Observations of Thermally Driven Circulations in the Pyrenees: Comparison of Detection Methods and Impact on Atmospheric Composition Measured at a Mountaintop
Résumé
The atmospheric composition measured at the Pic du Midi high altitude observatory (2875m asl) in the French Pyrenees is frequently affected by upward transport of boundary-layer air during anabatic circulations at different scales. The Pyrenean Platform for Observation of the Atmosphere (P2OA) includes two observatories located 28-km apart: at the Pic du Midi, and at a low altitude site (580 m asl) located in the plain north of the mountain chain. Based on a ten-year long data series collected at P2OA, three different methods are used to detect thermally-induced circulations. The methods are based on observations collected independently at three key locations in the plain-mountain circulation cell: within the altitude return flow above the plain, close to the surface in the plain, and at the mountain top. The main aims are (i) to present and compare the three detection methods, and (ii) to evaluate the impact of thermally-driven circulations on in situ air composition measurements at the Pic du Midi. The first method uses radar wind measurements at 3000m and 5000m above the plain in order to detect the return flow of the plain-mountain circulation. The second, based on surface wind data from the plain site, reveals days during which surface thermally-induced winds occur locally. The third method, based on surface data at the mountain top, focuses on diurnal moisture cycles in order to rank days with decreasing anabatic influence. We then compare the three independent detection methods, discuss possible connections between thermal circulations at different scales and locations, and finally present an evaluation of their impact on in situ atmospheric composition measurements at Pic du Midi.
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