engleski

The impact of downslope winds and the urban heat island on fog formation over the Zagreb area

This study investigates fog development over a wider Zagreb area. According to the climatological data, haze and fog occur frequently over the Zagreb-airport area and cause severely low visibility that can last for several days. Zagreb airport is located in a flat terrain south of Zagreb near the Sava river at a height of 108 m above sea level (asl). To the north, the city of Zagreb (~120 m asl) is a main source of urban pollution including condensation nuclei. There are heavy traffic roads around the airport which also generate pollutants. North of Zagreb, the Medvednica mountain rises up to 1000 m asl in a relatively short distance of about 10 km, with a very well defined downslope forest area. We focus on a case study of a long-lasting fog event that took place during 6-8 November 2013 to understand of the dynamic processes of fog development and fog persistence. The selected case was analyzed by means of available measurements and numerical simulations performed by the WRF-ARW high-resolution numerical model in several model setups. The model was able to reproduce this fog event with small differences among the various model runs. The results revealed the roles of: (i) the downslope wind which usually occurred over city when the net radiation over the Medvednica slopes becomes negative and (ii) the effect of urban Zagreb area which adds pollution to the downslope flow and consequently decreases fog duration over the city. The effect of the downslope flow was not apparent in the surface layer over the airport because it occurred above the thermal inversion contributing to the fog persistence. The influence of the Sava river has been also estimated and discussed. This comprised an additional calculation of backward trajectories and simulations of the water vapor dispersion (a passive scalar in the atmosphere) by the FLEXPART Lagrangian particle dispersion model driven by the WRF output.

fog; downslope wind; turbulence; WRF

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