10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
The Topography Contribution to the Influence of the Atmospheric Boundary Layer at High Altitude Stations
Martine Collaud Coen, Elisabeth Andrews, Dominique Ruffieux, FRANCISCO NAVAS-GUZMÁN, Federal Office of Meteorology and Climatology
Abstract Number: 1187 Working Group: Aerosol Transport and Transformation
Abstract High altitude stations are often emphasized as free tropospheric measuring sites but they remain influenced by atmospheric boundary layer (ABL) air masses due to convective transport processes. In this study, a topography analysis is performed allowing calculation of a newly defined index called ABL-TopoIndex. The ABL-TopoIndex is constructed in order to correlate with the ABL influence at the high altitude stations and long-term aerosol time series are used to assess its validity. Other important parameters influencing the aerosol load such as the wind, the soil state and the synoptic weather conditions were not taken into account.
We analysed 43 high altitude stations representative of 5 continents. The topography was taken from GTopo30. The ABL-TopoIndex relies on the criteria that the ABL influence will be low if 1) the station is one of the highest points in the mountainous massif, 2) there is a large altitude difference between the station and the valleys, plateaus or the average domain elevation, 3) the slopes around the station are steep, and 4) the «drainage basin» for air convection is small. These principles are implemented by the calculation of 5 parameters involving the hypsometric curve, the steepness of the slopes around the station and the drainage basin for convection. The geometrical mean of these five parameters is the ABL-TopoIndex and allows ranking of the stations as a function of the ABL influence due to convection.
The first observation is that all stations on volcanic islands (in this study) have very low ABL-TopoIndex (i.e., low BL influence), whereas the stations in the Himalaya and the Tibetan plateau have high ABL-TopoIndex. Mount Pico in the Azores and the Teide Observatory in Izãna rank as the stations with the lowest ABL influence because both islands comprise the highest summit in the archipelago and both stations are just below the summits. In contrast, PYR lies at the foot of Mount Everest and at the confluence of several valleys leading to a high ABL-TopoIndex. Mount Moussala, Bulgaria, Mount Helmo, Greece and Mount Washington Observatory, USA are stations situated at the top of their mountainous ranges, leading to ABL-TopoIndex similar to the volcanic islands.
Statistically significant correlations between the ABL-TopoIndex and the aerosol parameters measured at 28 high altitude sites allow validation of the methodological approach. The greatest correlations are found with the minima of the aerosol parameters that represent the most likely FT air masses. The maxima of aerosol parameters are more representative of the intensity of aerosol sources and of advection of air masses with high aerosol concentrations. There are also strong anticorrelations between the slope local steepness and the particle number concentration, suggesting that new particle formation could be largely influenced by this topographical parameter. The amplitude of the diurnal cycle of the absorption coefficient is also correlated with the ABL-TopoIndex and is, thus, likely to be representative of ABL influence. The strength of the diurnal cycles of the scattering coefficient and the number concentration are, however, mostly explained by the latitude of the station, leading to the conclusion that the insolation drives the aerosol diurnal cycle.
Acknowledgements All station managers having shared aerosol datasets to validate the ABL-TopoIndex are associated as co-authors and gratefully acknowledged.