The spatial structures of turbulent flow in the atmospheric boundary layer (ABL) are complex and diverse. Multi-point spatial correlation measurements can help improve our understanding of these structures and their statistics. In this context, we investigate Taylor’s hypothesis and the statistics of spatial structures on the microscale. For the first time, simultaneous horizontally distributed wind measurements with a fleet of 20 quadrotor UAS (unmanned aerial systems) are realized. The measurements were taken at different heights and under different atmospheric conditions at the boundary layer field site in Falkenberg of the German Meteorological Service (DWD). A horizontal flight pattern has been specifically developed, consisting of measurements distributed along and lateral to the mean flow direction with separation distances of $$5\ldots 205$$ 5 … 205  m. The validity of Taylor’s hypothesis is studied by examining the cross-correlations of longitudinally distributed UAS and comparing them with the autocorrelations of single UAS. To assess the similarity of flow structures on different scales, the lateral and longitudinal coherence of the streamwise velocity component is examined. Two modeling approaches for the decay of coherence are compared. The experimental results are in good agreement with the model approaches for neutral atmospheric conditions, whereas in stable and convective ABL, the exponential approaches are not unconditionally valid. The validation results and the agreement with the literature on coherence in the ABL underline the potential of the UAS fleet for the purpose of spatial turbulence measurements.

The spatial structures of turbulent flow in the atmospheric boundary layer (ABL) are complex and diverse. Multi-point spatial correlation measurements can be used to improve the understanding of these structures and their statistics. In this context, we investigate Taylor’s hypothesis and the statistics of spatial structures on the microscale in this study. For the first time, simultaneous horizontally distributed wind measurements with a fleet of 20 quadrotor UAS (unmanned aerial systems) are enabled. The measurements were taken at different heights and under different atmospheric conditions at the boundary layer field site in Falkenberg of the German National Meteorological Service (DWD). An adaptable horizontal flight pattern has been especially developed, consisting of measurements distributed along and lateral to the mean flow direction with separation distances of 5...205 m. The validity of Taylor's hypothesis is studied by examining cross-correlations of longitudinally distributed UAS and comparing them with autocorrelations of single UAS. To assess the similarity of flow structures on different scales, the lateral and longitudinal coherence of the streamwise velocity component is examined. Two modeling approaches for the decay of coherence are compared. The experimental results are in good agreement with the model approaches for neutral atmospheric conditions, whereas in stable and convective ABL, the exponential approaches are not unconditionally valid. The validation results and the agreement with the literature on coherence in the ABL underline the potential of the UAS fleet for the purpose of spatial turbulence measurements.

Abstract. Future air traffic using (green) hydrogen (H2) promises zero carbon emissions but the effects of contrails from this new technology has hardly been investigated. We study contrail formation behind aircraft with H2 combustion by means of the particle-based Lagrangian Cloud Module (LCM) box model. Assuming the absence of soot and ultrafine volatile particle formation, contrail ice crystals form solely on atmospheric background particles mixed into the plume. While a recent study extended the original LCM with regard to the contrail formation on soot particles, we further advance the LCM to cover the contrail formation on ambient particles. For each simulation, we perform an ensemble of box model runs using the dilution along 1000 different plume trajectories that are based on 3D Large Eddy Simulations using the FLUDILES solver. The formation threshold temperature of H2 contrails is by around 10 K higher than for conventional contrails (which form behind aircraft with kerosene combustion) due to a factor of 2.6 higher energy-specific water vapor emission. Therefore, contrail formation becomes primarily limited by the homogeneous freezing temperature of the water droplets formed on the ambient particles such that contrails can form at temperatures down to around 234 K. The number of formed ice crystals varies strongly with ambient temperature even far away from the contrail formation threshold. The latter is because the water-supersaturation in the plume lasts longer for colder conditions and, hence, more of the entrained aerosol particles can form droplets and ice crystals. The contrail ice crystal number clearly increases for a higher ambient aerosol number concentration. The increase becomes weaker for higher number concentrations (>≈ 200 cm−3) and lower ambient temperatures (< 230 K). The ice crystal number decreases significantly for ambient particles with mean dry radii <≈ 10 nm due to the Kelvin effect. Besides simulations with one aerosol particle ensemble, we analyze contrail formation scenarios with two co-existing aerosol particle ensembles that differ either in their mean dry size or hygroscopicity parameter. We compare them to scenarios with a single ensemble that is the average of the two ensembles. We find that the total ice crystal number can differ significantly between the two cases, in particular if nucleation mode particles are involved. Due to the absence of soot particle emissions, the ice crystal number in H2 contrails is typically reduced by more than 80–90 % compared to conventional contrails. The contrail optical thickness is significantly reduced and H2 contrails either become later visible than kerosene contrails or are not visible at all for low ambient particle number concentrations. On the other hand, H2 contrails can form at lower flight altitudes where conventional contrails would not form.

&lt;p&gt;Exchange and transport processes in the atmospheric boundary layer (ABL) are driven by turbulence on a wide range of scales. Their adequate parameterization in numerical weather prediction (NWP) models is essential for a high predictive skill of forecasts. In heterogenous and complex terrain, the common simplification of turbulence to statistical models does not necessarily hold. Coherent structures such as convective cells, secondary circulations, gusts, slope and valley flows can be summarized to sub-mesoscale structures which are not well represented in models. A reason for the lack of understanding of these flow features is the challenge to adequately sample their three-dimensional, spatio-temporal structure and their contribution to the energy budget of the ABL.&lt;br&gt;We present a system to achieve simultaneous spatial measurements with a fleet of multirotor unmanned aircraft systems (UAS). The major benefit of this approach is, that true simultaneous measurements can be obtained without the need of expensive infrastructure such as masts or lidar instruments. In field campaigns with more than 1000 single flights at the Meteorological Observatory Lindenberg - Richard A&amp;#223;mann-Observatory (MOL-RAO), the system was validated in 2020 and 2021 to provide reliable measurements of the horizontal wind vector. We showed that turbulent eddies can be resolved with a time resolution of up to 2~Hz, unless the overall TKE level is below the noise threshold of the UAS measurements, which can be the case in stable atmospheric stratification. Additionally to the wind vector estimation that is based only on avionic data from the autopilot, pressure, temperature and humidity sensors are carried by each UAS.&lt;br&gt;In future, within the project ESTABLIS-UAS, the fleet of UAS shall be expanded and capabilities for flights beyond visual line of sight and throughout the whole ABL shall be developed. The project includes a three-fold approach to validate single UAS measurements, fleet observations and methods to derive spatial averages and fluxes. Wind tunnel tests, field experiments and virtual measurements in numerical simulations will be performed to gain confidence in the achievable accuracy in a wide range of conditions. Also, measurement strategies are to be developed that allow the derivation of meaningful fluxes in the mountain boundary layer (MoBL).&amp;#160;&lt;br&gt;The UAS fleet is planned to be deployed in two campaigns in the framework of the TEAMx research programme. The ESTABLIS-UAS measurements will fill observational gaps in the sub-mesoscale. The analysis of the UAS fleet data in synthesis with continuous ground observations and remote sensing will provide unprecedented new insights into the complex MoBL flow. The results will foster the development of new and better parameterization of the ABL in complex terrain.&lt;/p&gt;