Atmos. Chem. Phys., 14, 4263-4278, 2014
www.atmos-chem-phys.net/14/4263/2014/
doi:10.5194/acp-14-4263-2014
© Author(s) 2014. This work is distributed
under the Creative Commons Attribution 3.0 License.
Atmospheric boundary layer top height in South Africa: measurements with lidar and radiosonde compared to three atmospheric models
K. Korhonen1,2, E. Giannakaki1, T. Mielonen1, A. Pfüller1, L. Laakso3,4, V. Vakkari3,5, H. Baars6, R. Engelmann6, J. P. Beukes4, P. G. Van Zyl4, A. Ramandh7, L. Ntsangwane8, M. Josipovic4, P. Tiitta2,4,9, G. Fourie7, I. Ngwana8, K. Chiloane10, and M. Komppula1
1Finnish Meteorological Institute, P.O. Box 1627, 70211, Kuopio, Finland
2Department of Applied Physics, University of Eastern Finland, P.O. Box 1627, 70211 Kuopio, Finland
3Finnish Meteorological Institute, P.O. Box 503, 00101, Helsinki, Finland
4Unit for Environmental Sciences and Management, North-West University, Potchefstroom, South Africa
5Department of Physics, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland
6Leibniz Institute for Tropospheric Research, Permoserstrasse 15, 04318, Leipzig, Germany
7Sasol Technology R{&}D (Pty) Ltd., P.O. Box 1, Sasolburg, South Africa
8South African Weather Service, Pretoria, South Africa
9Department of Environmental Sciences, University of Eastern Finland, P.O. Box 1627, 70211 Kuopio, Finland
10Eskom Holdings SOC Ltd; Sustainability Division; Research, Testing and Development, South Africa

Abstract. Atmospheric lidar measurements were carried out at Elandsfontein measurement station, on the eastern Highveld approximately 150 km east of Johannesburg in South Africa throughout 2010. The height of the planetary boundary layer (PBL) top was continuously measured using a Raman lidar, PollyXT (POrtabLe Lidar sYstem eXTended). High atmospheric variability together with a large surface temperature range and significant seasonal changes in precipitation were observed, which had an impact on the vertical mixing of particulate matter, and hence, on the PBL evolution. The results were compared to radiosondes, CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) space-borne lidar measurements and three atmospheric models that followed different approaches to determine the PBL top height. These models included two weather forecast models operated by ECMWF (European Centre for Medium-range Weather Forecasts) and SAWS (South African Weather Service), and one mesoscale prognostic meteorological and air pollution regulatory model TAPM (The Air Pollution Model). The ground-based lidar used in this study was operational for 4935 h during 2010 (49% of the time). The PBL top height was detected 86% of the total measurement time (42% of the total time). Large seasonal and diurnal variations were observed between the different methods utilised. High variation was found when lidar measurements were compared to radiosonde measurements. This could be partially due to the distance between the lidar measurements and the radiosondes, which were 120 km apart. Comparison of lidar measurements to the models indicated that the ECMWF model agreed the best with mean relative difference of 15.4%, while the second best correlation was with the SAWS model with corresponding difference of 20.1%. TAPM was found to have a tendency to underestimate the PBL top height. The wind speeds in the SAWS and TAPM models were strongly underestimated which probably led to underestimation of the vertical wind and turbulence and thus underestimation of the PBL top height. Comparison between ground-based and satellite lidar shows good agreement with a correlation coefficient of 0.88. On average, the daily maximum PBL top height in October (spring) and June (winter) was 2260 m and 1480 m, respectively. To our knowledge, this study is the first long-term study of PBL top heights and PBL growth rates in South Africa.

Citation: Korhonen, K., Giannakaki, E., Mielonen, T., Pfüller, A., Laakso, L., Vakkari, V., Baars, H., Engelmann, R., Beukes, J. P., Van Zyl, P. G., Ramandh, A., Ntsangwane, L., Josipovic, M., Tiitta, P., Fourie, G., Ngwana, I., Chiloane, K., and Komppula, M.: Atmospheric boundary layer top height in South Africa: measurements with lidar and radiosonde compared to three atmospheric models, Atmos. Chem. Phys., 14, 4263-4278, doi:10.5194/acp-14-4263-2014, 2014.
 
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