Elevated aerosol stratification above the Rhine Valley under strong anticyclonic conditions

A backscatter lidar study of the aerosol stratification in the planetary boundary layer (PBL) and lower troposphere over the Rhine valley is presented in this work. The observations have been made under anticyclonic conditions from 6 to 14 September 1999, during the Special Observation Period of the campaign FORM (Foehn in the Rhine Valley in Mesoscale Alpine Program). The lidar site was in Trübbach, Switzerland (47°04′N, 9°28′E, 490 m above sea level (a.s.l.)). The lidar was operated alongside with a wind-temperature radar, a set of surface meteorological stations and radiosonde observations. A daily series of range corrected lidar signal and its gradient are given, showing the diurnal cycle of aerosol layer development in the PBL and lower troposphere over the valley. Averaged altitude distributions of the aerosol backscatter coefficient and the lidar signal gradient are also presented. The results show that the aerosol distribution below the surrounding mountain peaks or below the valley top (approx. 2000–2400 m a.s.l.) have the diurnal pattern following from the thermal wind cycle and the temperature inversions in the valley. This diurnal cycle is characterized with a dynamic aerosol stratification containing mixed and detached (accumulation) aerosol layers. Above the valley top we observe an elevated aerosol stratification extending to approx. 4500–5000 m a.s.l. This elevated aerosol stratification correlates well with radiosonde observations presenting evidences of elevated residual layer over a larger Alpine region during the studied period.