Observations of Thermally Driven Circulations in the Pyrenees: Comparison of Detection Methods and Impact on Atmospheric Composition Measured at a Mountaintop

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) includes 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 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 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 circulations at different scales and locations, and present an evaluation of their impact on in situ atmospheric composition measurements at Pic du Midi.

Two-year measurements of Black Carbon properties at the high-altitude mountain site of Pic du Midi Observatory in the French Pyrenees

“…In winter, FT-and PBL-influenced conditions occurred roughly 74% and 26% of the analyzed time, respectively, against 48% and 52% for summer, respectively. These results are broadly in agreement with the previous study by Hulin et al (2019) at PDM, which quantified around 47 % of the days as PBL-influenced over a 10 years period. The PBL-influenced coditions occured mostly around 15:00 UTC, consistent with the dynamics at mountain sites where plain-tomountain winds and along-valley winds become the strongest in the afternoon (Whiteman, 2000).…”

“…PDM was found to have a low ABL-Topoindex, similar to other Alpine high altitude stations. PDM is thus a suitable site to study both the background lower free troposphere (FT) over long timescales and injection of air masses from the PBL (Hulin et al, 2019;Tsamalis et al, 2014;Fu et al, 2016;Marusczak et al, 2017). In this article the instrumentation and methodology are presented in section 2.…”

Multivariate statistical air mass classification for the high-alpine observatory at the Zugspitze Mountain, Germany

“…In the winter half year, photochemical O 3 production was assumed to play a minor role at high-alpine sites because of the generally low solar irradiance and the weak thermally induced uplift. This argument is supported by the trajectory residence time statistics of Kaiser et al (2007), which demonstrated that the O 3 mixing ratio at European high-alpine sites was generally lower in winter if the air mass originated from lower altitudes, whereas in summer, this was often not the case.…”

Semi‐idealized simulations of wintertime flows and pollutant transport in an Alpine valley. Part II: Passive tracer tracking

“…Rather, we showed that the tracers affect the south‐facing basin sidewalls due to asymmetric thermal forcing, even at relatively high elevation (up to 2000 m asl) while pollutants have been emitted at 560 m asl. Similar dynamics have already been observed in the Austrian Inn Valley during a wintertime pollution episode (Gohm et al ., ; Harnisch et al ., ) and should be considered carefully since it may influence the atmospheric composition measured at high‐altitude stations (Bukowiecki et al ., ; Hulin et al ., ). At night, the downstream transport appears as a second effective mechanism of material transport, but it mainly concerns the Sallanches tracers while Passy tracers tend to accumulate within the central curved sector of the BASIN volume.…”