Sharav dust storm

Bou Karam D., C. Flamant, J. Cuesta, J. Pelon, E. Williams (2010):

Dust emission and transport associated with Saharan depressions: The February 2007 case.

Abstract


The dust activity over North Africa associated with the Saharan depression event in February 2007 is investigated by mean of spaceborne observations and ground based measurements. 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 spatio-temporal 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 Mountain on the afternoon of February 20 in response to midlatitude trough intrusion. It migrated eastward with a speed of 11 m s-1 and reached Libya on February 22 before exiting the African continent toward the Mediterranean Sea on February 23. The horizontal scale of the cyclone at the surface varied between 800 km and 1000 km during its lifetime. On the vertical the cyclone extended over 8 km and a potential vorticity of 2 PVU was reported on its centre at 3km in altitude. The cyclone was characterised by a surface depth of 9 hPa, a warm front typified at the surface by an increase in surface temperature of 5°C, and a sharp cold front expressed by a drop in surface temperature of 8°C and an increase in 10m wind speed of 15 m s-1.

The cyclone provided a dynamical forcing that led to strong near-surface winds and produced a major dust storm over North Africa. Heavy dust load was seen along the cold front and the southeastern edge of the cyclone accompanied by a deep cloud band along its northwestern edge. The dust was transported all around the cyclone leaving a clear eye on its centre. 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.

Key Words: Mediterranean cyclone, CloudSat, ECMWF, North Africa, Sharav cyclone, CALIPSO.

Bou Karam D., C. Flamant, J. Cuesta, J. Pelon, E. Williams (2010): Dust emission and transport associated with a Saharan depression: The February 2007 case , J. Geophys. Res., doi:10.1029/2009JD012390.

Dry cyclogenesis and dust lofting over Sahara-Sahel

Diana Bou Karam, Cyrille Flamant, Pierre Tulet, Martin C. Todd, Jacques Pelon and Earle Williams: Dry cyclogenesis and dust mobilization in the Inter Tropical Discontinuity of the West African Monsoon: a case study, submitted to JGR.



Abstract

Three-dimensional mesoscale numerical simulations were performed over Niger in order to investigate dry cyclogenesis in the West African Inter Tropical Discontinuity (ITD) during the summer, when it is located over the Sahel. The implications of dry cyclogenesis on dust emission and transport over West Africa are also addressed. The study focuses on the case of 7th July 2006, during the African Monsoon Multidisciplinary Analysis (AMMA) Special Observing Period 2a1.

Model results show the formation of three dry cyclones along the ITD during a 24-hour period. Simulations are used to investigate the formation and the development of one of these cyclones over Niger in the lee of the Hoggar-Aїr Mountains. They show the development of the vortex to be associated with (a) strong horizontal shear and low-level convergence existing along the monsoon shearline and (b) enhanced northeasterly winds associated with orographic blocking of cool air masses from the Mediterranean Sea. The dry cyclone was apparent between 0700 and 1300 UTC in the simulation and it was approximately 400 km wide and 1500 m deep. Potential vorticity in the centre of vortex reached nearly 6 PVU at the end of the cyclogenesis period (1000 UTC).

The role of the orography on cyclogenesis along the ITD was evaluated through model simulations without orography. The comparison of the characteristics of the vortex in the simulations with and without orography suggests that the orography plays a secondary but still important role in the formation of the cyclone. Orography and related flow splitting tend to create Low Level Jets (LLJs) in the lee of the Hoggar and Aїr mountains which, in turn, create conditions favorable for the onset of a better defined and more intense vortex in the ITD region. Moreover, orography blocking appears to favor the occurrence of a longer-lived cyclone.

Furthermore, model results suggested that strong surface winds (~11 m s-1) enhanced by the intensification of the vortex led to the emission of dust mass fluxes as large as 3 µg m-2 s-1. The mobilized dust was mixed upward to a height of 4-5 km to be made available for long-range transport.

This study suggests that the occurrence of dry vortices in the ITD region may contribute significally to the total dust activity over West Africa. The distribution of dust over the Sahara-Sahel may be affected over areas and at time scales much larger than those associated with the cyclone itself.

Key Word: AMMA, cyclone, dust emission, MesoNH, Intertropical Discontinuity.