Guy Cautenet

Quantifying uncertainty in estimates of mineral dust flux: An intercomparison of model performance over the Bodélé Depression, northern Chad

Mineral dust aerosols play an important role in the climate system. Coupled climate-aerosol models are an important tool with which to quantify dust fluxes and the associated climate impact. Over the last decade or more, numerous models have been developed, both global and regional, but to date, there have been few attempts to compare the performance of these models. In this paper a comparison of five regional atmospheric models with dust modules is made, in terms of their simulation of meteorology, dust emission and transport. The intercomparison focuses on a 3-day dust event over the Bodele depression in northern Chad, the worlds single most important dust source. Simulations are compared to satellite data and in situ observations from the Bodele Dust Experiment (BoDEx 2005). Overall, the models reproduce many of the key features of the meteorology and the large dust plumes that occur over the study domain. However, there is at least an order of magnitude range in model estimates of key quantities including dust concentration, dust burden, dust flux, and aerosol optical thickness. As such, there remains considerable uncertainty in model estimates of the dust cycle and its interaction with climate. This paper discusses the issues associated with partitioning various sources of model uncertainty.

Dust as a tipping element: The Bodélé Depression, Chad

Dust plays a vital role in climate and biophysical feedbacks in the Earth system. One source of dust, the Bodélé Depression in Chad, is estimated to produce about half the mineral aerosols emitted from the Sahara, which is the worlds largest source. By using a variety of new remote sensing data, regional modeling, trajectory models, chemical analyses of dust, and future climate simulations, we investigate the current and past sensitivity of the Bodélé. We show that minor adjustments to small features of the atmospheric circulation, such as the Bodélé Low-Level Jet, could profoundly alter the behavior of this feature. Dust production during the mid-Holocene ceased completely from this key source region. Although subject to a great deal of uncertainty, some simulations of the 21st century indicate the potential for a substantial increase in dust production by the end of the century in comparison with current values.

Thermal Impact of Saharan Dust over Land. Part I: Simnulation

Abstract Simulations are carded out to verify a mesoscale model in order to perform sensitivity tests of satellite response to atmospheric dust content. The model chosen is the mesoscale model of Colorado State University with a modified radiation parameterization in order to take atmospheric dust content into account. Downward and upward longwave irradiances are estimated using a 25-interval model. The shortwave pan of the spectrum is processed by a very fast, highly parameterized, single-interval code. Tests using experimental data gathered during the Etude de la Couche Limite Atmospherique Tropicale Seche (ECLATS) experiment performed during the 1980 dry season near Niamey (Niger, West Africa) prove that dust content is satisfactorily handled. Three 24-h simulations performed under various meteorological and turbidity conditions show that ground surface energy exchanges are satisfactorily described, so that surface temperature is predicted with a standard deviation of about 1°C. Vertical profiles of co...

Mesoscale modeling of aeolian dust emission during the BoDEx 2005 experiment

The Bodele depression (Northern Chad) is believed to be the single largest source for the Saharan dust transported over the Atlantic Ocean, especially that emerging in the Gulf of Guinea in boreal winter. During the Bodele Dust Experiment 2005 (BoDEx 2005), which was performed in February-March 2005, a severe dust event was observed and some of its main characteristics (surface wind, dust concentrations, radiation) were recorded. This paper tests the capability of a mesoscale model coupled online with a dust production model (DPM) to reproduce the small scale features associated to this dust event. These simulations clearly show that a spatial resolution of 10 km x 10 km is required to reproduce satisfactorily the observed surface winds and the main characteristics of the dust plume.

Thermal Impact of Saharan Dust over Land. Part II: Application to Satellite IR Remote Sensing

Abstract The use of the mesoscale model described and qualified in Part I is arranged with radiative transfer codes for the simulation of the thermal infrared response of Meteosat from a Sahelian target. The sensitivity of the satellite response to various atmosphere and surface parameters, either relevant or extraneous to dustiness, is analyzed and physically interpreted throughout the daily cycle, considering especially the thermal impact of the dust at the ground surface. The most significant parameters, according to this criterion of sensitivity, are the amount of dust in the atmosphere and its radiative characteristics, and the ground surface emissivity in the satellite channel. If neglected, the atmospheric water vapor content may be a large source of error for the retrieval of dustiness from the satellite data. The theoretical results are discussed and compared with earlier published experimental work.

Modeling of a Saharan Dust Event

We have coupled a nonhydrostatic mesoscale model with a simple but comprehensive mineral aerosol source scheme, along with a spectral sedimentation scheme. We present a simulation of a Saharan dust transport event (4 days), including mass uptake estimates, 3D transport and dry deposition. The model is initialized with ECMWF data. Meteosat imagery is used to check the dust cloud uptake and trajectory.

Computation of Ground Surface Conduction Heat Flux by Fourier Analysis of Surface Temperature

Abstract A method for computing the ground surface heat flux density is tested at two places in West Africa during the rainy season and during the dry season. This method is based upon the Fourier analysis of the experimental ground surface temperature. The only required parameter is the soil thermal inertia. The results of these calculations agree with the measurements. This method avoids the use of empirical formulas relating the ground heat flux density to other terms of the surface energy budget. It is shown that these relations are not universal.

An Investigation of the Dynamical and Physico-Chemical Features of Desert Aerosol over West Africa and Eastern Atlantic

The possible strong climatic influence of mineral aerosol (almost 1 gigaton of desert dust annually mobilized by wind erosion from the arid or semi-arid areas of Northern Africa is transported towards Mediterranean or Atlantic basins) encourages to develop mesoscale modeling description of all the aspects of the dust life cycle: wind uptake, transport, removal, direct or indirect radiative forcing. This poster deals with a mesoscale description of some of these questions. We have simulated a dust outbreak over Atlantic (at the beginning of May 1997) with the mesoscale model RAMS (Regional Atmospheric Modeling System). The main aspects addressed here are (i) dynamics: checking of dust trajectories using satellite imagery and (ii) an attempt to assess the radiative response using local surface measurements (at Sal Island, Cabo Verde).