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Synoptic scale dust emissions over the Sahara desert initiated by a convective cold
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pool in early August 2006
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Diana Bou Karam1, Earle Williams2, Matthew Janiga3, Cyrille Flamant1, Michael McGraw-
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Herdeg2, Juan Cuesta4, Antoine Auby1 and Chris Thorncroft 3
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Parsons Laboratory, Massachusetts Institute of Technology, Cambridge, MA, USA.
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LATMOS/IPSL, CNRS and Université Pierre et Marie Curie, Paris, France.
Department of Earth and Atmospheric Sciences, University at Albany, NY, USA.
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LISA/IPSL, CNRS and Université Paris 12, Créteil, France.
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Abstract
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This paper is concerned with a dust-raising cold pool over the Sahara desert that occurred on August
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3-5, 2006. Both the quantity of the uplifted dust and its spatio-temporal evolution are examined using
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satellite observations and a high-resolution numerical simulation. The dust emission during this event
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was initiated by a mesoscale cold pool emanating from mesoscale convective systems (MCSs) that
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developed over northern Niger and Mali on August 3. This event is one of several exceptional
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northward extensions of the West African monsoon during the 2006 wet season. We examine the
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propagation of the cold pool and associated dust lofting using high temporal resolution false color dust
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product images from the Meteosat Second Generation Spinning Enhanced Visible and Infrared Imager
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(MSG-SEVIRI). Observations from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation
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(CALIPSO) are used to characterize the vertical structure of the dust cloud as it spreads over the
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Sahara and across the Atlantic coast. The European Centre for Medium-Range Weather Forecasting
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African Monsoon Multidisciplinary Analysis (ECMWF-AMMA) special reanalysis was used to
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describe the synoptic conditions that accompanied this event. Furthermore, a numerical simulation
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using the mesoscale model MesoNH was performed to quantify the emissions and the westward
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transport of dust during this event.
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The dusty cold pool covered southern Algeria and a large part of northern Mali and Western Niger
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attaining a total area close to 500,000 km2. It extended over 2-3 km in altitude and had an aerosol
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optical depth on the order of 1.5 and a total dust load of about 2 Tg on average. Following daytime
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heating, the dusty cold pool and associated northward surge of moisture favored the development of
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additional precipitation and dust lofting. The dusty cold pool and monsoon surge were accompanied
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by a collapse of the Saharan heat low. About 0.4 Tg of dust produced during this event was subjected
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to westward transport over the Atlantic Ocean after being mixed upward by the diurnal heating over
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the Sahara to altitudes as high as 5 km.
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Comments: You mentioned that you preferred a methodology then results order to the abstract, that is
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fine. I suggest reordering the sentences in the second paragraph so that if follows a chronological
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order.
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.
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Key Words: West African Monsoon, Mesoscale convective systems, CALIPSO, SEVIRI, ECMWF,
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mineral dust, MesoNH.
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1. Introduction
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Satellite observations consistently indicate the most widespread and persistent aerosol plumes found
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on Earth are associated with dust [e.g. Prospero et al., 2002]. Dust emitted from arid and semiarid
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regions and transported in the atmosphere has been recognized to be an important component of the
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Earth’s climate system because of its influence on the Earth’s radiation budget [e.g., Haywood et al.,
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2003; Forster et al., 2007]. In many regions, they are the biggest contribution to atmospheric optical
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thickness [Tegen et al., 1997],. This is particularly true over North and West Africa, the world’s
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strongest summertime dust source. Here emission and transport have been shown to be highly variable
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in time and space although they are typically controlled by turbulent processes within the Saharan
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boundary layer small-scale high-wind events [e.g. Prospero et al., 2002; Engelstaedter and
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Washington, 2007]. There is now evidence that dust emission over the Sahara and Sahel is associated
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with different meteorological processes occurring on a variety of scales, from small scale features like
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dust devils through mesoscale phenomena including mesoscale convective features to synoptic scale
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forcing features (like midlatitude cold fronts). These forcing mechanisms for dust emission over
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Sahara and Sahel have been identified in recent work particularly in the framework of the African
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Monsoon Multidisciplinary Analyses program [Redelsperger et al., 2006].
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Comments: Above two paragraphs merged into one. I think its important to provide contrast between
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what typically occurs over the Sahara and the big events like our case study which are the topic of the
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next paragraph.
