Modifications of a valley wind system by an urban area - Experimental results

The Austrian city of Graz at the south-eastern edge of the Alps frequently experiences wintertime stagnations during anticyclonic flow conditions, lea ding to high local concentrations of primary pollutants. This paper investi gates the dominant three-dimensional local flow structures in the Graz regi on during a representative January stagnation period in 1998 using data obt ained 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. Th e main modifications to the along-valley wind system in the Mur valley that runs through Graz from north to south are caused by nearsurface temperatur e field differences between a warmer north and a cooler south part of the c ity and the regular development of a nighttime down-valley low-level jet an d its upward lifting when crossing the city centre.