Investigating the planetary boundary layer height at the CLOUDLAB field site
The planetary boundary layer (PBL) is the bottom-most layer of the atmosphere. It is directly influenced by the Earth’s surface and responds to surface forcing within about an hour. The flow in the PBL is characterised by turbulence during the day, which means that the PBL is usually well-mixed. The PBL is also the part of the atmosphere in which humans live. Determining its properties and particularly its height (PBLH) is therefore of great relevance. For example, the PBLH can influence air quality by controlling the volume of air into which pollutants released near the ground are mixed. In my project, I investigated the PBLH at the CLOUDLAB field site in Eriswil, Switzerland. The goal was to compare instrument and method performances under different atmospheric conditions.
The colour shade in the figure shows the attenuated backscatter measured by a ceilometer. Ceilometers are usually used for measuring cloud base heights (white triangles) but the attenuated backscatter can also be used as a proxy for aerosol concentration. The aerosol concentration is expected to be higher in the PBL than in the free troposphere above, so the strong change in backscatter can be used to estimate the PBLH. A number of algorithms exist to detect this change: red circles show the PBLH determined by the manufacturer algorithm installed on the ceilometer, whereas black and yellow circles show PBLH estimations from the open-source research algorithm ‘STRATfinder’.
The PBLH can also be estimated from temperature and humidity profiles. Orange symbols show a detected temperature inversion, where the temperature increases with height. Temperature inversions form a cap on vertical mixing so can be used as a marker of the PBLH. Blue symbols show the PBLH estimated from the relative humidity profile. The relative humidity is expected to be higher in the well-mixed PBL than in the free troposphere, so a minimum in the vertical relative humidity gradient can be used as a marker of the PBLH. The PBLH can also be detected from a maximum peak in the potential temperature gradient (purple symbols). This method is based on the expectation that the free troposphere is more stably stratified than the PBL.
I applied these thermodynamic methods to temperature and humidity profiles measured by radiosondes (diamond symbols), unmanned aerial vehicles (triangle symbols) and a microwave radiometer (square symbols). A microwave radiometer is a remote sensing instrument which measures microwave emission from molecules in the atmosphere and uses this to infer the temperature and humidity profiles.
I compared the different methods and instruments using case study days such as the one shown, as well as systematic comparisons over a year of observations. On this day, the thermodynamic and aerosol methods show good agreement. There is a well-defined PBLH with a clear temperature inversion and strong aerosol concentration gradient. However, on a systematic level, the agreement between the PBLH estimated by different methods and instruments was rather poor. In particular, the presence of low-level clouds can lead to non-ideal PBL structure and evolution, in which case the features detected by the thermodynamic and aerosol methods are often not at the same height.
Figure from the master thesis by Heather Corden, 2023.
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