资源环境科技发展态势分析平台

This study investigates fog development over the greater Zagreb area; a long-lasting fog event that took place from 6 to 8 November 2013. According to climatological data, haze and fog occur frequently over the Zagreb airport area and cause severe low visibility that can last for several days. The Zagreb airport is located on a flat terrain south of Zagreb near the Sava River. North of Zagreb, Medvednica Mountain rises up to 1000 m above sea level over a relatively short distance of approximately 10 km, with a very well-defined downslope forest area.

The here selected case was analyzed by means of available meteorological and air quality measurements and numerical simulations performed by the WRF-ARW high-resolution numerical model. The model was able to reproduce this fog event with some differences among the various model setups. The influence of the Sava River was also estimated by the additional calculation of backward/forward trajectories.

The results revealed the roles of (i) downslope wind, which usually occurred over the city when the net radiation over the Medvednica slopes became negative, and (ii) the urban Zagreb area. Maximum wind speeds occurred at a height of approximately 300 m above ground level, which was slightly above the temperature inversion at the intersection between the slopes and the city. When the downslope flow experienced a hydraulic jump at the lower part of the slope, the downslope flow did not progress significantly over the Sava River or to the south. Then the fog is shallow and dense, staying away from the city. Under certain conditions, the extent of the katabatic flow can reach the airport area. Then the effect of the downslope flow was not apparent in the surface layer over the airport; however it existed above the thermal inversion layer, which contributed to fog persistence.

The possible influences of the Sava River and urban heat island (UHI) on fog were relatively small. The largest river effect is most likely at stations that are located northeast of the river. The UHI can contribute to the formation of katabatic winds under clear-sky conditions in the afternoon/early evening. Still during temperature inversion conditions with fog, the UHI can affect the magnitude of the temperature inversion by reducing it.