Dust Storm Descends on ARM Mobile Facility in West Africa
A dust storm in early March significantly reduced visibility at the ARM Mobile Facility deployment site in Niamey, Niger. The instruments in the picture measure surface meteorology and the energy fluxes associated with both solar and thermal longwave radiation.
On Tuesday, March 7, the ARM Mobile Facility (AMF) recorded the onset of a large Saharan dust storm in Niamey, Niger. The dust storm approached the site at about 0930 UTC and rapidly reduced visibility. A thicker dust cloud, several kilometers in height, rolled in about 1230 UTC, reducing visibility even further. The dust storm continued unabated for two more days and then gradually dissipated over the next several days. This was the largest dust storm recorded in the area in the past two years. Satellite images indicate the storm originated in the central Sahara, driven by a cold air outbreak from Europe.
On March 7, dust optical depth in the morning was about 0.3, which is a typical value for Niamey. At 0930, the optical depth rose to roughly double that amount, and then at 0100 jumped to greater than 1.5. The next day, March 8, dust optical depth increased to about 3, an order of magnitude greater than typical levels. Data curves for different solar wavelengths (shown in the upper panel) all lie on top of each other, indicating the presence of large dust particles. (Figure courtesy of J. Barnard and E. Kassianov, Pacific Northwest National Laboratory.) Click for a larger version.
In this lidar (pulsed laser) image of the dust storm, the red color indicates significant scattering of the laser light. Note the very dark red color appearing at about 0930 as the dust storm arrives. The layer grows in height with time. Because the dust attenuates the laser light, the actual height of the layer cannot be accurately determined at times, but it extended to about 2 km. (Figure courtesy of Connor Flynn, Pacific Northwest National Laboratory.) Click for a larger version.
The Total Sky Imager instrument provides a hemispheric view of sky cover above the AMF near local noon in Niamey. The reddish tinge most likely comes from the presence of hematite in the blowing dust.
The AMF instrument suite measures atmospheric properties important for understanding climate. One of these properties, optical depth, is a measure of how much a specific layer in the atmosphere attenuates solar radiation. Typical values of particle optical depth in U.S. cities are less than a few tenths; the values in Niger during the first two months of 2006 were measured around 0.3 to 0.5. The recent storm had optical depths in excess of 3, which decreased the solar radiation reaching the surface by about 150 W/m2, or 15% of its usual value, and converted the incident solar radiation from direct to diffuse, which is typical of cloudy skies.
Optical depth was approximately constant with visible wavelength, indicating the dust particles were relatively large in size. Average dust particle size decreased with time, suggesting the larger particles fell to the surface, leaving a smaller size distribution. The thermal infrared emission spectrum of the dust can be used to infer its mineralogical content. Preliminary analysis confirmed the dust composition was dominated by clay minerals with a small, but important, fraction of hematite (an iron-oxide compound). This is consistent with some earlier studies, but differs from a recent study in Senegal indicating a silicate dust composition. The difference is likely due to different regions of origin within the southern flank of the Sahara.
Interestingly, winds during this storm were relatively light near Niamey. The dust was lofted into the atmosphere by strong winds over the Sahara desert to the northeast of Niamey, and then carried to Niamey by wind pattern that meteorologist refer to as a "density current." Air near the surface of the Sahel region (on the southern edge of the Sahara) is hotter at this time of year than the Saharan desert air, especially during this cold air outbreak from Europe. The cooler Saharan air flows downhill from the higher desert area and undercuts the warmer air over Niamey. The result is a relatively shallow layer (about 2 km) with very large particle concentrations. The dust arriving in Niamey was reddish in color, probably due to the presence of hematite.
The false color image from a high resolution spectral imager (upper panel) on the Meteostat Second Generation satellite shows the Saharan dust outbreak on March 8, 2006. The effect of the dust on the longwave radiation budget is measured by the Geostationary Earth Radiation Budget instrument on the same satellite (lower panel). The dust reduces the outgoing longwave radiation by more than 50 W/m2 compared to surrounding regions of the Sahara. (Figure courtesy of A. Slingo, University of Reading, UK.) Click for a larger version.
The Sahara desert is one of the major sources of windblown dust. Saharan dust has been measured frequently in the Caribbean and as far away as Brazil, and satellite images show large dust outbreaks moving over the Atlantic Ocean. The AMF measurements permit scientists to study the properties of dust near their source region, and the impact of dust on climatically important quantities, such as the surface radiation budget. Scientists involved with the AMF deployment in Niamey expect that this event is only the first of several dust storms that will occur in the next month or two preceding the onset of the monsoon in June.
