The use of unmanned aerial vehicles (UAVs) to conduct measurements in the atmospheric surface layer presents new possibilities for obtaining a spatial description of the structure and organization of turbulence. For example, a UAV possesses the ability to spatially sample the flow field with reduced reliance on Taylor's flow hypothesis, can sample at altitudes above the reach of tower-based sensors and below that of manned aircraft, and can sample quantitatively more data than a fixed-point measurement within the same time period. Finally, a UAV can be very portable and has the potential to measure in locations and conditions where the use of ground-based instrumentation is prohibitive.
In this measurement campaign UAVs and other ground-based micrometeorological sensors were used to measure the atmospheric surface layer properties during a total eclipse, with atmospheric property data acquired up to 100 m from the surface. The data was collected in clear conditions at the Russellville, KY Regional Airport on August 21, 2017 from 10:30 to 14:30 local time. Weather at this location was cloudless, with recorded low and high temperatures of 27 C and 33 C respectively, with an average sea level barometric pressure of 1020 hPa (falling slightly over the course of the day).
Profiles of pressure, temperature, humidity and the horizontal components of wind velocity were measured using a quadrotor aircraft equipped with a sonic anemometer. Simultaneously, two autonomous fixed wing aircraft, outfitted with multi-hole pressure probes capable of measuring all three components of velocity, as well as pressure, temperature and humidity sensors, flew horizontal transects at 50 m and 100 m. These aircraft were complimented by a ground-based sonic anemometer measuring the horizontal components of wind and temperature at 7 m, as well as soil temperature sensors. Additional measurements were obtained at 2 m using a portable weather station which measured wind speed, direction, pressure, temperature, humidity and solar radiation.
Solar radiation data from the portable weather station is presented in Fig.1(a) and provides a clear timeline of the eclipse. First contact occurred at 11:58 with totality lasting 2m 28 s initiating at 13:26. Final contact occurred at 14:53.
To provide an overview of the atmospheric surface layer processes, data from the vertically profiling aircraft is presented in Fig.1 (b)-(d). During the day, this aircraft flew 15 flights, with 13 of those yielding usable data. Each flight consisted of the aircraft climbing and descending between 10 m to 100 m at approximately 2 m/s over a period of 10 to 15 minutes. To produce Fig.1(b) and (c) the average value of atmospheric properties during each flight was determined at 1 m intervals. The profiles from the 13 flights were then interpolated to produce the surface layer evolution with virtual potential temperature and mean wind velocity shown in Fig.1(b) and (c) respectively.
The virtual potential temperature indicates well mixed layer conditions in place by the start of the profiling measurements. These conditions persist until approximately 13:00, well into the initial partial eclipse. At this time, a temperature inversion forms, which grows with altitude until just after totality. Ground temperature measurements (not shown) indicate that the soil had cooled by 4 C at this initiation of the temperature inversion. Soil temperature cooled by another 3 C, reaching a minimum at 13:40, approximately 15 minutes after totality. At this point, the temperature inversion is strongest near the surface. The ground temperature recovered as insolation increased, causing a decay in the temperature inversion with time until the resumption of mixed layer conditions by the cessation of flights at 14:30.
The mean wind velocity shown in Fig. 1(b) has had the coordinate system rotated to be in the direction of the mean wind from 50 to 100 m. As with the virtual potential temperature, the measurements indicate mixed layer conditions were present prior to first contact. Between 12:30 and 13:00 a velocity gradient formed near the surface, as thermal forcing decreased. By 13:40 near-surface winds had become calm with a slight wind shear evident in the lowest 50 m of the surface layer. As with the virtual potential temperature, by 14:30, mixed layer conditions had resumed.
The wind velocity and potential temperature from the flight showing the greatest effect of the eclipse on the atmospheric surface layer are detailed in Fig. 1(d). Here, the measured velocities from individual climbs and descents have been interpolated to produce the visualizations shown in this figure. This, more detailed view of the wind and temperature evolution show that the greatest effects of the eclipse were observed in the lowest 50 m of the surface layer, manifesting in the strong wind shear and formation of a stable layer near the surface. Notably, the temperature profiles show evidence of Kelvin-Helmholtz waves forming at the interface between the stable layer and the residual layer above it.
In summary, these results show the formation of a strong stable layer in the lowest 50 m of the atmospheric surface layer, initiating just prior to totality, but strongest approximately 15 minutes after totality, and persisting for approximately 20 minutes. Preliminary turbulent kinetic energy spectra and turbulence statistics calculated from the fixed-wing aircraft, which will be included in the extended abstract, show that the turbulent kinetic energy experiences stronger decay during the same period, consistent with flux and gradient Richardson numbers (not shown) indicating a drop in buoyant production. Work is ongoing on the estimation of the corresponding turbulent kinetic energy and surface heat budgets using the data collected during the eclipse.
This work was supported by the National Science Foundation through grant #CBET-1351411 and by National Science Foundation award #1539070, Collaboration Leading Operational UAS Development for Meteorology and Atmospheric Physics (CLOUDMAP).
- Indicates paper has been withdrawn from meeting
- Indicates an Award Winner