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Over the Sahara, dust storms have been documented to be associated with high near-surface wind
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speeds resulting from the downward mixing and dissipation of momentum from the nocturnal low-
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level jets (LLJs) during the morning build-up of the planetary boundary layer. Such LLJs are most
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active during the summer and are associated with the Saharan heat low (SHL). Dust outbreaks occur in
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response to collapses of the SHL [e.g. Knippertz, 2008] or are connected to LLJs generated in the lee
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of complex terrain as is the case in the Bodélé region of Northern Chad [e.g. Washington and Todd,
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2005; Todd et al., 2008]. Dust emission over the Sahara has also been shown to be connected to the
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low level dynamics associated with the penetration during spring of an upper-level trough to low
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latitudes [e.g. Jankowiak and Tanré, 1992]. Lastly, emission can occur in response tomesoscale
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density currents [e.g. Wakimoto, 1982; Simpson, 1997] caused by strong evaporational cooling of
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precipitation along cloud-bands on the Saharan side of the Atlas Mountain chain in southern Morocco
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[Knippertz et al., 2007].
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Over the Sahel, cold pools associated with the outflows of mesoscale convective systems (MCSs) have
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long been known to produce large outbreaks of dust called ‘haboobs’ particularly at the beginning of
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the monsoon season, before the growing vegetation begins to reduce local dust emission and are
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frequently observed during the afternoon and nighttime [Sutton, 1925, 1931; Williams et al., 2009;
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McGraw-Herdeg, 2010]. They represent an efficient mechanism for dust mobilization by virtue of
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their strong near-surface winds. This can be followed by mixing associated with daytime heating
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which lofts the dust to altitudes favourable for long-range transport in summertime [e.g. Sterk, 2002;
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Flamant et al., 2007; Bou Karam et al., 2008; Marsham et al., 2008]. Furthermore, Bou Karam et al.,
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[2008] and [2009] have identified another mechanism for dust emission over the Sahel during summer,
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in which highly turbulent winds at the leading edge of the monsoon nocturnal flow in the Inter
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Tropical Discontinuity (ITD) region generate dust uplifting and transport to high altitude.
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However, the relative importance of dust activity in Sahelian and Saharan sources for dust export from
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Africa during summertime is currently debated. Also, our understanding of the factors controlling the
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dust cycle and the interannual variability of the cross-Atlantic dust transport from sources in these two
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regions, activated by meteorological processes discussed above, is still limited. Thus, accurate
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simulation of dust generating mechanisms during summertime over Sahara and over Sahel is crucial
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for quantifying the total dust load observed annually and estimating the relative contribution of
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Sahelian and Saharan source region to the overall dust transport out of Africa.
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Despite the importance of MCS outflows in terms of dust emissions [Tetzlaff and Peters, 1986], only a
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few studies [e.g. Knippertz et al., 2009] have documented their behavior over the Sahara desert during
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summertime, where the surface is unprotected by vegetation and deflatable materials are often
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abundant. Hence, the dust activity linked to cold pools over the Sahara may have significant
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contribution to the total dust production over North and West Africa in summertime.
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Taking advantage of the recent availability of convection-permitting numerical models, we aim in this
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study to quantify for the first time the dust uplifted over the Sahara by a MCS cold pool of synoptic
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scale. The main objectives of this study are: 1) To examine, using satellite observations with high
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spatio-temporal resolution, dust emission and transport over and out of the Sahara in connection with
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the MCS outflows, 2) to evaluate the impact of such features on the atmospheric conditions over the
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Sahara particularly in terms of supply of moisture and temperature perturbation, 3) to evaluate the
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ability of a mesoscale atmospheric model (MesoNH) to represent the dust lifting by the cold pool over
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the Sahara and 4) to estimate the emissions and the westward transport of dust during this event
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The paper is organized as follows: Section 2 provides an overview of the data sources used in this
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study. In section 3, the meteorological conditions that accompanied this event are addressed. The
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spatio-temporal evolution of the dusty cold pool, including its impact on the atmospheric conditions
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over the Sahara Desert is treated in section 4. Section 5 describes the main characteristics of the dusty
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cold pool including the estimate of the dust load and transport. Finally, discussion and conclusions are
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treated in section 6.
