The topography contribution to the influence of the atmospheric boundary layer at high altitude stations

Abstract

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. The local and meso-scale topographical features around the station are involved in the convective boundary layer development and in the formation of thermally induced winds leading to ABL air lifting. The station altitude is not a sufficient parameter to characterize the ABL influence. 20Topography data from the global digital elevation model GTopo30 were used to calculate 5 parameters for 46 high altitude stations situated in five continents. The geometric mean of these 5 parameters determines a topography based index called ABL-TopoIndex which can be used to rank the high altitude stations as a function of the ABL influence. To construct the ABL-TopoIndex, we rely on the criteria that the ABL influence will be low if the station is one of the highest points in the mountainous massif, if there is a large altitude difference between the station and the valleys or plateaus, if the slopes around 25 the station are steep, and finally if the drainage basin for air convection is small. All stations on volcanic islands exhibit a low ABL-TopoIndex whereas stations in the Himalaya and the Tibetan Plateau have high ABL-TopoIndex values.Spearman's rank correlation between aerosol optical properties and number concentration from 28 stations and the ABLTopoIndex, the altitude and the latitude are used to validate this topographical approach. Statistically significant (s.s.) correlations are found between the 5 and 50 percentiles of all aerosol parameters and the ABL-TopoIndex whereas no s.s. 30 correlation is found with the station altitude. The diurnal cycles of aerosol parameters seem to be best explained by the station latitude although a s.s. correlation is found between the amplitude of the diurnal cycles of the absorption coefficient and the ABL-TopoIndex. Finally, the main flow paths for air convection were calculated for various ABL heights.Atmos. Chem. Phys. Discuss., https://doi

Seasonal Variation of Aerosol Size Distribution Data at the Puy de Dôme Station with Emphasis on the Boundary Layer/Free Troposphere Segregation

“…Chauvigné et al (2018) [57], also reports time spent in the FT for the Chacaltaya (CHC) (5240 m a.s.l, Bolivia) site, and found FT prevalence for 45% of the time as a yearly average, with no marked seasonal variations. The difference between the PUY and these sites is probably that the PUY is at a lower altitude, but also because the configuration of the topography is different (North-South orientated single mountain chain) [58].…”

Meteorological Applications Benefiting from an Improved Understanding of Atmospheric Exchange Processes over Mountains

“…Many monitoring stations for air chemistry and greenhouse gases are located on mountain tops, which can be affected by these mechanisms [118]. This has led to the development of approaches to distinguish local effects due to atmospheric exchange processes from background concentrations measured at mountain top locations [119,120]. These approaches would greatly benefit from new datasets to explicitly characterize these exchange processes.…”

Reply to Referee#1

“…Exceptions are, however, observed, for example, at CBW (western Europe) the median SAE reaches values of around 2.1. Indeed, both polluted air masses from the industrialized zones of the Benelux countries and clean air masses from the sea contribute to the presence of aerosol particles at this site (Crumeyrolle et al, 2010). Moreover, CBW is surrounded by several large cities at distances of approximately 20-40 km from the station, which may have contributed to the high SAE values measured in this geographical location.…”