Investigating Impacts of the Wind Field and Unknown Flying Objects on High-Res Ceilometer Profiles during ScaleX 2016

In June and July 2016, the TERENO pre-alpine observatory at Fendt, Germany, hosted the ScaleX campaign. TERENO is a large-scale integrative observation program designed to determine the long-term ecological and climatic impact of global environmental change at regional scales (http://teodoor.icg.kfa-juelich.de/overview-en?set_language=en); ScaleX added a variety of in-situ and remote sensing instruments to the existing equipment in order to address a research question that is especially demanding in such a complex terrain: How well can our observations close the balances of energy and matter fluxes and constrain uncertainties associated with their modeling? To list just a few, the remote sensing instruments involved three wind lidars, two ceilometers, a RASS, and a microwave radiometer. In addition to these continuously operating systems, several research groups operated fixedwing and multicopter UAVs covering vertical ranges up to 1000 m.

The IMK-IFU institute of the Karlsruhe Institute of Technology (KIT) coordinated all ScaleX activities and provided most of the instruments involved (http://www.imk-ifu.kit.edu/). Instead of its standard settings reporting attenuated backscatter profiles up to 15400 m range with 10 m resolution, accumulated over 30 s, one of the two IMK-IFU standard commercial laser ceilometers had been operated in a high resolution mode, switching between 550 m range accumulated over 0.2 s, and 3850 m range accumulated over 5 s. In order to obtain a higher vertical resolution, laser pulse length had been reduced from 110 ns to 70 ns, and the instrument had been tilted by 25° from the vertical.

This presentation concentrates on two aspects of the high-res ceilometer measurements, seeking answers to the questions whether reflections from large flying objects in altitudes below 500 m can have a significant impact on the results of aerosol layer detection, and whether signal variance and short-term trends are correlated to wind profiles reported by a near-by wind lidar.

The “large flying objects” can be birds, bats, insects, or leafs. Depending on the site, also objects operated by humans might be the cause for a sudden signal increase in just one or two range gates; some of the UAVs operated during ScaleX caused exceptionally high reflection signal at the receiver of the high-res ceilometer. An algorithm has been developed that is detecting all these unknown flying objects. The attached graph shows the evaluation result of a 2 min interval. Three UFOs had been detected (white circles). Aerosol layer height detection based on 2 min averaging including the UFO profiles would report a false layer at 160 m; filtering out those profiles avoids such false detections. Most of the UFO encounters appear in the hour around sunset; a detailed statistical investigation will be included in the presentation.

Wind lidar comparison will present some case studies, in particular a study of nocturnal wind shear and morning convection. For the latter, also the 3850 m range profiles of attenuated backscatter and signal variance are investigated in order to characterize differences between upwards and downwards moving air parcels. Such movements with vertical wind velocities exceeding 1 m/s have been registered directly by the wind lidar and indirectly by the ceilometer, examining fluctuations in the attenuated backscatter profiles.

Ceilometers are affordable, robust, continuously operating mini lidars with low maintenance costs. They are increasingly used for scientific applications. This investigation shows that adding high-res operating modes widens these areas. Manufacturers should keep this in mind when designing future systems.