Characterizing Rotations in Thermally-Driven Winds Above a Valley Floor Using Observations From the 21 August 2017 Solar Eclipse and a Climatological Record

Thermally-driven winds in complex terrain are characterized by a diurnal cycle with upvalley winds during the day, downvalley winds at night, and transitional periods around sunrise and sunset. These winds are important for air pollution dispersion in valleys, and can also affect weather patterns on a regional scale. However, the winds are not always resolved in weather and climate models, and poor parameterization schemes result in inaccurate weather forecasting over mountainous regions. Observations of thermally-driven winds during transition periods will help improve model parameterizations, and ultimately lead to more accurate weather forecasting in complex terrain.

The 21 August 2017 solar eclipse provided a unique opportunity to observe thermally-driven winds in a valley. The rapid decrease and subsequent increase in solar radiation created two transitionary periods in the middle of the afternoon. The wind patterns that emerged during and after the eclipse were similar to those during the morning and evening transitions, but with distinct differences due to the sun's position during the eclipse. The high elevation and low azimuth angles during the eclipse caused the wind over the valley floor to rotate between upvalley, downslope, and downvalley flows. The direction of these rotations (clockwise vs. counterclockwise) was strongly influenced by terrain features in the valley. A period of northeasterly wind between these rotations was unexpected as it does not align with the main southeast-northwest valley axis. Wind rotations over valley sidewalls have been described in previous studies, but rotations over valley floors are not well-documented in the literature.

A year-long dataset was examined in order to further investigate the significance of the the wind rotations and northeasterly wind observed during the eclipse. First, the data were filtered to identify thermally-driven days at the investigation area. Wind rotations and periods of northeasterly winds were flagged in this filtered dataset. Small wind rotations with magnitudes of 30 degrees or less occur frequently during the day and the night. Large rotations of 135 degrees or more occur much less often, and happen most frequently around sunrise and sunset. Northeasterly winds occur with the highest frequency during the morning transition period and coincide with large clockwise wind rotations. Hence, the northeasterly wind on the afternoon of 21 August was a direct result of the solar eclipse. Wind rotations appear frequently over the valley floor, and follow a diurnal cycle on thermally-driven days. These wind rotations could have important implications for air pollution dispersal in valleys.