Towards a Better Understanding of the Early Morning Boundary Layer Transition, Using Observations of Small Unmanned Aerial Vehicles (UAV)

Regarding the morning transition of the atmospheric boundary layer (ABL) textbooks and articles mainly focus on the growth phase of the convective boundary layer (CBL), while details of the transition between the nocturnal inversion layer (NIL) and the CBL are not well described in the theoretical models. Especially the onset of turbulence is subject to uncertainties. In summer 2013, a series of experiments was carried out to study the early-morning transition of the ABL by the working group for environmental physics at the University of Tübingen. Measurement flights with the small UAV MASC were carried out at two separate experimental sites in Southern Germany, both in complex and heterogenous terrain. On eight convective days between May and September, vertical soundings up to 500 m were done in 30 minute intervals, from about one hour after sunrise, until the CBL depths reached the top altitude of the flights to record the evolution of the thermal stratification in the early morning. In between the soundings, flight legs of up to 1 km length were performed to measure turbulent statistics and sensible heat fluxes at a constant altitude of 100 m. This way, in the course of the experiment, fluxes at different heights relative to the boundary layer depth are measured. With the help of surface flux measurements of a sonic anemometer, methods of similarity theory could be applied to the UAV flux measurements to compare the transitional behaviour of the boundary layer to literature. It becomes evident that standard scalings for the CBL are not applicable for the observed transition phase. The experiment and the obtained results are a good example for prospects and limitations of boundary layer research with a single UAV at the present state of the art. The flexible operation of the UAV allows to investigate ABL processes in areas that are usually not considered for meteorological observations and thus can help to deepen the understanding of microscale effects. However, operation of a single UAV provides only scarse information about the development of the stratification of heat flux and turbulent kinetic energy throughout the boundary layer. In future, experiments with multiple UAV operating at the same time at different altitudes are planned in order to be able to simultaneously measure the heat flux in different heights of the ABL and further improve the understanding of the early morning boundary layer transition processes.