Observing continental boundary-layer structure and evolution over the South African savannah using a ceilometer

Abstract

The atmospheric boundary layer (BL) is the atmospheric layer coupled to the Earth’s surface at relatively short timescales. A key quantity is the BL depth, which is important in many applied areas of weather and climate such as air-quality forecasting. Studying BLs in climates and biomes across the globe is important, particularly in the under-sampled southern hemisphere. The present study is based on a grazed grassland-savannah area in northwestern South Africa during October 2012–August 2014. Ceilometers are probably the cheapest method for measuring continuous aerosol profiles up to several kilometers above ground and are thus an ideal tool for long-term studies of BLs. A ceilometer-estimated BL depth is based on profiles of attenuated backscattering coefficients from atmospheric aerosols; the sharpest drop often occurs at BL top. Based on this, we developed a new method for layer detection that we call the signal-limited layer method. The new algorithm was applied to ceilometer profiles which thus classified BL into classic regime types: daytime convective mixing, and a double layer at night of surface-based stable with a residual layer above it. We employed wavelet fitting to increase successful BL estimation for noisy profiles. The layer-detection algorithm was supported by an eddy-flux station, rain gauges, and manual inspection. Diurnal cycles were often clear, with BL depth detected for 50% of the daytime typically being 1–3 km, and for 80% of the night-time typically being a few hundred meters. Variability was also analyzed with respect to seasons and years. Finally, BL depths were compared with ERA-Interim estimates of BL depth to show reassuring agreement.