The Importance of Mineral Dust
Atmospheric aerosols which include mineral dust, have an important impact on the climate system but the details of that impact are still unclear.
Dust directly and indirectly influences the Earth’s radiation budget. The direct influence involves the scattering and absorbing radiation. Indirect influences include interactions with cloud droplets which in turn impacts the climate.
We need to know about the role of dust in the climate system so that we can represent the effects of the dust on climate processes in climate models which are our prime tool for weather forecasting and climate change prediction.
Some of the effects of dust are immediate, such as the reduction in solar radiation available to heat the earth’s surface. Accurate representation of the dust in models therefore requires that the emission of material and transport is understood and well simulated otherwise the dust in the forecast or climate model imposes its impacts in the wrong place and at the wrong time. For this reason it is crucial to know what is going on in the very large dust sources. The Bodélé depression in Chad is widely regarded as the largest single source of dust in the world.
Bodélé Depression, Chad
Research on mineral aerosols in the African Climate Group at Oxford began with analysis of the TOMS Aerosol Index data from the Nimbus 7 satellite following the encouragement of Professor Andrew Goudie (R Washington, et al., Annals of the Association of American Geographers 93 (2), 297-313 https://doi.org/10.1111/1467-8306.9302003). The instrument flying on Nimbus 7 was designed to detect ozone not dust, but it turned out that differencing two of the channels of radiation identified absorbing aerosols such as dust. Plots of this data pointed to a bullseye over the Bodélé Depression in Chad (Washington et al., 2003 see Figure 1 above).
At the time we weren’t sure if data from TOMS was providing a realistic view because as the surface of the Bodélé depression is so white and highly unusual it often causes difficulty in remote sensing. To find out for sure, we set up the Bodélé Dust Experiment (BoDEx) which was funded by the Royal Geographical Society. That funding took us to Chad and started a 20 year research programme on dust.
Working with Samuel Mbainayel from the Direction des Resources en Eau et de la Meteorologie (DREM) in N’Djamena, Chad, we set off from the capital of Chad to the Bodélé depression. BoDEx2005 took place over 14 days at the leading edge of the dust emission in the Bodélé Depression at a place marked on the map as Chicha. All that was there was a lone tree and a lot of ordinance lying around from the Toyota Wars of the 1980s.
We released pilot balloons 9 times a day to track the boundary layer winds and set up two automatic weather stations as well as a Cimel photometer and aerosol sampling equipment. The AWS measured 2-minute averages of temperature, dew point temperature, wind speed and direction, pressure, solar and UV radiation at 2m (Washington et al., 2006). These are the first data and as far as we can tell, the only met data from the dustiest place on earth.
The data is analysed in this paper: Dust and the low‐level circulation over the Bodélé Depression, Chad: Observations from BoDEx 2005 https://doi.org/10.1029/2005JD006502
A few years ago the Prime Minister of Chad kindly invited us to N’Djamena to discuss the famous Bodélé Depression dust source.
Key Findings on Bodélé Dust
From: Dust and the low‐level circulation over the Bodélé Depression, Chad: Observations from BoDEx 2005
- The observations of winds showed a large diurnal cycle in wind speed on dusty and dust free days with a peak in the mid-morning.
- The core of the fast winds is part of the Bodélé Low Level Jet and lie between ~100m and 900m above the surface
- The maximum wind on dust free days was 10ms-1 during the day and 2ms-1 at night
- The maximum surface wind speed increased to 16ms-1 on dusty days and these values were sustained through the dust event
- The observations confirmed that the dust plumes captured in MODIS satellite imagery originate from the Bodélé depression
From: Atmospheric controls on mineral dust emission from the Bodélé Depression, Chad: The role of the low level jet
- We found that the reason for such high levels of dust emission from Chad is the co-location of a very fast wind which we called the Bodélé low-level jet (LLJ) and the highly erodible diatomite sediment of the Bodélé depression The ridging of the Libyan High-Pressure system spins up the Bodélé Low Level Jet which is part of the north-easterly Harmattan winds so that most of the dust emission events from the Bodélé occur through this mechanism
- The Bodélé Low Level Jet is evident in reanalysis data as an unusual wind in the cross section of longitude with height across West Africa
- The ridging of the Libyan High-Pressure system spins up the Bodélé Low Level Jet which is part of the north-easterly Harmattan winds so that most of the dust emission events from the Bodélé occur through this mechanism
From Links between topography, wind, deflation, lakes and dust: The case of the Bodélé Depression, Chad
- There are long term links between the topography, wind deflation and dust source in the depression which have led to the maintenance of the dust source. The topography of the region is the key controlling agent in the formation of the dust source, the Tibesti and Ennedi mountains cause the funnelling of the erosive wind regime.
- The Sahel region has experienced a variety of climates throughout geological history, during dry phases (for example the Last Glacial Maximum) the enhanced deflation caused a shallow to form over the Bodélé depression, which then filled in wetter phases to produce paleolake MegaChad.
- This resulted in a habitat in which diatoms lived during the wetter phases, such as the Holocene pluvial. Therefore, rather than a coincidental dust source the Bodélé depression has been produced through a combination of long-term atmospheric, hydrological and geomorphological processes
From: Dust as a tipping element: The Bodélé Depression, Chad
- Minor adjustments to small features of the atmospheric circulation could profoundly alter the behaviour of this feature
- The Bodélé depression is a small region and is a transport limited region meaning that any changes to the LLJ could lead to substantial changes in dust production
- Simulations of the 21st century indicate the potential for a substantial increase in dust production by the end of the century in comparison with current values.
More recent research on the Bodélé depression shows that there are still substantial underestimations of the LLJ wind speeds in reanalysis data. Despite reanalysis models now showing the diurnal cycle of the wind regime, the higher winds which are most crucial for dust entrainment are greatly underestimated when compared to observational data.