Counter-current and channelling effect under stable stratification in the area of Karlsruhe

Summary

The topic of this study is the investigation of channelling of the airflow and the so-called counter-current within the area of Karlsruhe. These phenomena of a strong turning of the wind direction with respect to the height generally develop under conditions of stable stratification and are of great importance in air pollution modelling.

First a case study of a channelling event with data from the 200 m high IMK-tower is analysed in detail. During this channelling event a deviation angle, i.e. a difference of the wind direction between the upper (200 m) and lower levels (40 m) of about 100° exists.

In the second part, the data collected at the IMK-tower over a period of 16 years are used to evaluate from the climatological point of view the frequency of channelling events in conjunction with the meteorological conditions. Concerning the deviation angle it can be found that there is an increase of the mean deviation angle at the transition from unstable stratification to stable stratification, but only a slight dependence on the degree of stable stratification. In contrast a considerable dependence on the wind speed is found, i.e. there is a decrease of the deviation angle with increasing wind speed. The mean deviation angle for a gradient Richardson number Ri < 0 is about 10°, while for Ri > 0 this value increases up to 50°.

Finally, numerical simulations are carried out with the meso-scale model KAMM. The simulated deviation angles are in good agreement with the observations. Especially from these simulations the dependence of the deviation angle on the wind speed can be explained.