By: Keri Nicoll
The popularity and availability of Unmanned Aerial Vehicles (UAVs), has led to a surge in their use in many areas, including aerial photography, surveying, search and rescue, and traffic monitoring. This is also the case for atmospheric science applications, where they are used for boundary layer profiling, aerosol and cloud sampling and even tornado research. It is often the case that a human pilot is still required for safety reasons (even though many systems are mostly flown under autopilot), but the reliability of satellite navigation and autopilot software now means that fully autonomous flights are now possible, even being used in operational weather forecasting.
In the Department of Meteorology, we have been developing small science sensors to fly on UAVs for cloud and atmospheric electricity research. Atmospheric electricity is all around us (even in fine weather), and charge plays an important role in aerosol and cloud interactions, but is rarely measured. Over the past few years, our charge sensors have been flown on several different aircraft as part of two separate research projects to investigate charged aerosol and cloud interactions, briefly discussed in this blog.
The first flight campaign took place in Lindenberg, Germany, with colleagues from the Environmental Physics Group at the University of Tubingen. This flight campaign was to investigate the vertical charge structure in the atmospheric boundary layer (lowest few km of the atmosphere), and how it varied with meteorological parameters and aerosol. Four small charge sensors which we developed (see Figure 1(a): 1 and 2) were flown in special measurement pods attached to each wing of a 4 m wingspan fixed wing UAV (known as MASC-3). MASC-3 also measured temperature, relative humidity, 3D wind speed vector (using a small probe mounted in the nose of the aircraft) and aerosol particle concentration. Data was logged and saved on board the aircraft at a sampling rate of 100 Hz, and MASC-3 was controlled by an autopilot in order to repeat measurement patterns reliably. Since charge measurements from aircraft are notoriously difficult to make, it was important to minimise the effect of the aircraft movement on the charge measurement. This was done by flying carefully planned, straight flight legs, and developing a technique to remove the effect of the aircraft roll on the charge measurements. Multiple flights were performed during fair weather days, at different intervals throughout the day (from sunrise to sunset), to observe how the vertical charge structure changed throughout the day as the boundary layer evolved. Full results from the campaign are reported in our paper.
Figure 1: (a) Charge sensor pod for MASC-3. Charge sensor (1, 2), painted with conductive graphite paint, and copper foil to reduce the influence of static charge build up on the aircraft. (b) MASC-3 aircraft with charge sensor pods mounted on each wing (8). The meteorological sensor payload is in the front for measuring the wind vector, temperature, and humidity (9). Figure from Schön et al, 2022.
The second UAV flight campaign took place as part of our project: “Electrical Aspects of Rain Generation” funded by the UAE Research Program for Rain Enhancement Science. Watch our video on this project here. This involved instrumenting UAVs with specially-developed charge emitters which could release positive or negative ions on demand. The UAVs were flown in fog to investigate whether the charge released affected the size and or concentration of the fog droplets. This is an important first step in determining whether charging cloud droplets might be helpful in aiding rainfall in water stressed parts of the world. To perform these experiments, we worked with engineers from the Department of Mechanical Engineering at the University of Bath. Skywalker X8 aircraft with a 1.2 m wingspan were instrumented with our small charge sensors and cloud droplet sensors, along with temperature, and relative humidity sensors (as shown in Figure 2, and discussed in Harrison et al, 2021). Our specially developed charge emitters were mounted under each wing of the UAV, and under pilot control to be switched on and off whenever required by the flight scientist in a known pattern. The UAV flights took place at a private farm in Somerset, in light fog conditions (making sure that we could see the UAVs at all times, for safety reasons), flying in small circles around a ground based electric field mill, which was used to detect the charge emitted by the aircraft. Our results (reported recently in Harrison et al, 2022) demonstrated that the radiative properties of the fog differed between periods when the charge emitters were on and off. This demonstrates that the fog droplet size distribution can be altered by charging, which ultimately means that it may be possible to use charge to influence cloud drops and thus rainfall.
Figure 2:. (a) Skywalker X8 aircraft on the ground. (b) X8 aircraft in flight, with instrumentation labelled. (c) Detail of the individual science instruments: (c1) optical cloud sensor, (c2) charge sensors, (c3a) thermodynamic (temperature and RH) sensors, (c3b) removable protective housing for thermodynamic sensors, and (c4) charge emitter electrode. Figure from Harrison et al, 2021.