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2. Data sources
2.1 Ground based measurements
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Hourly surface measurements are used to describe the meteorological conditions accompanying the
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advance of the dusty gust front to the north. Surface wind (10 m above ground level, agl), temperature
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(2 m agl) and visibility were obtained from meteorological station instruments deployed at
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Tamanrasset (Algeria; 22.80°N, 5.52°E) and In Salah (Algeria; 27.19°N, 2.47°E). The location of
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these stations is shown in Figure 1. These data are from the Integrated Surface Hourly (ISH) database
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(dataset 3505), provided by the National Climatic Data Center (NCDC).
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2.2 Space-borne observations
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The horizontal distribution of the dust associated with the cold pool is described using the Spinning
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Enhanced Visible and Infra-Red Imager (SEVIRI) images of the Meteosat Second Generation (MSG)
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computed from a combination of three infrared channels (8.7, 10.8, and 12 μm).. MSG-SEVIRI is in
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geostationary orbit at 0°W over the equator and provides images of Africa with 15-minute and 0.03°
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resolution. False-color images are created using an algorithm developed by the European organization
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for the Exploitation of Meteorological Satellites (EUMETSAT) which colors red the difference in
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brightness temperature (BTD) between the 12.0 and 10.8 μm channels, green the BTD between the
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10.8 and 8.7 μm channels and blue the 10.8 μm channel [e.g. Schepanski et al., 2007]. On these
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composite images, dust appears pink or magenta and clouds appear orange or brown. The effect of the
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dust on the BTD depends on its altitude [e.g. Pierangelo et al., 2004] suggesting that these composite
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images may favor the dust which is elevated so that its radiating temperature differs significantly from
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the ground.
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Additionally, aerosol optical depth (AOD) obtained from the Moderate Resolution Imaging
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Spectroradiometer (MODIS) /AQUA Deep Blue Collection 5.1 over desert surfaces (MYD08_D3
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product) with horizontal resolution of 1°x1° is used to characterize the optical depth of the dust in the
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cold pool. In addition, the 3B42 TRMM Satellite product [Huffman et al., 2007] is used to document
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the precipitating convection over the Sahara Desert where no other radars are available.
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Finally, information about the vertical distribution of dust during the event under scrutiny is provided
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from attenuated backscatter profiles at 532 nm retrieved from the space-borne Cloud-Aerosol LIdar
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with Orthogonal Polarization (CALIOP) on board the CALIPSO (Cloud-Aerosol Lidar and Infrared
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Pathfinder Satellite Observation; Winker et al., [2003]) satellite with vertical and horizontal resolution
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of 60 m and 12 km, respectively. It is worth noting that MODIS and CALIPSO are part of the A-Train.
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The A-train orbit crosses North Africa twice a day, once during the daytime (between 1230 and 1430
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LST) and once during the nighttime (between 0030 and 0230 LST), and has a revisit time period of 16
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days for the same orbit [Stephens et al., 2002].
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2.3 Model data
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2.3.1 Reanalyses
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The analyses used to provide the synoptic-scale context for this event came from the 0.5º European
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Centre for Medium-Range Weather Forecast (ECMWF) African Monsoon Multidisciplinary Analysis
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(AMMA) Reanalysis. This analysis was achieved by running the ECMWF data assimilation system
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using the radiosonde data collected and archived during AMMA [Augusti-Panareda et al., 2009].
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However, over the Sahara, the ECMWF AMMA Ranalyses are mainly dominated by model physics
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and assimilated satellite data due to the paucity of synoptic stations and radiosondes data in this region
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[e.g. Knippertz et al., 2009].
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Finally, toward understanding the fate of the dust raised by this event, the calculation of the back
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trajectories was performed using the FLEXPART model [Stohl et al., 2005] which is a Lagrangian
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particle dispersion model driven by the ECMWF AMMA Reanalyses that simulates the synoptic scale
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and mesoscale transport of tracers released from a defined point [e.g. De Villiers et al., 2010]. In the
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following, the term ‘ECMWF analysis’ denominates the ECMWF AMMA Reanalysis data.
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2.3.2 Numerical simulations
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Simulations have been carried out using MesoNH [Lafore et al., 1998], a non-hydrostatic mesoscale
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atmospheric model with an on-line dust emission and transport module. MesoNH includes
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parameterizations of various processes including cloud microphysics [Cohard and Pinty, 2000],
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turbulence [Bougeault and Lacarre, 1989], deep and shallow convection [Bechtold et al., 2001],
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chemical aerosol [Tulet et al., 2005] and dust aerosol [Grini et al., 2006].MesoNH is coupled to an
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externalised surface model which handles heat and water vapour fluxes between the low-level
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atmosphere and four types of surface: vegetation, towns, oceans and lakes [Masson et al., 2003].
