Thermally driven wind systems and high-altitude ozone concentrations in Yosemite National Park

A small meteorological field campaign was conducted during August 2003 to better understand the role of local circulations on O3 transport and concentration throughout Yosemite National Park. Although, ozone and meteorological measurements are continuously measured at lower elevations of the Park, this study focused on a short intensive observation period that was aimed at determining transport mechanisms in higher, more alpine regions. During this period two additional ozone monitors were installed at Tuolumne Meadows and Tioga Pass. Four portable meteorological towers were installed along a transect of the Sierra Nevada, with one collocated at each ozone monitor. At Tuolumne Meadows, a Sintec MFAS Sodar ran continuously during the period. In addition, a 3-D sonic anemometer was installed at Turtleback Dome above Yosemite Valley. Seven Hobo temperature dataloggers were also installed in the area, including a transect from Tioga Pass up the west ridge of Mt. Dana.

Preliminary analyses show a distinct diurnal pattern of O3 concentration at Tuolumne Meadows that correlates well with the breakup of the nocturnal stable layer and the onset of upvalley winds. At Tioga Pass, the diurnal variation of ozone is less and the wind regime there is more complex than at Tuolumne Meadows. Another interesting feature observed during the experiment was a nocturnal down-mixing event. This occurred at Tuolumne Meadows after the initial buildup of the nocturnal stable layer. Sodar profiles showed southerly winds aloft that mixed downward to the surface destroying the surface inversion. At this same time a strong spike in O3 concentrations occurred indicating that either an elevated layer of higher O3 concentrations was mixed vertically downwards to the surface or that higher concentrations were advected horizontally by the southerly winds. Numerical simulations are being conducted to investigate the role of elevated pollutant layers on these mixing events and the overall role of thermally-driven circulations of ozone transport in the Sierra Nevada.