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Saharan dust events contribute significantly to the aerosol load in spring and autumn. These events can be captured with continuous measurements on the Jungfraujoch and at the aerological station in Payerne.
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Mineral dust is a major component of atmospheric aerosols. It comes mainly from deserts, but can also come from soil erosion and agriculture. The Sahara is the biggest source of mineral dust, releasing between 60 and 200 million tonnes of dust into the atmosphere every year. Whereas larger particles fall back to the ground quickly, smaller particles can be carried thousands of kilometres, reaching every part of Europe.
Detection of Saharan dust events on the Jungfraujoch
Mineral dust can be identified by the reddish-yellow colour that it lends to rain, or by deposits on snow, ice or soil. Continuous measurements of the optical qualities of aerosols have been taken since 2001 at the high-Alpine research station on the Jungfraujoch, where the occurrence of mineral dust from the Sahara is recorded with an hourly temporal resolution. If a Saharan dust event occurs, it is shown by MeteoSwiss under measurement values at meteorological stations. The most recent Saharan dust events are published.
Laser remote sensing of Saharan dust
Saharan dust events can be detected with LIDAR instruments. These instruments, which emit laser beams into the atmosphere, are used at the MeteoSwiss aerological station in Payerne to measure the height and thickness of mineral dust clouds.
For example, LIDAR instruments detected a layer with high aerosol concentration (dark red) at an altitude of 4 to 6 km above Payerne from 00:00 hours on 30th March 2014. During the day, this layer sank (due to subsidence) to lower altitudes (3 to 5 km). At the same time, the concentration also decreased (colour gradient changed from red to yellow). In measurements carried out during the same time frame on the Jungfraujoch, these aerosols were identified as Saharan dust. A second aerosol layer originating in the Po Valley was registered at an altitude of 2 to 3 km from 02:00 hours. This dusty layer also sank during the day, eventually mixing with the planetary boundary layer, the lower part of the Earth’s atmosphere.
Climatology of Saharan dust events
Saharan dust events have been recorded on the Jungfraujoch since 2001. These tend to make a major contribution to the aerosol load over Switzerland, peaking during spring (March to June) and in October and November. There are usually only a few significant events in summer, and only very brief ones in winter. Almost half of the Saharan dust events last only a few hours, while a quarter persist for more than a day. Analyses of the path of dust clouds show that the time it takes for mineral dust to be carried from the Sahara to the Jungfraujoch is between two days and one week. Between 10 and 35 events are registered each year. This is the equivalent of between 200 and 650 hours during which Saharan dust is recorded on the Jungfraujoch.
The various aerosol types can be distinguished by their optical qualities. One of these qualities is the single scattering albedo, which describes the ratio of light reduction due to scattering. This is an important parameter for obtaining local estimates of the direct radiative forcing by aerosols. The detection method is based on the reversed wavelength dependence of the single-scattering albedo in the presence of mineral dust. Using the example of Saharan dust events in mid-June 2002, this effect is clearly recognizable: The highest backscattering was measured during the event at the highest wavelength (950 nm), while the backscattering in normal cases is greatest with smaller wavelengths.
Publikation
Saharan dust events at the Jungfraujoch: Detection by wavelength dependence of the single scattering albedo and first climatology analysis, 4, 2465-2480, 2004.