Influence of regional and local winds on urban ventilation in Cologne, Germany

Abstract

This study examines the influence of regional and local winds on urban ventilation during clear and calm summer nights with low exchange weather conditions, taking the city of Cologne in the Cologne Bay (Germany) as an example. Results of this study on cold air penetration in the city show a sensitive interaction between the structure and direction of open rural areas and the temporal pattern of local cold air movement and regional wind. During the first half of calm summer nights, directly linked ventilation areas are unable to ensure the horizontal exchange of locally formed cold air between the surrounding countryside and the urban area as a result of the long distance from the city centre and large obstacles obstructing the ventilation areas. Time-scale analyses of the wind field carried out at nine meteorological stations show that the ventilation areas function independently from each other. In rural ventilation areas a flow of cold air from neighbouring slopes is observed during the whole night. During the second half of calm summer nights, the wind direction in the ventilation areas follows a path running downstream parallel to the River Rhine, caused by advected cold airformed and accumulated in the upstream section of the Cologne Bay. Advected cold air is superimposed on microscale circulation of cold air in the ventilation areas, allowing a horizintal transport of air from the surrounding countryside into the city. Depending on the direction and extent of the ventilation areas, regional wind can strengthen or replace the local air transport in the city of Cologne.

Keywords

• urban ventilation
• cologne
• germany
• time-scale analyses
• wind field
• köln
• kaltluftbewegungen