High-Resolution Image

“The dust falls in such quantities as to dirty everything on board, and to hurt people’s eyes; vessels even have to run on shore owing to the obscurity of the atmosphere”. Noted by Charles Darwin in 1832 when he began his famous voyage aboard the Beagle off the west coast of Africa (Cape Verde Islands), this is probably one of the earliest sightings recorded of wind-blown desert dust over the Atlantic Ocean.

Every year, millions of tons of dust are ejected from the arid/semi-arid soils of North Africa, including the Saharan desert. Strong gusty winds and intense solar heating of the desert surface cause dust particles to be lifted from their parent soils and transported across the Atlantic Ocean in a layer of hot and dry air known as the Saharan Air layer (SAL). Over a period of about 5-6 days, the dust makes its way across the Atlantic Ocean reaching the East Coast of the United States, the Caribbean Islands and even as far as the Amazon Rain Forest in South America. The impact of dust is certainly felt world-wide.

Global dust is continuously being tracked through a dense network of satellite/ground-based sensors, to better understand the complex interactions of dust with earth’s natural systems, including the oceans, and our weather and climate. This was the case during the summer of 2006, when NASA conducted the NASA African Monsoon Multidisciplinary Analyses (NAMMA) Field Campaign in the Cape Verde Islands, to examine the development of hurricanes and the role that dust might have on these systems.

An example of a large dust plume over Cape Verde (white circle) is observed by MODIS/Terra on 06 Sept [Fig. 1(a)]. The aerosol optical thickness (AOT - τ), a measure of the scattering/absorptive properties of aerosols like dust, is shown for the same scene at 0.55µm (green wavelength), retrieved using the NASA Deep Blue algorithm [Fig. 1(b)]. The AOT is ~ ≥ 0.80 over Cape Verde (white circle). For comparison, clear skies have an AOT of zero. The angstrom exponent (α), a measure of particle size, is also shown for the same scene over Cape Verde (Fig. 1(c) - white circle). Values are near zero, indicative of large sized particles like dust.

In September 2006, the SMART-COMMIT ground-based laboratories were deployed to Sal Island [Fig. 1(d)]. One of the instruments, the Atmospheric Emitted Radiance Interferometer [AERI – Fig. 1(e)], detects/tracks airborne dust by measuring the longwave scattering and thermal emissions from the atmosphere. The more the dust, the greater will be the detected signal intensity. Figure 1(f) shows wave-like pulses of dust over Cape Verde observed by AERI. Also shown are time series of AOT [Fig. 1(g)] and surface forcing [Fig. 1(h)] in the IR for a 3-day dust period. See image text for details. AERI will be used to further characterize the IR radiative impact of dust near the surface.

R. Hansell, Q. Ji, MJ Jeong, S. Bell, C. Wang, C. Li, C. Salustro, C., Bettenhausen, C. Hsu, and S.C. Tsay