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.

Near dawn airborne lidar and dropsonde observations acquired on 7 July 2006, during the African Monsoon Multidisciplinary Analysis (AMMA) Special Observing Period 2a1, were used to investigate dust mobilisation, lifting and transport in the inter-tropical discontinuity (ITD) region over western Niger.

Atmospheric reflectivity data from the LEANDRE 2 lidar system enabled us to analyse the structure of dust plumes in the context of wind and thermodynamic information provided by the WIND lidar system and dropsondes.

Dust mobilisation was mainly observed in two locations: (a) Within the monsoon flow as the result of the passage of a density current issued from a mesoscale convective system over southwest Niger. (b) At the leading edge of the monsoon flow where the near-surface winds and turbulence were strong, because the monsoon flow was behaving as an intrusive density current. The circulation in the head of the monsoon density current lifted the mobilized dust towards the wake, along an isentropic surface. Away from the leading edge, some of the mobilized dust was observed to mix across the monsoon-harmattan interface, due to the existence of mechanical shear above the monsoon layer, and to become available for long-range transport by the harmattan.

Because dust sources are widespread over the Sahel and presumably active on many days when the ITD is located in this region during summer, dust emissions associated with the described mechanism may influence the radiation budget over West Africa.

Key words: Monsoon, Harmattan, Density current, AMMA, Airborne lidars, Dropsondes, ECMWF analyses.

The aerosol vertical distribution is also controlled by dynamics, as the penetration of the winter monsoon flux to the north, and northern winds bring mineral dust leads to a frontal discontinuity location close to Djougou latitude. During the early dry season, the aerosol vertical distribution was observed to be structured in two layers, the lower being the boundary layer reaching altitudes up to 2 km, and an upper layer reaching 4 to 5 km.

Lidar data are used to retrieve the time evolution and vertical profile of extinction, and discuss transport processes during the period analyzed. As the monsoon flux during the dry season is steadily progressing to the north, but also moving back and forth according to shorter time scale forcings, biomass burning particles are transported from the boundary layer into the upper troposphere. This transport has a strong impact on the distribution of aerosol particles on the vertical, and extinction values larger than 0.3 km-1 have been retrieved at altitudes close to 3 km.

A particular event of biomass burning air mass outbreak associated with a synoptic forcing is studied, where satellite observations are used to discuss observations of biomass burning particles over Djougou and at the regional scale.

(1)  Service d’Aéronomie / IPSL, Paris, France
(2)  Météo-France, CNRM/GMEI, Toulouse, France
(3) Laboratoire d’Aérologie, UPS and CNRS, Toulouse, France
(4) Deutsches Zentrum für Luft- und Raumfahrt, Wessling, Germany

Airbone lidar observations acquired with the LEANDRE 2 system during 3 flights of the SAFIRE Falcon 20 in the framework of the AMMA Special Observing Period (SOP) 2a1 (July 2006) over western Niger, revealed the existence of desert dust uptakes in the region of the inter-tropical discontinuity (ITD) in the morning hours. Complementary observations provided by dropsondes released from the same platform as well as airborne wind measurements made from another platform (the DLR Falcon 20, flying in coordination with the SAFIRE Falcon 20) evidenced that the lifting was associated with the leading edge of the monsoon low level jet, and to be transported southward by the harmattan, above the monsoon layer.

A 10-day numerical simulation, using the mesoscale model Meso-NH (including the dust emission box Dust Entrainment And Deposition model), was conducted to assess the representativity of the observed phenomenon as well as the mechanisms associated with the Sahelian dust emissions. The Meso-NH simulation (initialized by and nudged with ECMWF analyses) was carried out on a 2000 km x 2000 km domain (20-km horizontal resolution) centered at 20°N and 7°E, that included the Falcons flight track, as well as numerous AMMA-related ground-based measurement sites (Tamanrasset, Agadez, Niamey/Banizoumbou, etc..) for validation purposes.

In the simulation, large dust uptakes associated with the leading edge of the monsoon flow, with a dust concentration reaching 2000µg/m3, and to be transported southward by the harmattan, above the monsoon layer, were well reproduced. On the other hand, the simulation suggested the existence of dust emissions associated with the harmattan flow which were not observed by airborne lidar measurements. The reason for the discrepancy between the model results and the lidar observations is investigated.

