Observations of Thermally-driven Circulations in the Pyrenees: Detection Methods and Impact on Atmospheric Composition Measured at a Mountain Top

Atmospheric composition measured at the Pic du Midi high altitude observatory (2875m asl) in the Pyrenees is frequently affected by upward transport of boundary-layer air during anabatic circulations at different scales. The Pyrenean Platform of Atmospheric Observation (P2OA) includes two observatories located 28-km apart form each other: the Pic du Midi, and a low altitude site (600 m asl) located in the plain north of the chain.

Based on a long data series (about 10 years) collected at P2OA, different methods are carried out to detect thermally-induced circulations from observations at three key locations in the plain-to-mountain circulation cell: within the altitude return flow above the plain, close to the surface in the plain, and at the mountain top. The first method uses radar wind profiler measurements at 3000m above the plain in order to detect the return flow of the thermally-induced plain-mountain circulation. The second, based on surface wind data from the plain site, reveals days during which surface breeze blows towards the entrance of the closest major valley. The third method, based on surface data at the mountain-top, focuses on moisture diurnal cycles in order to detect and rank days with decreasing anabatic influence.

We compare those three independent detection methods, discuss the articulation between the local scale and larger scale of the circulations, and give a preliminary evaluation of their impact on in situ atmospheric composition measurements at Pic du Midi.