Master Thesis
Determination of the mixed layer height by ceilometer measurements and its influence on large-scale convective energy exchange
Katrin Kohnert (06/2012-03/2013)
Support: Thomas Foken
Die Arbeit wird am Karlsruher Institut für Technologie, Atmosphärische Umweltforschung (KIT: IMK-IFU) unter Teilbetreuung von JunProf. M. Mauder durchgeführt.
A major part of global change research is concentrating on atmospheric processes that are related to global warming or air pollution. In both cases, research is dependent on profound knowledge of these processes in the atmosphere and on parameters influencing them. This is essential in order to develop atmospheric models on regional and global scale. This study analyses the mixed layer height (MH), which is an important parameter in air pollution studies, for modelling atmospheric turbulences and for research on energy conversion, as it is the part of the atmosphere that is bordering at the earth’s surface. So far, the energy balance at the earth’s surface has, however, not been closed by experimental data. This study aims to contribute to ongoing efforts to overcome current lacks of knowledge, firstly regarding the temporal and small scale spatial variation of the MH in complex terrain, and secondly, by validating a recent parameterisation for the energy balance closure problem including the MH as input parameter. The study is based on data from three study sites in the German pre-Alps that are imbedded in the Terrestrial Environmental Observatories (TERENO) network. The sites are located within a north-south distance of about 30 km, with the southernmost being located in a valley surrounded by mountains up to 1000 m agl, whereas the two northern sites are surrounded by gentle hills. The MH is derived via the minimum gradient method applied on data from ceilometer measurements conducted from July to October 2012. At each site a highly significant decrease of the MH is observed over the study period, which is only partly related to the decreasing global radiation. A comparison between the monthly data at the sites reveals that at the southernmost site the MH is significantly elevated compared to the northern ones. This is interpreted as an effect of the surrounding mountains that elevate 1000 m above the study site. A comparable influence of surrounding topography is found at the northern site related to wind directions. In the morning and at noontime in each month the MH is significantly higher during west wind situations compared to cases with wind from other directions. This shows the influence of an elevated plateau west of the site. An influence of mesoscale circulations existing between the Alps and the foreland on the MH is not found. The application of the energy balance closure parameterisation suggested by Huang et al. (2008), that includes the MH as input parameter, shows that it is not transferrable on the TERENO sites. The TERENO data exceed the suggested fits and do not follow the proposed pattern. It is concluded that this parameterisation neglects important effects from heterogeneous surfaces and that the large-eddy simulation (LES) is based on a too coarse resolution to cover processes close to the surface correctly. This parameterisation should not iv be applied on experimental data. Overall, the results of this study suggest that the conversion of components of the energy balance is still not fully understood. Moreover, the results show that the MH is highly variable on a small spatial scale and over time, depending on surrounding topography and wind direction which makes it indispensable to consider differences when including the MH on regional or global models. Knowing about this variability is essential when addressing questions related to the distribution of air pollutants which directly influences the health and wellbeing of the population.