Schween, J. H., Hirsikko, A., Löhnert, U., and Crewell, S.

The height of the mixing layer (MLH) determines not only the dispersion of pollutants in the atmospheric boundary layer. It also influences whether convective clouds form, how frequent they are and how large they will grow.

Common techniques for the determination of the MLH are based on profiles of temperature, wind and/or humidity or aerosol backscatter. All of these are proxies to the underlying process i.e. the vertical movement of air parcels.

Temperature profiles give information about the height where rising parcels become non buoyant. But due to their inertia they will rise further while they are dissipated by entrainment and wind shear. The influence of wind shear can be incorporated by combining temperature and horizontal wind speed in the Richardson- or Bulk-Richardson number. But the combination of dynamics and dissipation makes it difficult to give an accurate estimate for the height convective plumes can reach. This results in great uncertainty about the 'critical Richardson number' which should identify this height.

Profiles of constituents like water vapor and aerosol which originate from the surface show in general a sharp decrease towards the 'clean' free troposphere at the top of the boundary layer. But they rather reflect the history of the mixing than the current state. Especially in the morning hours when the mixing layer grows into the residual layer a detection of the height of this interface is challenging.

A Doppler lidar measures the velocity along its beam thus providing the opportunity to measure directly the vertical movement i.e. the mixing process itself. We present a method to derive MLH from profiles of vertical velocity investigate it in comparison with other methods and its sensitivity to the used threshold.