Links between topography, wind, deflation, lakes and dust: The case of the Bodélé Depression, Chad

The Bodélé Depression, Chad is the planet's largest single source of dust. Deflation from the Bodélé could be seen as a simple coincidence of two key prerequisites: strong surface winds and a large source of suitable sediment. But here we hypothesise that long term links between topography, winds, deflation and dust ensure the maintenance of the dust source such that these two apparently coincidental key ingredients are connected by land-atmosphere processes with topography acting as the overall controlling agent. We use a variety of observational and numerical techniques, including a regional climate model, to show that: 1) contemporary deflation from the Bodélé is delineated by topography and a surface wind stress maximum; 2) the Tibesti and Ennedi mountains play a key role in the generation of the erosive winds in the form of the Bodélé Low Level Jet (LLJ); 3) enhanced deflation from a stronger Bodélé LLJ during drier phases, for example, the Last Glacial Maximum, was probably sufficient to create the shallow lake in which diatoms lived during wetter phases, such as the Holocene pluvial. Winds may therefore have helped to create the depression in which erodible diatom material accumulated. Instead of a simple coincidence of nature, dust from the world's largest source may result from the operation of long term processes on paleo timescales which have led to ideal conditions for dust generation in the world's largest dust source. Similar processes plausibly operate in other dust hotspots in topographic depressions

Remote sensing of cirrus clouds and aerosols by a sun photometer in Tunisia

International audienceSome ground based measurements of solar radiation by using a sun photometer, have been conducted in Tunisia during the period of November 2000–February 2002. Five key measurement sites were selected: Three Sites (Tunis, Sousse, Gabes) are located on the Mediterranean coast and Two sites (Gafsa, Tozeur) on the boarder of Sahara. Over a total of 149 measurement days, 21 days are identified as clear sky, 114 days as Cirrus clouds and 14 days as aerosols. Aerosols and Cirrus clouds Optical Thickness (AOT) are derived from photometric measurements at 532 nm wavelength. Spatial and temporal variabilities of AOT are presented and discussed in this paper. Cirrus clouds were frequently observed at Gafsa and Tozeur where saharan aerosol events are expected to be more frequent than cirrus clouds. The mediterranean sea and saharan aerosols are suspected to have the main role in cirrus clouds formation, by providing water vapor and high concentrations of cloud condensation and ice forming nuclei

HEAVY METAL VAPORIZATION IN FLUIDIZED BED COMBUSTION OF SOLID WASTE AND COAL

Solid samples, either of realistic waste model or coal, and spiked with Cd, Pb or Zn, were burned in an electrically-heated fluid bed reactor coupled to a customized ICP spectrometer, for on-line analysis of vaporized metals. For waste samples, a single kinetic law (whatever the metal), predicting the vaporization characteristic time and the time course of the metal concentration in the solid, was obtained. Tests with burning coal samples, spiked with Cd (at 820°C) and Zn (temperature range 680°C to 820°C), proved that this law is still valid, with a slight tendency to underestimation for both Cd and Zn vaporization rates. The transient metal concentration in burning coal was also very well predicted

Crossbreeding the Sardinian and East Friesian breeds in Sardinia

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Aspects quantitatifs de la production laitière des brebis. VII. – Précision d'un contrôle laitier alterné (AT) et d'un contrôle laitier d'alternance quelconque corrigé pour les écarts moyens entre les performances du soir et du matin (AC)

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Amélioration génétique de la production laitière des brebis Sardes II. – Facteurs de variation génétiques et non génétiques des performances de brebis ayant réalisé 2 lactations

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