Low-level jet wind speed profiles from Bordj-Badji Mokhtar averaged for (a) dusty days (b) dust-free days.
Low-level jets, streams of strong winds only a few hundred metres above the Earth's surface, are common in desert regions. When low-level jets occur over dust sources, the strong winds can lead to dust emission, such as over the highly-active Bodélé Depression in the eastern Sahara. In the central Sahara - the dustiest place in the world in summer - they are also thought to be common, but until recently there have been no field observations in this remote and hostile region. In June 2011 however, multiple sites across the central Sahara were instrumented as part of the Fennec project, an international consortium led by Professor Richard Washington from the School of Geography and the Environment. Detailed examination of lidar, radiosonde and surface instrument measurements at the Algerian site of Bordj-Badji Mokhtar have provided the first ever observational analysis of low-level jets in the central Sahara.
We found that, whilst the low-level jets were present on most (21 out of 28) days, only the 5 very strongest jets (wind speeds ≥16 m/s) led to dust emission. On other days, the jets were too weak and the convective boundary layer was too shallow to impart momentum to the surface for dust emission. Low-level jets were most frequently embedded in either the south-westerly monsoon flow or the dry north-easterly Harmattan flow, and this orientation was controlled by the position of the Saharan Heat Low, a synoptic-scale feature that influences the climate of large areas of North and West Africa. A Met Office weather forecasting model, the Africa-LAM, was particularly successful at simulating the Harmattan jets, whilst the lower resolution but commonly-used ECMWF reanalysis product significantly underestimated their strength.
The journal article reporting these findings has recently been highlighted as a 'Research Spotlight' in Eos, Transactions American Geophysical Union.