The rapid "sunrise/sunset" experienced during the eclipse
Turner, David D. — NOAA- Global Systems Laboratory
Area of research:
Atmospheric Thermodynamics and Vertical Structures
The solar eclipse that occurred on 21 August, 2017 simulated a rapid sunset and sunrise event. 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 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 provides 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.
This unique data set should prove instrumental in evaluating and improving the representation of these transitions in both weather and climate models.