Weather regimes and related atmospheric composition at a Pyrenean observatory characterized by hierarchical clustering of a 5-year data set
Résumé
Atmospheric composition measurements taken at many high-altitude stations around the world, aim to collect data
representative of the free troposphere and of an intercontinental scale. However, the high-altitude environment favours vertical
mixing and the transportation of air masses at local or regional scales, which has a potential influence on the composition
of the sampled air masses. Mixing processes, source-receptor pathways, and atmospheric chemistry may strongly depend on
local and regional weather regimes, and these should be characterized specifically for each station. The Pic du Midi (PDM) is
a mountaintop observatory (2850 m a.s.l.) on the north side of the Pyrenees. PDM is associated with the Centre de Recherches
Atmosphériques (CRA), a site in the foothills ar 600 m a.s.l. 28 km north-east of the PDM. The two centers make up the
Pyrenean Platform for the Observation of the Atmosphere (P2OA). Data measured at PDM and CRA were combined to form a
5-year hourly dataset of 23 meteorological variables notably: temperature, humidity, cloud cover, wind at several altitudes. The
dataset was classified using hierarchical clustering, with the aim of grouping together the days which had similar meteorological
characteristics. To complete the clustering, we computed several diagnostic tools, in order to provide additional information
and study specific phenomena (foehn, precipitation, atmospheric vertical structure, and thermally driven circulations). This
classification resulted in six clusters: three highly populated clusters which correspond to the most frequent meteorological
conditions (fair weather, mixed weather and disturbed weather, respectively); a small cluster evidencing clear characteristics
of winter northwesterly windstorms; and two small clusters characteristic of south foehn (south- to southwesterly large-scale
flow, associated with warm and dry downslope flow on the lee side of the chain). The diagnostic tools applied to the six clusters
provided results in line with the conclusions tentatively drawn from 23 meteorological variables. This, to some extent,
validates the approach of hierarchical clustering of local data to distinguish weather regimes. Then statistics of atmospheric
composition at PDM were analysed and discussed for each cluster. Radon measurements, notably, revealed that the regional
background in the lower troposphere dominates the influence of diurnal thermal flows when daily averaged concentrations are
considered. Differences between clusters were demonstrated by the anomalies of CO, CO2 , CH4 , O3 and aerosol number
concentration, and interpretations in relation with chemical sinks and sources are proposed.
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