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Natural land surfaces are described by interactions treated in the Soil Biosphere and Atmosphere
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model (ISBA) [Noilhan and Mahfouf, 1996].
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The dust emission scheme is the Dust Entrainment And Deposition (DEAD) model [Zender et al.,
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2003], implemented as a component of MesoNH [Grini et al., 2006], that calculates dust flux from
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wind friction velocity. DEAD includes entrainment thresholds for saltation, moisture inhibition and
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saltation feedback. The ORILAM model [Tulet et al., 2005] simulates transport and loss processes by
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following the evolution of two moments of three lognormal modes defined by Alfaro and Gomes
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[2001]. Dust advection and diffusion are treated using the transport processes of MesoNH including
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mixing within the planetary boundary layer, shallow convective transport and advection by winds.
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MesoNH uses the radiative scheme of the European Centre for Medium-range Weather Forecasts
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(ECMWF), which computes shortwave and longwave radiative fluxes. Shortwave radiative fluxes are
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computed for 6 wavelengths using the extinction coefficients, asymmetry factors and single scattering
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albedo provided by look-up tables for spherical aerosol particles.
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In this study a five-day simulation (3-7 August 2006) was performed. The study area covering Niger,
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Eastern Mali and southern Algeria forms a domain of 3000 km x 2500 km (centred on 20°N / 7°E, see
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Fig. 1) with a horizontal mesh size of 10 km. In the vertical, 72 levels were used with 35 of them
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within the planetary boundary layer. The lowermost level is at 10 m above the ground, while the
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highest level is at 28 km above the ground. Initial and lateral boundary conditions were taken from the
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ECMWF analyses. The ability of MesoNH to simulate dust emission and transport over West Africa
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has been highlighted in several recent studies [e.g., Grini et al., 2006; Chaboureau et al., 2007; Tulet
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et al., 2008; Crumeyrolle et al., 2008; Bou Karam et al., 2009a; Bou Karam et al., 2010].
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3. The synoptic situation
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Figure 2 shows Hovmoller plots (Latitude versus time) of Mean Sea Level Pressure (MSLP),
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geopotential at 850 hPa, meridional winds and specific humidity at 925hPa derived from the ECMWF
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AMMA reanalysis and averaged over 5 °W-5° E are used to address the synoptic situation between 2
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and 8 August 2006 (Figure 2). These fields have been smoothed with a 2 day low-pass Lanczos filter
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to remove the substantial variability associated with the diurnal cycle. The analysis of these Hovmoller
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plots suggest three distinct periods based on the large-scale flow and thermodynamic environment:
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pre-dust outbreak, rapid northward dust movement, and a more gradual northward dust movement
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associated with reduced dust emission.
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The first period, between 2 August at 0000 UTC and 4 August at 0000 UTC preceded the dust event.
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This period was characterized by a deep surface low (~1004 hPa) between 19 and 22° N located over
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southern Algeria and northeastern Mali (Fig. 2a). The intertropical front (ITF), the interface between
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two opposing surface winds: the cool moist south-westerly monsoon flow and the warm dry
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northeasterly Harmattan [e.g. Hastenrath, 1988; Bou Karam et al., 2008; Pospichal et al., 2009] was
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found around 20°N (Fig. 2c and 2d) with a core of intense southerlies between 15 and 20 °N (Fig. 2c).
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. The second period. between 4 August at 0000 UTC and 5 August 0000 UTC coincided with the
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rapid northward movement of the dusty cold pool. The surface low of 1004 hPa moved westward and
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northward to northern Mauritania and western Algeria (Fig. 2a). At low-levels (i.e. 925hPa), strong
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southerlies were seen between 14 and 24°N (Fig. 2c) and the ITF moved northward to 22°N (Fig. 2d).
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A dramatic northward surge of the monsoon air (up to 24°N) was observed during this period (Fig.
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2d). At this latitude, the total column water vapor (TCWV) increased from approximately 15 to 25 g
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kg-1. Decreases in the 925 hPa potential temperature of 2-3 K were observed during this period over a
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large region centered at 20°N.
