Blanc SIMI 6 - Sciences de l'information, de la matière et de l'ingénierie : Système Terre, environnement, risques

The Saharan climate system – FENNEC


The Saharan Climate System and its impact at the regional scale

Why focus on Saharan climate?

<br />The Saharan heat low plays a pivotal role in the monsoon system of West Africa and in the exchanges of moisture and aerosols between the Sahara, the Mediterranean and the Sahel. It is also located in regions with the highest aerosol loads of earth's atmosphere during the northern summer. The desert dust are a crucial component of the climate which direct and indirect effects on the radiation balance are poorly understood. Biases and errors in the modeled radiation budget above the Sahara are very important and it is now tought to have significant impact on the modeling of atmospheric dynamics far beyond the Sahara, with consequences on the forecast of the development of hurricanes in the Atlantic or on the understanding of atmospheric circulation over the Mediterranean.<br /><br />The FENNEC project aims to better understand the Saharan climate, and is particularly interested in the mesoscale physical and chemical processes in the region of the central Sahara. This project will characterize the dynamics, thermodynamics and particle composition above the desert, in the Saharan atmospheric boundary layer and in the free troposphere, and evaluate their representation in global and regional models. A second objective is to study the mechanisms for lifting the desert aerosols in the sources, depending on surface conditions and weather conditions to improve forecasts of dust storms in the region of the central Sahara. A third objective is to make reliable «aerosols« products from space observation in the Saharan region.

The project FENNEC is articulated around field campaigns involving ground-based facilities and airborne means, the development of a platform dedicated to forecasting dust events over the Sahara being planned in support of the airborne campaign. Field campaigns are an essential component of the project since they will enable to acquire a unique data set on which most of the work conducted under the project FENNEC will be based.

Modeling of the Saharan atmosphere is a central component of the project FENNEC, and must fulfill two roles: (i) assist in the interpretation of measures «soil« and airborne and (ii) understand how the representation «simplified« process key in this region leads to a bias in the simulations and errors forecasts, and how these errors can be corrected. Once properly configured for this region, the simulations will quantify the direct and semi-direct radiative impact of aerosols in the Saharan region and the radiative effect on atmospheric dynamics.

The first part of the Fennec experimental campaign was conducted in June 2011 including: (i) the detachment of the Falcon 20 UMS SAFIRE, (ii) the installation of a remote sensing station in southern Spain and (iii) the installation of four GPS stations in Mali and Morocco.

For the first time, the ALADIN and AROME models (5 and 24 km horizontal grid size, respectively) have been implemented operationally to provide dusty forecasts.

The second part of the field campaign took place in June 2012 with the implementation of the desert aerosols sampling platform of LISA and the flux station of GET at In Salah (Algeria) in collaboration with the Algerian National Office of Meteorology.

Results (to be consolidated) half-way through the project
1. Highlighting the impact of intra-seasonal variability of the position of the Saharan heat low on the composition and thermodynamics of the Saharan atmospheric boundary layer,
2. Demonstration of the ability of AMSU-A observations (temperature) to detect the seasonal and intra-seasonal cycles of the Saharan heat low,
3. Demonstration of the ability of the operational AROME model to predict the convection in the Sahara: this good behavior led to the accurate forecast of a convective initiation event on the Atlas and the propagation of a density current (and uprisings of dust associated ) above the Sahara. This helped to make unique airborne observations over Mauritania and Mali,
4. Highlighting the impact of dust on the development of the Saharan atmospheric boundary layer (observations and numerical simulations).

The scientific exploitation of data acquired during the 2011 SOP is in progress. Early results have given place to a score of communications in international conferences (AGU, EGU, ILRC, AMMA, ...) as shown in the list of conference abstracts. A number of papers are also under preparation to be submitted soon. The exploitation of the 2102 SOP data will start soon.

Beyond the project period, we will continue to work with African Meteorological Offices on the impact of Saharan climate and Saharan dust on the weather in Western and northern Africa.

