Observations of the Marine Atmospheric Boundary Layer's Response to the April 8, 2024, Solar Eclipse during the WFIP3 Project

Total solar eclipses are infrequent natural events that offer unique opportunities to study the response of the atmospheric boundary layer (ABL) to rapid changes in solar and terrestrial radiation. Previous studies have documented the effects of solar eclipses on various meteorological parameters, including near-ground temperature (Eugster et al., 2017), wind speed and direction (Higgins et al., 2019), and various turbulence parameters (Turner et al., 2018). A review of studies conducted during the occurrence of an eclipse over coastal regions (Founda et al., 2007) revealed differing atmospheric responses compared to onshore events, particularly in terms of temperature fluctuations due to the larger heat capacity of the sea.

The total solar eclipse that occurred on April 8, 2024, is a significant atmospheric event having the path of totality traversing the United States from Texas to Maine. An array of meteorological instruments, including sonic anemometers, scanning and profiling Doppler LiDARs, was deployed across several locations along the northeastern coast of the United States during the Third Wind Forecast Improvement Project (WFIP3). This setup provides a unique opportunity to observe the marine atmospheric boundary layer’s (MABL) response to the eclipse. To the best of the authors’ knowledge, this is the first time that the MABL’s response to an eclipse event was observed with multiple meteorological instruments over both oceanic and coastal sites.

High-frequency data from sonic anemometers allow for a detailed analysis of temperature, wind speed, wind direction, and turbulent fluxes. Specifically, temperature fluctuations at the Cape Cod and Rhode Island sites followed a typical pattern expected during an eclipse: the recorded temperature drop due to the eclipse was of 1.4 °C and 1.2 °C at heights of 4 m and 10 m, respectively. This indicates that the eclipse’s effect on temperature reduction decreases with altitude. However, simultaneous measurements collected at the air-sea interaction tower (ASIT) showed a different pattern, namely with minimal temperature variations that might be the result of the large heat capacity of the ocean. One-minute averaged data shows that the temperature was 8.7°C at the beginning of the eclipse at ASIT, dropping to 8.4°C with an 82-minute time lag from the maximum obscuration. A comparison of these observations with those available in the literature (Eugster et al., 2017) shows that the temperature reduction at the Cape Cod site and ASIT is less than 80% and 95%, respectively, of the reductions reported for other onshore studies. This analysis, thus, corroborates the diminishing effects of the coastal region and ocean on the atmospheric response during an eclipse.

Data from the sonic anemometers deployed at the Cape Cod and Rhode Island sites show that the variance of the vertical velocity fluctuations and TKE tends to decrease during the eclipse. Furthermore, the reduction in surface temperature also weakens the temperature gradient in the vertical direction.

The vertical turbulent flux of sensible heat turned negative just after the maximum obscuration while the Obukhov length was negative before the beginning of the eclipse (i.e., convective stability regime) at the Rhode Island and Cape Code sites. Just after the highest obscuration, the vertical heat flux changed to positive. This trend indicates a transition in atmosphere stability from convective to stable during the occurrence of the eclipse, then back to convective at the end of the eclipse.

Turbulence parameters at ASIT exhibited minimal variability during the occurrence of the eclipse. As previously discussed, the ocean’s larger heat capacity results in smaller effects on the ocean's surface temperature due to the eclipse. Consequently, there is little change in the sensible heat flux and other turbulence parameters, such as friction velocity and TKE over the ocean. In addition, the MABL is typically more stable and less sensitive to short-term changes in surface heating, as it is often influenced by the large-scale sea-air temperature difference rather than direct solar heating.

Higgins et al. 2019 attempted to detect the height at which the eclipse's effect on wind speed could be observed by deploying LiDARs, but they were unsuccessful due to a poor signal-to-noise ratio from one of the LiDARs. In this study, the spatial effect of the eclipse on wind speed was analyzed using four profiling LiDARs. The results indicate that the eclipse's effect on wind speed reduction is observable up to 140 meters at the MVCO coastal site, while only up to about 100 m at ASIT and Nantucket, which are deployed over the ocean or closer to the shoreline, respectively. This suggests that the effects of an eclipse on atmospheric turbulence vary rapidly in time but they can also have significant spatial variability in coastal regions.

REFERENCES

Eugster, W., Emmel, C., Wolf, S., Buchmann, N., McFadden, J. P., & Whiteman, C. D. (2017). Effects of vernal equinox solar eclipse on temperature and wind direction in Switzerland. Atmospheric Chemistry and Physics, 17(24), 14887-14904.

Higgins, C. W., Drake, S. A., Kelley, J., Oldroyd, H. J., Jensen, D. D., & Wharton, S. (2019). Ensemble-averaging resolves rapid atmospheric response to the 2017 total solar eclipse. Frontiers in Earth Science, 7, 198.

Turner, D. D., Wulfmeyer, V., Behrendt, A., Bonin, T. A., Choukulkar, A., Newsom, R. K., ... & Cook, D. R. (2018). Response of the land‐atmosphere system over north‐central Oklahoma during the 2017 eclipse. Geophysical Research Letters, 45(3), 1668-1675.

Founda, D., Melas, D., Lykoudis, S., Lisaridis, I., Gerasopoulos, E., Kouvarakis, G., ... & Zerefos, C. (2007). The effect of the total solar eclipse of 29 March 2006 on meteorological variables in Greece. Atmospheric Chemistry and Physics, 7(21), 5543-5553.