UAS-Based Observations of Eclipse Impact on the Atmospheric Surface Layer*

A solar eclipse provides a unique opportunity to observe the response of atmospheric turbulence to an approximate step change in solar radiation. To assess the unsteady changes in the atmospheric surface layer introduced by a solar eclipse, a measurement campaign was conducted in Bloomington, Indiana, on April 8, 2024. The weather at this location resulted in a high temperature of 24°C and fair conditions prior to the eclipse, resulting in a measurable impact of the eclipse. Several measurement systems were deployed, including a fixed-wing uncrewed aerial vehicle (UAV), a quadrotor UAV, radiosondes, and ground-based sensors.

The ground-based sensors implemented in this campaign measured the surface energy balance as well as the atmospheric state at 2 m and 10 m above ground level. The UAV platforms were outfitted with pressure, temperature, and humidity sensors, as well as a sonic anemometer on the rotorcraft and a five-hole probe on the fixed-wing aircraft to record wind speed and direction. The rotorcraft was used to capture the profile of the surface layer by flying a repeated vertical flight path between 10 m and 100 m, whereas the fixed-wing aircraft was flown at a constant pressure altitude of 40 m above ground level and was able to measure the turbulent state.

A review of the data revealed that the eclipse did have a significant effect on the atmospheric surface layer. Prior to the eclipse, mixed layer conditions were observed. Once the eclipse began, the air and surface temperature fell, altering the heat flux into the ground. This cooling of the Earth's surface caused a temperature inversion to occur, leading to the development of a stable layer. During the eclipse, turbulent kinetic energy reduced significantly, indicating a strong decay of atmospheric turbulence due to the loss of buoyant production. Additionally, the temperature profiles gave evidence of Kelvin-Helmholtz waves occurring at the interface between the stable layer and the residual layer above it.