Graph-based Tracing of the Atmospheric Boundary Layer by a Ceilometer: A Case Study

The atmospheric boundary layer (ABL) is an important factor in the atmospheric processes. It serves as an intermediary between the earth's surface and the atmosphere in the exchange of mass, energy, and momentum. ABL height (ABLH) also restricts the dispersion volume of atmospheric pollutants. The ABLH can be retrieved from lidar and ceilometer (low power lidar) backscatter profiles by tracing the increased aerosol loading associated with ABL. Numerous lidar-based algorithms are currently implemented, but their ability to trace ABL evolution is often limited. The main issues are related to the selection of the correct layer when multiple aerosol layers are present. To overcome the problem, we employ a graph-based approach to post-process already identified ABL candidates (determined by the gradient in ceilometer backscatter) in two cases. A simple non-parametric smoothing method was also utilized for comparison. Our findings indicate that the graph-based algorithm is superior to the smoothing method in obtaining a consistent tracing of the diurnal evolution of ABL over Sofia, although some issues were also encountered during the evening ABL transition.