Saharan dust uptakes associated with the inter-tropical discontinuity dynamics: Calipso observations and numerical modelling

D. Bou Karam(1), C. Flamant(1), J .Pelon(1), J.-P. Chaboureau(2), P. Tulet(3)

(1)Service d’Aéronomie / IPSL, CNRS and UPMC, Paris, France
(2)Laboratoire d’Aérologie, UPS and CNRS, Toulouse, France
(3)Météo-France, CNRM/GMEI, Toulouse, France

Over West Africa, the convergence line between monsoon and harmattan flows is called Intertropical Discontinuity (ITD). This boundary between dry, hot air to the north and warm, humid air to the south is an area of low pressure closely linked to other structures such as the Heat low, the African Easterly Jet, the Intertropical Convergence Zone and the African easterly waves. The latitudinal displacements of the ITD were associated to pressure changes in the harmattan and the monsoon air masses on both sides of it.

In this study, Saharan dust emission associated with the ITD structure and dynamics, are observed by means of an airborne Lidar, a satellite Lidar and by the Ozone Monitoring Instrument (OMI), and are analysed by mean of numerical simulations using the non hydrostatic mesoscale model MesoNH.

MesoNH is a regional model initialized by and nudged with ECMWF analyses, including a prognostic dust scheme allowing feedback studies between dynamics and radiation [Grini et al., 2006], and a dust emission box model,  the Dust Entrainment And Deposition (DEAD) model [Zender et al., 2003] which is implemented as a component of the MesoNH. DEAD describes dust sources and sinks while dust advection and diffusion are quantified by the transport processes and methods used in the host model. A series of simulations over ten days -between 2 and 12 of July 2006- was carried out, on a 2000 km² domain (20-km horizontal resolution) centred at 20°N and 7°E.

Airborne Lidar observations were carried out during the AMMA Special Observing Periods (SOPs) which took place in summer 2006, by means of the differential absorption lidar LEANDRE 2 system onboard the SAFIRE Falcon 20 between 3 and 10 july 2006 over northern Niger. Satellite Lidar and radiometer measurements were taken respectively by the backscattered Lidar of the satellite CALIPSO and the OMI on the Aura.

Large dust uptakes were observed to be associated with the leading edge of the monsoon flow and to be transported southward above the monsoon flow.
The vertical structure of dust storms was illustrated with the aid of MesoNH vertical fields and Lidar vertical profils, while MesoNH horizontal fields and OMI observations allowed us to follow the life cycle of these dust storms and their relations to the intertropical front, harmattan and monsoon dynamics and structure.

Saharan dust uptake over Sahel associated with the inter-tropical discontinuity dynamics: Lidar observations and mesoscale modelling

D. Bou karam(1), C. Flamant(1), P. Tulet (2), J.-P. Chaboureau (3)

(1)  Service d’Aéronomie / IPSL, Paris, France
(2)  Météo-France, CNRM/GMEI, Toulouse, France
(3) Laboratoire d’Aérologie, UPS and CNRS, Toulouse, France

The convergence line between monsoon and harmattan flows constitutes an interface which is called Intertropical Discontinuity (ITD). It represents the boundary between dry, hot air to the north and warm, humid air to the south, as these winds converge, dry and moist air are forced upward. It is an area of low pressure closely linked to other structures such as the Heat low and the Intertropical Convergence Zone.
In this presentation, the ITD structure and dynamics, and the mechanisms of the key processes associated with the Saharan dust emission, are analysed by means of Lidar observations,  dropsondes measurements and mesoscale modelling using the MesoNH model.

MesoNH is a regional model initialized by and nudged with ECMWF analyses, including a prognostic dust scheme allowing feedback studies between dynamics and radiation schemes [Grini et al., 2006], and a dust emission box model,  the Dust Entrainment And Deposition (DEAD) model [Zender et al., 2003] which is implemented as a component of the MesoNH

DEAD describes dust sources and sinks while dust advection and diffusion are quantified by the transport processes and methods used in the host model.
Lidar observations and dropsondes measurements were carried out during the AMMA Special Observing Periods (SOPs) which took place in summer 2006, respectively by means of an airborne lidar –the differential absorption lidar LEANDRE 2 system– and the AVAPS 4 –channel dropsonde system onboard the SAFIRE Falcon 20– between 3 and 10 july 2006 over northern Niger.

