent processes. The gravity‐inertia wave frequencies of most of the considered grids are also closer to the true solution than those on square grids. However, some hexagonal grids are not completely free of numerical inconsistencies. For example, one of the selected grids generates nonsymmetric roots of the gravity‐inertia solution that may lead to the unwanted simulation of the atmospheric process. The analysis indicates which hexagonal distributions can be an appropriate choice for use in atmospheric and ocean models.

Aridity in Vojvodina, Serbia

For investigating aridity in Vojvodina, two parameters were used: the De Martonne aridity index and the Pinna combinative index. These indices were chosen as the most suitable for the analysis of climate in Vojvodina (a region in northern part of Serbia). Also, these indices were calculated from data obtained from 10 meteorological stations for the period from 1949 to 2006. The spatial distribution of the annual and seasonal De Martonne and the Pinna combinative indices as well as the mean monthly values of the De Martonne index and aridity trends of these indices are presented. There were two, four, and five types of climate on a yearly, seasonal, and monthly basis in Vojvodina, according to the De Martonne climate classification which consists of a total of seven types. In addition, semi-humid and humid climate types were represented in the region, on a yearly basis. The winter season was dominated by wetter types of climate, while the summer season was characterized by drier ones. During the spring and autumn seasons, there were types of climate which range between both aforementioned types. Two out of three climate types, which can be identified using the Pinna combinative index, were registered in Vojvodina region. The most dominant climate type was the semidry Mediterranean with formal Mediterranean vegetation, while the humid type was only identified in one small part of southwestern Vojvodina. The calculated values of both aridity indices showed that there were no annual trends. Therefore, it can be considered that there were no recent aridity changes during the observed period. For paleoclimate, the general story is more complex. The lack of aridity trends in the recent period from 1949 to 2006 supports the fact that Vojvodina has very well preserved loess–palaeosol sequences from the Middle and Late Pleistocene, which indicates that crucial point for their preservation was caused by the weak aridity variability in the region.

A real data simulation of the Adriatic bora and the impact of mountain height on bora trajectories

SummaryThis study addresses simulation of the local bora wind and its properties as reflected on typical trajectories. Trajectory calculations are implemented in the Eta Model. The Eta Model has a vertical coordinate which permits a step-like representation of mountains and quasi-horizontal coordinate surfaces, the so-called eta coordinate. A realistic real data simulation of a bora wind case in achieved using the model with a 28 km horizontal resolution and 16 layers in the vertical. Numerical experiments with different mountain heights and shapes in the bora wind region are performed. These are motivated by observational indications and theoretically based expectations that a certain intermediate mountain elevation is required for generation of downslope windstorms with bora wind properties. Three-dimensional trajectories over various mountains mimicing real mountains but differing primarily in elevation are calculated and analysed. The maximum bora wind speed is predicted as expected through three-dimensional channels in the step mountain representations. The results illustrate and are in agreement with the observational evidence that mountain barriers of the elevation of about 1000 m are a necessary requirement for the occurrence of the bora-type downslope windstorms.

Spatial analysis of the temperature trends in Serbia during the period 1961–2010

The spatial analysis of annual and seasonal temperature trends in Serbia during the period 1961–2010 was carried out using mean monthly data from 64 meteorological stations. Change year detection was achieved using cumulative sum charts. The magnitude of trends was derived from the slopes of linear trends using the least square method. The same formalism of least square method was used to assess the statistical significance of the determined trends. Maps of temperature trends were generated by applying a spatial regression method to visualize the detected tendencies. The obtained results indicate a negative temperature trend for the period before the change year except for winter and a more pronounced positive trend after the change year. Besides being more pronounced, the vast majority of trends after the change year were also clearly statistically significant. Our estimate of the average temperature trend over Serbia is in agreement with those obtained at the global and European scale. Calculated global autocorrelation statistics (Moran’s I) indicate an apparent random spatial pattern of temperature trends across the Serbia for both periods before and after the change year.

