Pulsing dust release from the Bodele Depression

By HansPeter Roesli (EUMETSAT)

The dry bowl that forms the depression is marked by a series of dried lakes, many of which were last filled during wetter periods occurring over the last ten thousand years.

The primary cause for the large dust events that distinguish the region's climate is a high pressure system over North Africa that drives the northeasterly winds (Harmattan) over the Sahara. Forced through the gap between the Tibesti and Ennedi mountains and over the sloping terrain upwind of the Bodele Depression, the trade wind, in effect, becomes a low-level jet. According to observations by Todd et al. a pronounced diurnal cycle in surface wind speeds and dust release is observed with wind maxima close to 09:00 UTC. The same paper suggests that the wind speed peaks are due to a mixing of momentum from the low-level jet down to the surface by radiative heating in the early morning.

The way in which the diurnal cycle of surface winds modulates the dust release over the Bodele Depression is nicely illustrated by a sequence of 15-min Dust RGB images taken by Meteosat-9 and covering the period of 17–19 November 2009. The sequence clearly shows the regularly pulsing dust release from the Bodele Depression over three days that is marked by the dark patch in the upper right corner. The time of the maximum release agrees well with the observations by Todd et al.

The corresponding sequence of HRV images (17–19 Nov 2009) reveals the fine-structure of the dust plumes. In particular, a wavy dust top appears briefly before sunset signalling the presence of relatively strong wind shear at the top of the boundary layer (see also sequence of HRV images 17 November, 14:00–16:00 UTC).

In actual fact, there were pulsed releases of dust already occurring in the days leading up to the period shown above. A sequence of hourly Dust RGB images taken over a larger area during the period 13–18 November 2009 shows the partial dispersion of the dust over Niger in the hours following its release from the Bodele Depression.

Additional content

Animation HRV channel 17 November (14:00–16:00 UTC, AVI)
Animation Dust RGB 17-18 November (11:00–10:00 UTC)
Radiosounding Niamey 18 November 00:00 UTC (source: University of Wyoming)
 

References

Todd, M. C., R. Washington, J. V. Martins, O. Dubovik, G. Lizcano, S. M'Bainayel, and S. Engelstaedter (2007): Mineral dust emission from the Bodele Depression, northern Chad, during BoDEx 2005. J. Geophys. Res., 112, D06207, doi:10.1029/2006JD007170.

Schepanski et al. (2009): Meteorological processes forcing Saharan dust emission inferred from MSG-SEVIRI observations of subdaily dust source activation and numerical models J. Geophys. Res., 114, D10201, doi:10.1029/2008JD010325..