Vertical structure of the urban and rural boundary layer in Mülheim/Ruhr (Germany)

After 6 years of continuous measurements we have discovered a well developed heat island in the city centre of Muelheim/Ruhr. During clear radiation nights cold air is forming in the Rumbachvalley, and moves towards the city centre. This current of cold air makes its way from the valley into the city centre by following a wide tree lined street and continues along street canyons. . Weakening of this cold air flow is due to the increasing roughness of urban buildings and the asphalt surfaces that have been heavily heated by daytime radiation; the latter lead to continuous warming up and turbulent mixing of the cold air. However, it was observed repeatedly that cold air flow did not occur close to the ground surface in areas that had been heated up by incoming radiation during the day; cold air flow took place here in a slightly turbulent way about 6 –8 m above the street surface, making its way into the city centre through street canyons as one continuous flow. As the cold air approaches the city centre, its path is interrupted by wide crossroads and three 70 m high- story-buildings. After many measurement campaigns using smoke as tracer we found out that in 50 % of the radiation nights the katabatic cold air disappeared in this area due to turbulence. The turbulence is caused by: 1. The large area of hot surfaces (thermal turbulence). 2. The high- story-buildings reaching into the dominating field of air flow in higher layers (during periods with dominating high winds, mechanical turbulence is reaching down to street level); strength of this effect is dependent on the state and presence of such a dominating field of air flow at larger heights.

Contrary to this long period of research two tracer experiments have been carried out by the University of Essen. From the results of their measurements they concluded that the cold air from the Rumbachvalley is already sliding atop the urban heat island in the outer suburban areas. This is proposed to be the reason for mixing of cold rural air in the upper levels of the boundary layer.

To clarify this important question – especially in order to answer questions about air pollution – we made measurements using a SODAR for about a week in summer 1999. In co-operation with the University of Wroclaw, Poland we measured the vertical structure of the atmosphere above rural and above urban area as well as the structure in the outer suburban areas. This poster presents the first results of these measurements as well as the results of another measurement campaign in April and Mai 2000.