Dust Source, Vertical Profile and Climate Impact by RegCM3 Regional Climate Model over West Africa during 2006

This study aims to evaluate dust impact on climate parameters over the Sahel region by RegCM3 regional model during 2006. Indeed, aerosols are one of the main uncertainties in climate models. The aerosol optical depth (AOD) derived from RegCM3 model has been validated with various observed da-tasets. The aerosol sources are identified over North Algeria and East of Sahel (Bodele depression). Discrepancies are noted when considering dust temporal and spatial distribution. Dust season extends between March and October, with two peaks of AOD recorded in March (spring) and June (summer). The dust vertical distribution showed that the mineral aerosol layer is located between 850 hPa and 300 hPa (1.5 km to 7 km). The RegCM3 model simulates fairly well the transport in the upper layers, especially in the Saharan Air Layer (SAL) during the summer. However, RegCM3 simulates poorly the transport and sedimentation of particles in the lower layers (below 2 km). The investi-gation of dust radiative impact shows a general cooling. The maximum of radiative forcing is located around 18˚N - 20˚N, with values of about −80 W/m 2 in June - August (JJA) and −40 W/m 2 at the surface during March - May (MAM). This study also showed the indirect effect of dust with a decrease in precipitation about −0.7 mm/day around 15 - 20˚N during the rainy season.

emissions when considering tropospheric aerosols [1] [2] [3] [4] [5] and particles from biomass burning. The West African region is characterized by dust outbreaks throughout the year [6] and biomass burning in winter [7]. The African continent is not well equipped in terms of measurement tools for assessing the climate and its evolution. However, it is the continent where climate change impacts are felt the most (drought in 1970). Indeed, global models do not highlight regional specificities such as Sahara dust. Therefore, to better understand and study the variations of climate over Africa, the use of regional climate models is of great importance. These types of models have the advantage of using fine resolutions and taking into account local meteorological processes.
In West Africa, the years of continuous rain deficits are characterized by abnormalities in the dynamics of West African monsoon system, and a continuous increase in dust emissions. Several measurement campaigns (ABAT, AMMA) were conducted to study the impact of dust on regional climate [8]. In this study, we use the regional climate model RegCM3 to assess the impact of desert aerosols on climate over the Sahel in 2006. The model outputs have been validated with ground instruments and satellite observations. This validation study is followed by an estimation of dust impact on Sahel climate.

The Regional Climate Model RegCM3
Unlike the regional climate models, many global models were developed to simulate the summer cycle of desert dust [9] [10] [11]. Few dust studies using the regional climate models are available in the literature [12]. RegCM3 is a regional hydrostatic model developed at Abdu Salam Centre for Theoretical Physics (ICTP) [13]. Zakey et al. (2006) [14] have developed and tested in 2004 a dust module included in the ICTP Regional Climate RegCM3 [13] [15]. This dust module is widely described in [14] [16]. In this study, two sets of runs have been done: a control run (no dust effects, AOD = 0) and a dust run which takes into account dust radiative effects. The resolution of the model is 60 km.

Validation Data
AERONET network data are the main validation products used in this study [7].

Annual Seasonal and Daily Pattern of Dust
We begin by showing in Figure 2, the annual average of aerosol optical depth (AOD) obtained with models (RegCM3 and GOCART) and by two radiometers on orbit (MODIS and MISR) in 2006. This annual AOD is used to identify the aerosol sources. Two source areas have been identified: one is the border between Mauritania (East) and North-Western Mali and the other is from Niger (North-East) to the Western of Chad near Bodele depression. These maximum are located between 15 and 20˚N latitude and longitude about 8˚W according to RegCM3; then GOCART and MISR locate these sources in the band 15˚N -18˚N and 15˚E. Besides MODIS, all other remote sensing sensors indicate clearly these source areas. And finally, we note that the models tend to overestimate the optical thickness (observations) especially for RegCM3. Next we examine, the dust seasonal cycle in West Africa. Figure 3 shows the seasonal (March-April-May and June-July-August) cycle of AOD in West Africa using RegCM3 model and MISR measurements. Sources of aerosols (dust maximum) are mainly located at the border of Mauritania/Mali, southern Algeria and near to the Bodele depression (Chad). The seasonal distribution shows the predominance of dust during the wet season compared to dry season. Table 1 summarizes the mean annual AOD (550 nm) to MODIS, AERONET and RegCM3

Dust Vertical Distribution
In the following, we study the vertical distribution of dust with the model.  in the lower layers (1 -2 km) and the model has difficulties to reproduce this lower-layer transport. During the wet season (June), RegCM3 simulates fairly well the aerosol layer located in the Saharan Air Layer (3 -5 km). In January (winter), the Lidar detected two aerosol layers above Dakar. The first is located around 1 km while the second around 2 km. However, the model simulated only those located in the low layer at 1 km. Recent studies have shown that this layer is composed of biomass burning aerosols [18] [22]. Consequently, February 2006 is characterized by an anomaly in AOD that could be due to a lack of transport in the lower layers [18]. However, RegCM3 does not show this anomaly of air masses transport and simulate a layer between 1.5 and 3 km with a maximum suppression of 0.08 km 1 . For spring, the lidar has only worked for two months (March and April). In March, the model simulates a dust layer between 0.5 and 3.5 km with an extinction maximum of 0.14 km 1 . Because of a transport to a lower level, the Lidar is between 1.5 and 2.5 km with a maximum extinction of about 0.22 km 1 . However, RegCM3 simulates the data from April quite well. The dust layer is located, for the model and measurement, between 1 and 4 km. We note that the maximum of aerosol extinction observed (0.16 km 1 ) is twice as important as the RegCM3 simulation (0.08 km 1 ). In April the model simulates quite well the level of aerosol transport while underestimating the total AOD.
June was the month with the most important dust loading in Mbour in 2006, with a maximum extinction of 0.22 km 1 located at 3 km in the SAL (Saharan Air Layer). The Angstrom coefficient measured during month shows clearly a dust layer located between 2 and 5 km [21]. RegCM3 simulates well the shape of dust extinction profile in June. However, it detects dust concentrations below those of Lidar. The model estimates the maximum of extinction at 0.012 km 1 . From July to August, the model simulates very well the dust layer between 2 and 5 km. The extinction maximums are detected for both products (RegCM3 and Lidar) with a value of 0.12 km 1 in July and 0.10 km 1 in August located around 3.5 km. However, aerosols in the lower layer are not detected by the model. The same analysis is reproduced in autumn than in summer. Finally, the analysis cannot be made in the month of December because of unavailable data.

Dust Climate Impact by RegCM3 Regional Climate Model
In recent decades, several efforts were made to determine the radiative impact of dust on climate in West Africa [24]- [33]. Figure 7 shows the radiative impact of dust (shortwave) at the surface during  [34]. Ultimately, we examine the impact of dust on meteorological parameters such as precipitation and surface temperature. Figure 8 illustrates the impact of dust on surface temperature (left) and rainfall (right) over 17˚W and 20˚N during the wet season (JJA) of the latitudes (Lon = 17˚W -10˚E). The cooling causes a decrease in surface temperature over Sahel region. The maximum cooling is located near 21˚N with an intensity about to −1.7˚K. The increase of dust (can serve as cloud condensation nuclei) favored the indirect effect. Rainfall has decreased by 0.6 mm/day between 5˚N and 15˚N during JJA. These results are consistent with recent work indicating that the dust causes a reduction in temperature [35] [36] [37] by their direct effect and a drought (rain reduction) of their indirect effect [38] [39] [40].