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The period following 5 August at 0000 UTC was characterized by a more gradual northward
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movement of the dust front. The low-level southerlies persisted between 14°-24°N until 6 August at
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1200 UTC. The weakening southerly flow seen in the Hövmoller was associated with a continued
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westward movement and reduction in the strength of the SHL and southerlies. Continued but gradual
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increases in TCWV and reductions in 925 hPa temperature were observed over the Sahel. Following
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the cessation of southery flow on 6 August the ITF retreated back to ____ N.
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Comments: I have tried to better define this three period idea although I’m not sure which figure you
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are going to use. I think its very important to have each period have its own paragraph and a “name”
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I’ve termed them “pre-dust”, “rapid surge”, and “gradual surge followed by cessation”. Also, there
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needs to be specific mention of how we are denoting the ITF. Lastly, the conservation naturally moves
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in and out of the Hovmoller diagram as there is information that is not apparent on it. I think when
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specific latitudes are mentioned we need to be very clear as to whether these are in the Hovmoller (ie
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a longitudinal average) or if they are determined by a separate examination of maps.
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4. Spatio-temporal evolution of the dusty cold pool
4.1 The initiation phase: 3-4 August 2006
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On 3 August, an isolated cumulonimbus cloud developed southwest of the Aїr Mountains (i.e. 8°E,
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17°N) and expanded to a MCS to the southwest of the Aїr Massif. At 1500 UTC, a second MCS began
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to develop over northeastern Mali. These two MCSs continued to grow and at 2100 UTC they merged
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and formed a squall line nearly 1000 km across (Fig. 3a). On 3 August at 1800 UTC, the first visible
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cold pool in SEVIRI imagery emanated from the MCS over western Niger, a region recognized as an
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active dust source during the monsoon season by virtue of its location in the vicinity of mountain
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foothills where fluvial sediment from weathering provides fine material for deflation [e.g. Bou Karam
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et al., 2008; Schepanski et al., 2009; Bou Karam et al., 2009b]. The strong surface winds associated
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with the MCS density current and the turbulent winds at the leading edge of the monsoon flow (e.g.
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Bou Karam et al., 2008) produced strong dust emission and the cold pool was visible on the SEVIRI
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images as an arc of dust (purple colors, Figs. 3a).
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The MCS density current continued to mobilize dust as it moved northward and by 4 August at 0600
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UTC the cold pool formed an arc of dust, over 1000km across, encompassing northern Mali, southern
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Algeria and western Niger (Fig. 3b).
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4.2 The northward transport of dust: 4-5 August 2006
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The large dust cold pool continued its northward expansion over Algeria. On 4 August at 0900 UTC,
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the dust front arrived over Tamanrasset (Fig. 3c) and reached In Salah on 5 August at 0600 UTC (Fig.
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3d). At Tamanrasset, the arrival of the dust front was associated with a dramatic reduction in visibility
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(< 500 m for several hours, Fig. 4a) accompanied with a short-term increase in surface wind speed to
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7.5 m s-1. A small decrease in surface temperature (3°C) was observed in Tamanrasset at the time of
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the dust front arrival together with an increase of about 2 hPa in surface pressure and of about 4 g kg-1
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in water vapor mixing ration (Fig. 4a). Moreover, this event seemed to perturb the diurnal cycle in
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surface temperature at Tamanrasset where the temperature during the morning hours on 5 August (9-
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12 UTC) was about 5°C less than the previous days. This could be due to the development of
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precipitating clouds over the Hoggar Mountains as will be discussed in section 4.3, or also to the
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shielding of shortwave radiation by the heavy dust cloud. At In Salah, the arrival of the dust front over
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the station was evident in the change in wind direction, the drop in visibility (less than 2 km visibility
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for most of 5 August) and a 1 hPa increase in surface pressure (Fig. 4b). However, no significant
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decrease in surface temperature was discernible in the surface observations at In Salah (Fig. 4d).
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Figure 5a shows a Hovmoller of SEVIRI observations (latitude versus time) at 2° E and Figure 5b
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shows the full space-time evolution of the dust front observable in SEVIRI imagery. From these
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figures, we can discern and characterise three phases during the northward advance of the dust front: i)
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through the lifetime of the MCS (i.e. between 3 August at 2100 UTC and 4 August at 0900 UTC), the
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dusty cold pool propagated with speeds up to 18 m s-1, (ii) between 0900 UTC on 4 August and 0300
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UTC on 5 August the propagation speed of the dust front decreased to 9 m s-1 on average, and (iii)
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between 0300 UTC and 1500 UTC on 5 August where a final northward surge of the dust front with a
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propagation speed of 5 m s-1 was observed (Fig. 5a and 5b).
