Abstract
Under high-pressure systems, the nocturnal atmospheric boundary layer in the Pannonian Basin is influenced by gravity flows generated at the mountain ranges and along the valleys, determining the variability of wind and temperature at a local scale and the presence of fog. The mechanisms at the mountain foothills are explored at Zagreb Airport using data from a sodar and high-resolution WRF-ARW numerical simulations, allowing identification of how the downslope flows from the nearby Medvednica mountain range condition the temperature inversion and the visibility at night and early morning. These flows may progress tens of kilometres away from the mountain ranges, merging with valley flows and converging in the central areas of the basin. The ECMWF model outputs allow us to explore the mesoscale structures generated in form of low-level jets, how they interact when they meet, and what is the effect of the synoptic pressure field over eastern Europe, to illustrate the formation of a basin-wide cold air pool and the generation of fog in winter.
Highlights
Large areas with low elevation relative to their surroundings in emerged land are rarely flat, even if they seem otherwise to the human eye
Very gentle slopes are normally present, and microscale topographical variability generates shallow depressed zones with local minimal temperatures. These almost-flat lands are usually confined by some significant topography over which slope flows develop in appropriate meteorological conditions, usually reaching the low lands and determining the dynamics of the atmospheric boundary layer (ABL) and the local climatology
The large flat area of the Pannonian Basin is surrounded by mountain ranges and the Transylvanian elevated area
Summary
Large areas with low elevation relative to their surroundings in emerged land are rarely flat, even if they seem otherwise to the human eye. Very gentle slopes are normally present, and microscale topographical variability generates shallow depressed zones with local minimal temperatures These almost-flat lands are usually confined by some significant topography over which slope flows develop in appropriate meteorological conditions, usually reaching the low lands and determining the dynamics of the atmospheric boundary layer (ABL) and the local climatology. The characteristics of the interaction of flows originating at mountain ranges and over adjacent plains in the Pannonian Basin at night and their role in the generation of fog are the subjects of this study This eastern European region has become the subject of detailed investigations under the umbrella of the PannEx initiative [1], that aims to study the basin in an integrated matter for agro-climatological [2,3,4,5] and hydrometeorological [6,7]. We aim to analyse, in detail, the phenomena in the area of the foothills of the Medvednica mountain range, where fog is a recurrent feature needing better understanding [8], and to see how flows organise in the plain, well away from the mountain ranges [9]
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