Eclipse Provides a Natural Laboratory for Studying Boundary Layer Transitions
Scientists taking measurements at the Atmospheric Radiation Measurement User Facility (ARM) Southern Great Plains site were able to use the unique circumstances of the solar eclipse to study the response of the boundary layer to changes in solar radiation.
Turbulent motions in the lower part of the atmosphere are driven strongly by the heating of the surface of the earth by the sun during the day time. The interactions and feedbacks between the land surface and the atmospheric boundary layer are quite uncertain, especially during the morning and evening transitions when the sensible heat flux changes sign due to changes in the shortwave radiative flux absorbed by the earth’s surface. The solar eclipse that occurred on 21 August, 2017 simulated a rapid sunset and sunrise event. The obscuration of the sun by the moon during the eclipse reduced the downwelling shortwave radiative flux by over 800 W/m2 during the eclipse. The response of the land surface, and its interactions with the atmospheric boundary layer, during this 3-hr event was well observed by the ARM and other observations collected during the Land-Atmosphere Feedback Experiment (LAFE). The observations showed the rapid collapse of the daytime boundary layer, the development of a nocturnal boundary layer, and the rapid redevelopment of a new daytime boundary layer as the moon moved across the sun.
Most numerical weather prediction and climate models are unable to properly simulate either the morning or evening transitions well. The solar eclipse provided a unique opportunity to evaluate how models handle these transitions, as the observations collected at the ARM Southern Great Plains (SGP) site during LAFE provide a full characterization of the evolution of the land-atmospheric interactions during this event. These observations show the evolution of the surface fluxes (radiative, sensible, and latent), and the resulting impact on turbulent motions in the boundary layer above the surface. In particular, after the collapse of the convective boundary layer during the peak of the eclipse, a nocturnal boundary layer developed that included both a dramatic decrease in the profile of turbulent kinetic energy and the development of a low-level jet. When the sun reemerged from behind the moon, a new convective boundary layer rapidly developed.
On 21 August 2017, a solar eclipse occurred over the continental United States resulting in a rapid reduction and subsequent increase of solar radiation over a large region of the country. The eclipse’s effect on the land-atmosphere system is documented in unprecedented detail using a unique array of sensors deployed at three sites in north-central Oklahoma. The observations showed that turbulent fluxes of heat and momentum at the surface responded quickly to the change in solar radiation. The decrease in the sensible heat flux resulted in a decrease in the air temperature below 200 m, and a large decrease in turbulent motions throughout the boundary layer. Furthermore, the turbulent mixing in the boundary layer lagged behind the change in the surface fluxes, and this lag depended on the height above the surface. The turbulent motions increased and the convective boundary layer was reestablished as the sensible heat flux recovered. This unique data set should prove instrumental in evaluating and improving the representation of these transitions in both weather and climate models.
National Oceanic and Atmospheric Administration
This research was supported as part of the Land-Atmosphere Feedback Experiment (LAFE). LAFE was funded by the U.S. Department of Energy Office of Science, Office of Biological and Environmental Research as part of the Atmospheric Radiation Measurement Program and Atmospheric System Research Program, the NOAA Oceanic and Atmospheric Research Office of Weather and Air Quality, the NASA Water and Energy Cycle Program, the German Federal Ministry of Education and Research (BMBF), and the University of Hohenheim.
Turner D.D., V. Wulfmeyer, A. Behrendt, T. Bonin, A. Choukulkar, R. Newsom, W. Brewer, and D. Cook. “Response of the Land-Atmosphere System Over North-Central Oklahoma During the 2017 Eclipse.” Geophysical Research Letters, 45(3), 1668-1675 (2018). DOI:10.1002/2017GL076908