Dust emission and transport associated with a Saharan depression: February 2007 case

Abstract

International audienceThe dust activity over North Africa associated with the Saharan depression event in February 2007 is investigated by mean of spaceborne observations, ground-based measurements, and mesoscale simulation with Meso-NH. The main characteristics of the cyclone as well as the meteorological conditions during this event are described using the European Centre for Medium-Range Weather Forecasts (ECMWF). The dust storm and cloud cover over North Africa is thoroughly described combining for the first time Spinning Enhanced Visible and Infra-Red Imager (SEVIRI) images for the spatiotemporal evolution and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) and CloudSat observations for the vertical distribution. The Saharan depression formed over Algeria in the lee of the Atlas Mountains on the afternoon of 20 February in response to midlatitude trough intrusion. It migrated eastward with a speed of 11 m s−1 and reached Libya on 22 February before exiting the African continent toward the Mediterranean Sea on 23 February. The horizontal scale of the cyclone at the surface varied between 800 and 1000 km during its lifetime. On the vertical the cyclone extended over 8 km, and a potential vorticity of 2 potential vorticity units (PVU) was reported at its center at 3 km in altitude. The cyclone was characterized by a surface pressure anomaly of about 9 hPa with respect to the environment, a warm front typified at the surface by an increase in surface temperature of 5°C, and a sharp cold front characterized by a drop in surface temperature of 8°C and an increase in 10 m wind speed of 15 m s−1. The cyclone provided dynamical forcing that led to strong near-surface winds and produced a major dust storm over North Africa. The dust was transported all around the cyclone leaving a clear eye at its center and was accompanied by a deep cloud band along the northwestern edge of the cyclone. On the vertical, slanted dust layers were consistently observed during the event over North Africa. Furthermore, the dust was lofted to altitudes as high as 7 km, becoming subject to long-range transport. The model was able to reasonably reproduce the structure, lifetime, and trajectory of the cyclone. Also, comparison with Moderate Resolution Imaging Spectrometer (MODIS) deep blue aerosol optical depths and CALIPSO-CloudSat observations suggests that the model can be reliably used to quantify the dust emissions associated with this event. The mean daily dust loads over the area influenced by the cyclone were simulated to range between 2 and 8 Tg during the lifetime of the Sharav cyclone (i.e., 5 days). This study suggests that dust emissions linked with Saharan cyclones may contribute significantly to the total dust load over West and North Africa observed annually

How important are atmospheric depressions and mobile cyclones for emitting mineral dust aerosol in North Africa?

“…Cyclones have to be identified in at least three consecutive nights reflecting a life time of 48 h as the minimum time period for a complete life cycle of a cyclone. This assumption complies with life times given in the literature (Hannachi et al, 2011;Bou Karam et al, 2010).…”

“…Maheras et al, 2001;Schepanski and Knippertz, 2011). Several studies suggest that cyclones can cause dust storms (Bou Karam et al, 2010;Hannachi et al, 2011;Schepanski et al, 2009;Schepanski and Knippertz, 2011), although a case study by Knippertz and Fink (2006) for the exceptionally strong and continental-scale dust storm in March 2004 gives evidence that a cyclone only produces one part of the associated dust emission. The remaining dust mobilization is linked to strong northeasterly Harmattan winds.…”

“…Migrating lee cyclones usually follow east-to northeastward trajectories with propagation speeds around 10 m s −1 (e.g. Alpert and Ziv, 1989;Alpert et al, 1990;Bou Karam et al, 2010;Hannachi et al, 2011). They can advect hot, dry and dusty air towards the eastern Mediterranean, but may also bring rainfall (Winstanley, 1972) with flood risk in Israel (Kahana et al, 2002).…”

Seasonal variation and light absorption property of carbonaceous aerosol in a typical glacier region of the southeastern Tibetan Plateau

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Online coupled regional meteorology chemistry models in Europe: current status and prospects

“…It was extended with the sectional aerosol model ORISAM (Cousin et al, 2004) as well as with the three-moments aerosol scheme ORILAM for the simulation of aerosol dynamics and secondary inorganic and organic aerosols (Tulet et al, 2005), which laid the foundation for studies of chemistrymeteorology feedbacks. MesoNH-C was subsequently employed for a wide range of applications to study the effect of biogenic emissions on regional O 3 levels (Solmon et al, 2004), the impact of convection on aerosol hygroscopicity (Crumeyrolle et al, 2008), for regional scale CO 2 source inversion (Lauvaux et al, 2009), sulfur transport and chemical conversion in a volcanic plume (Tulet and Villeneuve, 2011), or to investigate Saharan dust transport (Bou Karam et al, 2010) to name but a few.…”