Abstract
Rapid changes in solar radiative forcing influence heat, scalar and momentum fluxes and thereby shift the trajectory of near-surface atmospheric transitions. Surface fluxes are difficult to obtain during atmospheric transitions by either bulk or eddy-covariance methods because both techniques assume quasi-stationarity in atmospheric state and require sufficiently long blocks of data, typically on the order of 10-30 min, to obtain statistically significant results. These computational requirements limit the temporal resolution of atmospheric processes that researchers can examine using traditional measurement techniques. In this paper, we present a novel observational approach to calculate surface fluxes at sub-minute temporal resolution. High-frequency data from a horizontal, log-spaced array of nine time-synchronized ultrasonic anemometers were used to perform spatial-temporal ensemble averaging and obtain eddy-covariance turbulence fluxes at unprecedented time resolution. The 2017 Great American Solar Eclipse event provided a ‘natural experiment’ to test the ensemble-observation and averaging approach. A total eclipse is energetically well-constrained and, unlike day/night transitions, is a perturbation that quickly transitions from and back to a state of significant solar forcing, providing an ideal scenario for testing the space-timescales required for surface flux calculations. Additionally, two Doppler lidars and a vertically-oriented Distributed Temperature Sensing (DTS) system provided measurements to characterize near-surface atmospheric conditions. Results show that the ensemble-averaged sensible heat fluxes converged at timescales as short as 15 s. Additional analyses show that the timescale of the connection between the surface and the atmosphere is more rapid than previous measurements have been capable of showing, and is on the order of 10 minutes or less. This experiment demonstrates that ensemble-flux measurements are capable of resolving fluctuations in surface fluxes during rapid atmospheric transitions.
Highlights
Solar eclipses are a natural experiment (Harrison and Hanna, 2016) for which the land and atmosphere responses to changes in insolation, which are more rapid than during the morning and evening transitions, can be investigated (Antonia et al, 1979; Eaton et al, 1997; Foken et al, 2001)
Foken et al (2001) and Schulz et al (2017) measured turbulent fluxes with 5-min temporal averaging, the highest resolution we could find in the literature. Even this temporal resolution is insufficient to capture the evolution of the land-atmospheric interaction throughout a total solar eclipse, while it under-samples flux contributions of turbulence characterized by relatively longer time and larger length scales. To circumvent these practical shortcomings of traditional eddy-covariance techniques, we developed a novel spatialtemporal method to capture land surface fluxes during nonstationary events, wherein large-scale fluctuations are captured via the spatial extent of a log-spaced array of nine ultrasonic anemometers, and the small-scale turbulent fluctuations are captured with a rapid (20 Hz) sampling rate
The 2017 total solar eclipse was observed at a field site along the path of totality in Corvallis, Oregon
Summary
Solar eclipses are a natural experiment (Harrison and Hanna, 2016) for which the land and atmosphere responses to changes in insolation, which are more rapid than during the morning and evening transitions, can be investigated (Antonia et al, 1979; Eaton et al, 1997; Foken et al, 2001). The magnitude of this temperature change and timelag between the minimum solar radiation and the local minimum temperature is site- and event-dependent (e.g., on latitude, surface characteristics, synoptic forcing, cloud coverage, and time-of-day and duration of totality of the eclipse event), with an observed temperature drop of 1.5–10◦C (Stewart and Rouse, 1974; Anderson and Keefer, 1975; Aplin and Harrison, 2003; Aplin et al, 2016; Eugster et al, 2017), and with 15–20 min lags (Antonia et al, 1979; Aplin et al, 2016). Anfossi et al (2004) found that the TKE decay rate during a total solar eclipse follows a power-law relation with respect to dimensionless time
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