Observations of Thermally Driven Circulations in the Pyrenees: Comparison of Detection Methods and Impact on Atmospheric Composition Measured at a Mountaintop M. HULIN, F. GHEUSI, a M. LOTHON, V. PONT, AND F. LOHOU Laboratoire d’Aérologie, UPS Université Toulouse 3, CNRS (UMR 5560), Toulouse, France M. RAMONET AND M. DELMOTTE Laboratoire des Sciences du Climat et de l’Environnement, CEA-CNRS-UVSQ, Université Paris-Saclay, Gif-sur-Yvette, France S. DERRIEN, G. ATHIER, Y. MEYERFELD, AND Y. BEZOMBES Laboratoire d’Aérologie, UPS Université Toulouse 3, CNRS (UMR 5560), Toulouse, France P. AUGUSTIN Laboratoire de Physico-Chimie de l’Atmosphère, Université Littoral C^ ote d’Opale and Université Lille Nord de France, CNRS (UMR 8101), Lille, France F. RAVETTA UPMC Université Paris 6, CNRS (UMR 8190), LATMOS-IPSL, Paris, France (Manuscript received 2 October 2017, in final form 14 December 2018) ABSTRACT The atmospheric composition measured at the Pic du Midi high-altitude observatory (2875 m MSL) 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) in- cludes two observatories located 28 km apart: at the Pic du Midi and at a low-altitude site (580 m MSL) located in the plain north of the mountain chain. From a 10-yr-long data series collected at P2OA, three different methods are used to detect thermally induced circulations. The methods are based on obser- vations 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 mountaintop. The main aims are 1) to present and compare the three detection methods and 2) 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 3000 and 5000 m above the plain to detect the return flow of the plain–mountain circulation. The second, which is based on surface wind data from the plain site, reveals days during which surface thermally induced winds occur locally. The third method, which is based on surface data at the mountaintop, focuses on diurnal moisture cycles to rank days with decreasing anabatic influence. We then compare the three independent detection methods, discuss possible connections among thermal circu- lations at different scales and locations, and present an evaluation of their impact on in situ atmospheric composition measurements at Pic du Midi. 1. Introduction High-altitude sites are particularly suitable for the study of free-tropospheric composition and long-term trends. They are generally remote from regional emission sources and most often above the top of the continental boundary layer, allowing measurements to be made that are representative of continental to hemispheric scales (Keeling et al. 1976). Such measurements are a Current affiliation: Laboratoire d’Aérologie, Université de Toulouse, CNRS, UPS, France. Corresponding author: François Gheusi, francois.gheusi@ aero.obs-mip.fr APRIL 2019 HULIN ET AL. 717 DOI: 10.1175/JAMC-D-17-0268.1 Ó 2019 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). Unauthenticated | Downloaded 09/15/23 11:43 PM UTC