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The rapid northward surge of the dusty cold pool into the Sahara desert during the first phase was due
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to the strong density contrast caused primarily by the difference in temperature between the cold air of
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the convective downdraft and the hot Saharan air [e.g. Wakimoto, 1982; Simpson, 1997; McGraw-
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Herdeg, 2010].
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During the second phase, the active convection had stopped and the cold pool was no longer being
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replenished. Also, in the daytime, the turbulent mixing in the boundary layer tends to mix the cold
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layer with the environmental air thereby decreasing the density contrast across the density current-
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Saharan air interface and hence slowing the propagation of the dust front (Fig. 5b). During this phase,
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synoptic conditions played the major role in the advance of the dust cloud over the Sahara where the
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northern progression of the dust cloud was driven by a strong latitudinal pressure gradient and the
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associated pronounced southerly winds (i.e. Fig 2a and 2c). This analysis is supported by the absence
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of a distinct temperature drop (that usually characterizes the passage of density currents [e.g. Simpson,
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1997]) at In Salah when the dust cloud passed over.
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Finally, the last northward surge of the dust front was dominated by the emanation of outflows from
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new convection that developed over the original dust cloud over central Algeria (i.e. over In Salah Fig.
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5a and 6b) A diminishing role was likely played by the large-scale southerlies associated with the
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weakening and westward moving SHL duing this phase.. Similar behavior of the dust front was
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highlighted in the model results as well. A Hovmoller at 2° E of the vertically integrated dust
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concentration simulated by MesoNH (Fig. 5c) shows a dust front loaded by at least 1.2 g m-2 and
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advancing northward from 18°N on 3 August to 26 °N on 6 August 2006.
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4.3 The impact of the cold pool on the atmospheric conditions over the Sahara
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The intrusion of the cold pool into the Sahara desert impacted its environment mainly by the supply of
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moisture which helped on the development of new convection, resulting in weak precipitation.
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The transport of moisture into the Sahara was evident in the water vapor mixing ration (WVMR) fields
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simulated by MesoNH (Fig. 5c right panel). The area covered by the dust front seen in pink in Fig. 5a
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and in red in Fig. 5c (left panel) was associated with WVMR of about 12 g kg-1 at 925hPa and
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reaching 20 g kg-1 in some hours (Fig. 2d and Fig. 5c right panel) which is more than three times the
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WVMR over the desert (3 to 6 g kg -1, Fig. 5c right panel). A sharp gradient in WVMR was seen
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across the dust front-Saharan air interface with 10 g kg-1 difference over less than 1° in latitude (Fig.
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5c right panel). Moreover, in Tamanrasset an increase in WVMR from 8 to 12 g kg-1 was observed on
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4 August after midnight with the arrival of the dust front. The WVMR remained high (13 g kg-1) till
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the morning of the 6th of August (Fig. 4a). Also, with the advance of the cold pool into the Sahara,
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WVMR at In Salah increased by 6 g kg-1 on the morning of the 5th of August and remained high (15 g
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kg-1) till the evening of 6 August (Fig. 4b).
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Under these unusual conditions over the Sahara desert, new convective clouds developed at several
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locations over the Sahara (Fig. 6a and 6b). We also emphasize the faithful representation by MesoNH
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of the dust features seen in the SEVIRI observations as well as the occurrence of new convection over
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the Sahara (Fig. 6c and 6d). On 4 August at 1800 UTC (Fig. 6a and 6c), new convection developed
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over southern Algeria in response to the supply of moisture and favored by the orographic lifting
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associated with the Hoggar Mountains [e.g. Cuesta et al., 2010]. Based on the SEVIRI images, the
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dominant type of cloud associated with the convection over the Hoggar was thick mid-level clouds
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(Fig. 6a). From the model results, these clouds were around 6 km in altitude (Fig. 6c) and they resulted
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in localized precipitation over southern Algeria of about 2.5 mm per hour as reported in the TRMM
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observations (Fig. 6e). On 5 August at 1800 UTC, convection developed over central Algeria around 6
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km in altitude (Fig. 6b and 6d). This convection resulted on shallower precipitation than the one over
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the Hoggar and precipitation rates derived from TRMM did not exceed 2 mm per hour over central
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Algeria (Fig. 5f).
