Modifications of a Valley Wind System by an Urban Area – Experimental Results

Summary

The Austrian city of Graz at the south-eastern edge of the Alps frequently experiences wintertime stagnations during anticyclonic flow conditions, leading to high local concentrations of primary pollutants. This paper investigates the dominant three-dimensional local flow structures in the Graz region during a representative January stagnation period in 1998 using data obtained from a field experiment that supplemented the routine meteorological network with an array of sodars and tethersondes and a meteorological tower. Important modifications to the temperature and wind fields over Graz and its surroundings are attributed to both topographical and urban effects. The main modifications to the along-valley wind system in the Mur valley that runs through Graz from north to south are caused by near-surface temperature field differences between a warmer north and a cooler south part of the city and the regular development of a nighttime down-valley low-level jet and its upward lifting when crossing the city centre.