A series of simulations over ten days –between 2 and 12 of July 2006– was carried out, on a 2000 km² domain (20-km horizontal resolution) centered at 20°N and 7°E, 62 levels were used on the vertical resolution starting at 30 m above the ground.
Large dust uptakes were observed to be associated with the leading edge of the monsoon flow and to be transported southward above the monsoon layer during the 3 days of flight, with a dust concentration reaching 2000µg/m3.
MesoNH simulations show dust emission to be associated with the harmattan flow which is not the case in the lidar observations, this disagreement between model and lidar observations is probably due to the boundary layer parametrization in the model.

Numerical Modelling of Saharian dust impact on the atmospheric dynamics in the Bodele depression.

D. Bou karam(1), C. Flamant(1), P. Tulet(2), J.-P. Chaboureau(3) , R. Washington(4), M. Todd(5)

(1)Service d’Aéronomie / IPSL, Paris, France
(2)Météo-France, CNRM/GMEI, Toulouse, France
(3)Laboratoire d’Aérologie, UPS and CNRS, Toulouse, France
(4)Oxford University Centre for the Environment, University of Oxford, UK
(5)Department of Geography, University College London, UK

The historical record suggests that the Bodélé depression is a remarkably strong dust source on a global scale and it has been active for several hundred years. This region is undoubtedly the most intense dust source in the world [Prospero et al., 2002]. It is a unique dust source due to its location at a bottle neck of two large magmatic formations that serves as a ‘wind lens’, guiding and focusing the surface winds to the Bodélé depression. In the winter, the low level jet resulting from the constriction between the Tibesti and Ennedi massifs is responsible for the large dust loads observed by satellite, averaging more than 0.5Tg per day on 40% of the winter days [Koren et al, 2006 and Washington et al, 2006 ]. During the Bodélé Dust Experiment (BodEX) which took place in March 2005, mass flux of dust emission was estimated to be approximately 1.2Tg per day [Todd et al., in press].

In this presentation, the dynamics associated with the dust emission in this region, are analysed by means of observations and numerical modelling. Our goal is to evaluate the direct radiative effect of the aerosols lofted in this source area and the effects of this on the planetary boundary layer thermodynamics in the region using a combination of mesoscale simulations (model Meso-NH) and ground-based measurements acquired in the framework of BodEX.

MesoNH is a regional model initialized by and nudged with ECMWF analyses, including a prognostic dust scheme allowing feedback studies between dynamics and radiation [Grini et al., 2006], and a dust emission box model,  the Dust Entrainment And Deposition (DEAD) model [Zender et al., 2003] which is implemented as a component of the MesoNH. DEAD describes dust sources and sinks while dust advection and diffusion are quantified by the transport processes and methods used in the host model.

A series of simulations covering the whole BodEX period was carried out, on a 2000 km2 domain (20-km horizontal resolution). A nested domain (5-km resolution) was also implemented and has been activated on days when large dust events were observed (March 4 and 9 to 12). The 20-km resolution domain is centered at 16°53′ N, 18°33′ E which is the position of the site of observation, 72 levels were used on the vertical resolution starting at 10 m above the ground. The nested domain was centered at 17°40’N and 19°70’E, and also used 72 levels.

With the aim of better understanding the connection between the various processes concerned, and to evaluate the role of each one, we carried out 3 types of simulations: a simulation without dust aerosols, a simulation with prognostics dust aerosols, and a simulation with the characteristcs properities of the dust aerosols as measured during BoDEx. The 3 simulations were evaluated against the BoDEx data. For that purpose, surface measurements of meteorological parameters at 2m height (including temperature, wind speed, humidity, solar UV radiation and air pressure), and measurements of aerosols optical properities like aerosol optical depth and dust size distribution, were used.

Maximum wind speeds are underestimated by the model with a 2m/s difference approximatively, while the spatio-temporal evolution of dust cloud and daytime temperature at 2m are reasonably well reproduced.