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. ]]> http://www.drboukaram.net/sharav-dust-storm/feed 0 http://www.drboukaram.net/quantification-of-dust-load-in-the-itd-region http://www.drboukaram.net/quantification-of-dust-load-in-the-itd-region#comments Sat, 30 Jan 2010 09:58:16 +0000 admin http://www.drdiana.fr/?p=26 Bou Karam D., C. Flamant, P. Tulet, J-P. Chaboureau, A. Dabas, M. C. Todd (2009b): Estimate of Sahelian dust emissions in the Intertropical discontinuity region of the West African Monsoon, submitted to JGR.

Abstract

A three-dimensional mesoscale numerical simulation has been performed to investigate the dust emissions over Sahel associated with strong near-surface winds in the region of the West African Inter Tropical Discontinuity (ITD) during the summer, when the ITD is located over Niger and Mali around 18°N.

The study focuses on the period from 2 to 12 July 2006, in the framework of the African Monsoon Multidisciplinary Analysis (AMMA) Special Observing Period 2a1. A comparison with space-borne observations from the Spinning Enhanced Visible and Infra-Red Imager (SEVIRI) as well as airborne lidar observations acquired on 7 July 2006 demonstrates that the model is able to reproduce the complex vertical structure of the dynamics and aerosol field associated with the monsoon-harmattan interface.

This suggests that the model can be used reliably to analyse and quantify the dust emissions associated with the strong monsoonal surface winds blowing over the Sahelian dust sources during the rainy season. A comparison with the surface observations in Niamey between 2 and 12 July period indicates that the near surface characteristics of the monsoon to the south of the ITD are realistically reproduced over the period of the simulation.

The dust load in the ITD region simulated by the model is compared to the estimates derived from satellite observations using the method of Koren et al. [2006]. The daily mean values of dust load related to the strong winds on both side of the ITD, estimated from the simulation within the model domain (2°W-16°E, 12-28°N), are in excess of 2 Tg on some days and are found to be underestimated with respect to the observational estimates.

In the present case, the dust load associated with the strong winds south of the ITD accounts for approximately 1/3 of the total load over the entire domain on a given day, and is simulated to range between 0.5 and 0.8 Tg on average.

This indicates that the mean daily dust load estimated in the ITD region is close to the estimated daily rate of dust emission from the Bodélé depression. This study suggests that emissions driven by strong surface winds occurring on both sides of the ITD while it lies across the Sahel may contribute significantly to the total dust load over West and North Africa observed annually.

Keywords: Dust, MesoNH, airborne lidar, density current, AMMA

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.

Abstract

The diurnal cycle of the Inter-tropical discontinuity (ITD), i.e. the interface at the ground between the moist monsoon air and the dry Harmattan air, is an important factor in the West African monsoon system. During the whole year of 2006, high resolution ground-based remote-sensing measurements have been performed in the area of Djougou, Benin which made it possible to observe the ITD and the associated sharp gradient of temperature and humidity in detail.

In order to extend the point measurements to a 3D view of the ITD and to enhance the knowledge of the processes around the ITD, the mesoscale atmospheric model MesoNH has been run for a 84-hours period in April 2006. In addition Meteosat infrared observations have been used to determine the ITD position and its movement. From these observations a northward propagation of the moist air front (ITD) of 8-12 m s^-1 has been calculated.

The model turned out to match well with the observations: For example, the time of front arrival in Djougou is simulated with a maximum error of about 1 hour and the speed of the ITD is consistent with Meteosat images. This agreement suggests the use of the model to further describe processes in the lower atmosphere.

Keywords: AMMA, West African Monsoon, Low level jet, MesoNH

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