Synoptic circulation types related to the boundary layer height in Augsburg, Germany

The thickness of the planetary boundary layer is one of the most important factors determining vertical transport and concentration of pollutants near the surface. However, the boundary layer height (BLH) as well as its structure, especially the occurrence of stable layering, depends to a large extent on the synoptic situation, i.e. mainly on strength and direction of the synoptic wind, advection of air masses of different properties and cloudiness. Several former studies show an increasing trend of the BLH, especially during daytime, throughout the last decades. The study presented here evaluates the dependence of the BLH for a selected region around the city of Augsburg in southern Germany on synopticcirculation types in order to better understand short term as well as long term BLH changes and their effects on the urban air quality of Augsburg.

Boundary layer heights are retrieved from ceilometer measurement series starting in 2017 using a routine for estimating BLH from Vaisala CL51 ceilometer laser backscatter data. They are compared to hourly ERA5 BLH data (1940 to 2025) in order to evaluate the uncertainty when using the ERA5 reanalysis data as replacement for observation data at the considered location for long term studies.

The algorithm for determination of synoptic circulation and weather type patterns related to the BLH is based on the SANDRA algorithm (Simulated Annealing and Diversified Randomization) where the target variable multiplied by an empirically determined weight is included into the clustering process. Different synoptic field variables including geopotential height, wind components and temperature at different atmospheric heights as well as the influence of cloud cover are examined and their contribution to the explained variance of the boundary layer height is presented and discussed. Finally, the suitability of the prescribed correlations for establishing statistical short term prediction models is discussed.