Determination of the mixing layer height by a ceilometer

The mixing layer height (MLH) is a key parameter in studies of many atmospheric boundary layer processes, including air quality. It primarily determines the volume in which turbulence is active and into which constituents, which are emitted near the surface, are dispersed.

The presence of aerosol can be detected in the backscatter profiles of LIDAR cloud ceilometers. An algorithm was developed at KNMI to derive the mixing layer height from the backscatter profiles measured with a Vaisala LD-40 ceilometer. The method is based on the strong decrease in aerosol backscatter at the transition from the polluted mixing layer to the relatively clean free atmosphere. A quality index is assigned to indicate the reliability of the MLH detection. An analysis of mixing layer heights determined for a six year data set for De Bilt gives reasonable results with respect to availability and quality as can be inferred from the diurnal and seasonal cycle, although limitations certainly exist, e.g. deep convective boundary layers are often underestimated. To evaluate the mixing layer height algorithm and study the spatial and temporal variation, backscatter profiles from six ceilometers in the observational network of KNMI are stored and processed during a field test of two years. This research is carried out in the framework of the Dutch BSIK program “Climate for Space” which focuses on the effects of climate change.

Results of the MLH determination by the ceilometer network will be presented. Also intercomparison results with estimates obtained from the ceilometer algorithm applied to the RIVM backscatter lidar data and from measurements of the 1290 MHz wind profiler will be shown, so as to obtain a robust MLH estimate for CESAR (Cabauw Experimental Site for Atmospheric Research), where all instruments are operated.