Some aspects of the wind ‘Koshava’ in the lower troposphere over Belgrade

‘Koshava’ is a gusty wind of moderate to strong intensity, blowing from a south-easterly direction, over Serbia, Romania and Bulgaria. It is caused by the interaction between the synoptic circulation and the orography of the Carpathian and the Balkan mountains. This paper analyses wind data from an area where the influence the ‘Koshava’ wind is high during periods of maximum duration of ‘Koshava’. The focus of the paper is the examination of urban and suburban effects on ‘Koshava’ and the correlation between the instantaneous maximum wind speed and the hourly mean values. Various empirical distributions are fitted to the daily maximum wind speed and the best fit for each season and site is proposed. The vertical structure of the lower troposphere above a suburban site during the longest period of consecutive days of ‘Koshava’, which occurred in January and February 1972, is considered. Copyright

On some new statistical characteristics of the wind “Koshava”

Summary“Koshava” is a gusty wind of moderate to strong intensity, blowing from the south-eastern direction, over the area of the Republic of Serbia. It is caused by the interaction between the synoptic circulation and the orography of the Carpathian and the Balkan mountains. The “Koshava” wind can damage buildings, factories and industrial plants or city infrastructure. Therefore it is important to estimate its gust and the gustiness factor on the basis of the measured data.This paper discusses a statistical analysis of wind data in the maximum influence area of the “Koshava” wind in the periods of maximum duration of “Koshava”. The focus of the paper is the examination of urban and suburban effects on “Koshava” wind and the correlation between the instantaneous maximum wind speed and the hourly mean wind speed. The best fitting with various empirical distributions is proposed.

Improved bora wind simulation using a nested Eta Model

SummaryThis paper reports on improvements obtained in simulations of bora wind using a nesting technique in a primitive equations model. A version of the so-called Eta Model is used for both the lower resolution as well as for the higher resolution nested run. For nesting, a technique has been developed whereby for the boundary conditions “underground” model values are used. They are obtained by horizontal interpolation of model variables inside the models eta layers. Simulations are initialized using real data. This is done in order to assess the models performance in simulating the bora wind, an important mesoscale phenomenon caused by orography. Improvements achieved by nesting are illustrated by examples of precipitation as well as a trajectories forecast.

Water-soluble inorganic ions in urban aerosols of the continental part of Balkans (Belgrade) during the summer - autumn (2008)

Abstract Size-segregated aerosol samples were collected using six stages High Volume Cascade Impactor. Aerosol mass and water soluble ions concentrations were determined. The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model was used to study the origin of air masses arriving to Belgrade in the investigated period. The obtained results of aerosol mass and water-soluble ion concentrations have been divided into six sub-data sets based on air mass categories. The highest average mass concentration of the fine mode (Dp ≤ 0.49 μm) was found for air masses coming to Belgrade from the southeast and northwest directions, and of the coarse mode (3.0 < Dp ≤7.2 μm) for air masses arriving from the northwest direction. The highest concentrations of SO42– were found in the fine particles transported to the investigated area by air masses from southeast direction. The analysis of contribution of marine aerosol components (Na+ and Cl-), the Correlation and Cluster Analysis indicated the influence of marine aerosol on urban aerosol of the central Balkans coming from the Western Mediterranean and northern direction. NH4+ and SO42- and K+ dominated in the fine mode for all air mass categories. PCA demonstrated the dominant impact of secondary aerosol formation processes on urban aerosols. Graphical Abstract

Propagation of the Rossby waves on two dimensional rectangular grids

SummaryA simple two-dimensional quasi-geostrophic linearized model of the atmosphere is used to investigate the behaviour of the quasi-geostrophic modes for five horizontal rectangular grids. Numerical expressions for frequencies of Rossby waves for all grids are evaluated. It was found that the B and C grids produce only negative frequencies as well as the continuous case. The D grid has negative and zero frequencies. Finally, it was found that the A and E grids produce positive frequencies and eastward moving Rossby waves.

Seasonal prevailing surface winds in Northern Serbia

Seasonal prevailing surface winds are analyzed in the territory of Northern Serbia, using observational data from 12 meteorological stations over several decades. In accordance with the general definition of prevailing wind, two special definitions of this term are used. The seasonal wind roses in 16 directions at each station are analyzed. This study shows that the prevailing winds in Northern Serbia have northwestern and southeastern directions. Circulation weather types over Serbia are presented in order to determine the connections between the synoptic circulations and prevailing surface winds. Three controlling pressure centers, i.e., the Mediterranean cyclone, Siberian high, and the Azores anticyclone, appear as the most important large-scale factors that influence the creation of the prevailing winds over Northern Serbia. Beside the synoptic cause of the prevailing winds, it is noted that the orography of the eastern Balkans has a major influence on the winds from the second quadrant. It was found that the frequencies of circulation weather types are in agreement with those of the prevailing winds over Northern Serbia.