R. Washington et al., 2012 : Fennec - The Saharan Climate System: An overview, BAMS, in preparation

M. Todd et al., 2012: Meteorological and dust aerosol conditions over the Western Sahara observed at Fennec supersite-2 during the Intensive Observation Period June 2011, JGR, in preparation

K. Schepanski, C. Flamant, C. Kocha, J.-P. Chaboureau, C. Lavaysse, J. Perlon, F. Marnas, and P. Knippertz, 2012 : RAIN4DUST: An airborne study on the characterization of dust emission from alluvial sources, ACP, in preparation

S. Engelstaedter, R. Washington, C. Allen and C. Allen, 2012: The Saharan Heat Low and the West African Monsoon – Simultaneous Aircraft Observations from Fennec, JGR, in preparation

J.-P. Chaboureau, C. Flamant, C. Kocha, J.-P. Lafore, C. Lavaysse, F. Marnas, M. Mokhtari, J. Pelon, K. Schepanski, P. Tulet, 2012 : Verification of mesoscale dust forecasts over the Sahara during FENNEC 2011, JGR, in preparation.

C. Kocha, C. Flamant, F. Marnas, J.P. Chaboureau, D. Parker , J. Marsham, P. Knippertz and L. Garcia-Carreras 2012: How a dusty cold pool can change the evolution of the Saharan atmospheric boundary layer, QJRMS, in preparation.

The central Sahara has one of the most extreme climates on Earth. During the northern summer months, a large low pressure system caused by intense solar heating develops over a huge, largely uninhabited expanse of northern Mali, southern Algeria and eastern Mauritania. This Saharan heat low plays a pivotal role in the West African Monsoon. Moreover, the Saharan heat low region is dynamically coupled with the Mediterranean basin and the Sahel, two regions for which the predicted uncertainties associated with the impact of global forcing are quite significant.

This large zone is also where the thickest layer of dust anywhere in the Earth’s atmosphere is found. The direct and indirect effects associated with desert dust are still poorly quantified, and the biases and errors affecting the radiative budgets in models over the Sahara are very important. It is known that such errors in modelled dust radiatif effects lead to errors in key dynamical features beyond the Sahara, with consequences for tropical development in the Atlantic and the circulation over the Mediterranean basin.

The failure of climate models or numerical prediction models to capture main features of the Saharan weather is related to (i) the paucity of available data in this region, (ii) the difficulty to retrieve reliable space-borne “aerosol” products over deserts, and (iii) a lack of knowledge regarding the dynamics, thermodynamics and radiative processes in the Saharan atmosphere.

To date, there exist very sparse data sources in this region that can be used reliably to enhance knowledge in terms of mesoscale processes or model validation. Large uncertainties remain regarding the position of dust sources, the quantity and the properties of mineral dust emitted the albedo variability at the mesoscale and the impact of aerosol radiative forcing on the atmospheric dynamics in the region. Such uncertainties can only be thoroughly evaluated, and hopefully reduced, in the framework of an ambitious project aiming to make decisive progress in terms of dynamics, thermodynamics and on the structure and composition of the Saharan atmosphere, by means of observations over the Sahara.

Based on this, the interested French, British and German communities have decided to propose the FENNEC project which aims at (i) characterizing the Saharan atmospheric boundary layer, (ii) evaluating its representation in regional and global models, and (iii) improving “aerosol” products issued from space-borne observations. A key element of this programme is the organization of an international field campaign in the Saharan heat low region, which will include both ground-based and airborne detachments.

On important aspect of this proposal, which aims at enhancing the knowledge of processes controlling the Saharan climate system, is to provide a framework for the French groups eager to contribute to this type of research and to ask for the means for achieving the proposed goals. The FENNEC-France proposal is built on contributions from 11 laboratories.

The project benefits from support at the international level, amongst which support from the WMO program “Sand and Dust Storms Warning Advisory Assessment System” and the African Center of Meteorological Application for Development” (ACMAD).

Project coordinator

Monsieur Cyrille FLAMANT (CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - Délégation Ile de France Ouest et Nord) –

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.



Help of the ANR 740,000 euros
Beginning and duration of the scientific project: - 36 Months

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