Fifteen‐minute Meteosat Second Generation (MSG) Spinning Enhanced Visible and Infrared Imager (SEVIRI) infrared dust index images are used to identify dust source areas. The observations of dust source activation (DSA) are compiled in a 1° × 1° map for the Sahara and Sahel, including temporal information at 3‐hourly resolution. Here we use this data set to identify the most active dust source areas and the time of day when dust source activation occurs most frequently. In the Sahara desert 65% of DSA (March 2006 to February 2008) occurs during 0600–0900 UTC, pointing toward an important role of the breakdown of the nocturnal low‐level jet (LLJ) for dust mobilization. Other meteorological mechanisms may lead to dust mobilization including density currents initiated by deep convective systems which mobilize dust fronts (haboobs) occurring preferentially in the afternoon hours and cyclonic activities. The role of the nocturnal LLJ for dust mobilization in the Sahara is corroborated by regional model studies and analysis of meteorological station data.
[1] Atmospheric mineral dust has recently become an important research field in Earth system science because of its impacts on radiation, clouds, atmospheric dynamics and chemistry, air quality, and biogeochemical cycles. Studying and modeling dust emission and transport over the world's largest source region, the Sahara, is particularly challenging because of the complex meteorology and a very sparse observational network. Recent advances in satellite retrievals together with ground-and aircraft-based field campaigns have fostered our understanding of the spatiotemporal variability of the dust aerosol and its atmospheric drivers. We now have a more complete picture of the key processes in the atmosphere associated with dust emission. These cover a range of scales from (1) synoptic scale cyclones in the northern sector of the Sahara, harmattan surges and African easterly waves, through (2) low-level jets and cold pools of mesoscale convective systems (particularly over the Sahel), to (3) microscale dust devils and dusty plumes, each with its own pronounced diurnal and seasonal characteristics. This paper summarizes recent progress on monitoring and analyzing the dust distribution over the Sahara and discusses implications for numerical modeling. Among the key challenges for the future are a better quantification of the relative importance of single processes and a more realistic representation of the effects of the smaller-scale meteorological features in dust models. In particular, moist convection has been recognized as a major limitation to our understanding because of the inability of satellites to observe dust under clouds and the difficulties of numerical models to capture convective organization.Citation: Knippertz, P., and M. C. Todd (2012), Mineral dust aerosols over the Sahara: Meteorological controls on emission and transport and implications for modeling, Rev. Geophys., 50, RG1007,
About 40 million tons of dust are transported annually from the Sahara to the Amazon basin. Saharan dust has been proposed to be the main mineral source that fertilizes the Amazon basin, generating a dependence of the health and productivity of the rain forest on dust supply from the Sahara. Here we show that about half of the annual dust supply to the Amazon basin is emitted from a single source: the Bodélé depression located northeast of Lake Chad, approximately 0.5% of the size of the Amazon or 0.2% of the Sahara. Placed in a narrow path between two mountain chains that direct and accelerate the surface winds over the depression, the Bodélé emits dust on 40% of the winter days, averaging more than 0.7 million tons of dust per day.
We describe observations from the Fennec supersite at Bordj Badji Mokhtar (BBM) made during the June 2011 Fennec Intensive Observation Period. These are the first detailed in situ observations of meteorology and dust from the central Sahara, close to the center of the Saharan heat low and the summertime dust maximum. Historically, a shortage of such Saharan observations has created problems for evaluating processes, models, and remote sensing. There was a monsoon influence at BBM before 8 June and after 12 June, with dry Harmattan winds in between. A split boundary layer, generated by ventilation from the Atlantic, persisted during the drier phase. Extensive cold pools (haboobs) and microburst‐type events were regularly observed. Moisture reached BBM at night from the monsoon and the embedded haboobs. As well as the regularly occurring nocturnal low‐level jet (LLJ), a Saharan upper boundary layer (650 hPa) jet was observed, where winds feel drag from dry convection in the afternoon. This jet is linked to the diurnal cycles of moisture and cloud. Most dust was observed in the cloudier monsoon‐affected periods, and covarying dust and cloud amounts explain most of the variations in shortwave radiation that control the surface sensible flux. Dustiness is related to a standard parameterization of uplift using 10 m winds (“uplift potential”), and this is used to estimate uplift. Around 50% of uplift is nocturnal. Around 30% is from the LLJ, and 50% is from haboobs, which are mainly nocturnal. This demonstrates, for the first time from observations, the key role of haboobs, which are problematic for models.
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