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I think mention needs to be made that the convection did not develop until ~12 h after the passage of
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the dusty gust front. As Dave Schultz mentioned in his review one would not expect convection to
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develop over the cold pool as they are very stable. The soundings showed that the initial arrival of the
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cold pool (0600 UTC) resulted in a shallow layer of cool moist air (~50 hPa) and an inversion over
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Tamanrasset. However, throughout the day this inversion was mixed out providing a more favorable
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environment for the secondary episode of convection.
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Furthermore, the dusty cold pool appears to have had a significant impact on the Saharan Heat Low
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(SHL). An increase of 5 hPa in the mean sea level pressure was seen after the arrival of the dusty cold
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pool over the desert and a collapsing of the SHL from August 5 to 6 was observed after the dust front
326
reached the central Sahara (i.e. Fig 2a and 2b). This could be the result of the advection of cold air
327
within the southerlies into the SHL or a result of the dust and clouds radiative forcing by reflecting the
328
incoming shortwave radiation and thus decreasing the heating at the surface. Also, the new deep
329
convection that developed over the desert could have impacted the energy budget and played a role in
330
the SHL collapsing.
331
18
332
333
5. The characteristics of the dusty cold pool
5.1 Dust loads in the cold pool
334
On 5 August at 1200 UTC, the large arc-shaped plume of dust generated by the MCS cold pool lay
335
over southern Algeria, northern Mali and northwestern Niger (Fig. 7a). The MODIS Deep Blue
336
observations of AOD on 5 August at 1200 UTC showed the same horizontal extent of the dust cloud to
337
be associated with AODs between 0.5 and 1.5 (Fig. 7b). The largest AODs were observed over central
338
Algeria (Fig. 7b). Figure 7c shows the dust AOD at 550 nm simulated by MesoNH on 5 august at 1200
339
UTC. The model reproduced quite well the horizontal extent of the dust cloud as well as the AODs
340
associated with and observed in the MODIS Deep Blue data (Fig. 7c). In order to estimate the dust
341
load generated by the cold pool, we averaged the simulated dust mass over the area covered by the
342
dust cold pool and having a dust load of 0.5 g m-2 and more. An evolution between 3 and 7 August of
343
the dust load over the area affected by the cold pool is shown in Figure 7d. The total dust load was
344
simulated to range between 1.8 Tg and 2.8 Tg on 3 and 5 August with an average of 1.7 Tg for the rest
345
of the period (Fig. 7d).
346
Bou Karam et al., [2009b] have estimated the average dust load in the ITD region over a domain
347
encompassing 11°W-17°E, 11°-28°N to be on the order of 2.5 Tg and those associated with the
348
monsoon leading edge (only to the south of the ITD) to range between 0.5 and 0.8 Tg. This indicates
349
that the dust load produced during this event is significant compared to the dust emitted in the ITD
350
region.
351
19
352
5.2 Vertical distribution of dust in the cold pool
353
The vertical distribution of dust in the cold pool was analyzed using the CALIPSO observations during
354
two nighttime orbits of the spaceborne lidar on 4 and 5 August. Seven hours after the initiation of dust
355
emissions over western Niger by the MCS density current, CALIPSO passed over the arc of dust on 4
356
August at 0100 UTC (Fig. 8a). A sharp dust front was seen in the CALIPSO observations shown in
357
Fig. 8b (see the red marker). A dense dusty layer associated with lidar reflectivity at 532 nm in excess
358
of 7 x10-3 km-1 Sr-1 was observed between the ground and 2 km in altitude. The height of the dusty
359
layer decreased with latitude forming a sharp front at 21°N. Note that the shape of the dust front
360
observed here by CALIPSO is similar to the one shown by Williams et al., [2009] derived from a
361
vertically-pointing W-band radar data and by Flamant et al., [2007] and Bou Karam et al., [2008]
362
using an airborne lidar. The dust front was associated with potential temperature on the order of 312 K
363
with colder atmosphere behind (i.e. 308 K at 18°N) and was advancing into the desert characterized by
364
320 K in potential temperature over 5 km in altitude (Fig. 8b).
365
The following day, CALIPSO crossed over the part of the dust cloud located over northern Mali (Fig.
366
9a). On 5 August, the dust extended to higher altitude reaching 3 km (Fig. 9b). It was associated with
367
lidar reflectivity at 532 nm of about 4 x10-3 km-1 sr-1 and potential temperature on the order of 312 K.
368
Mid-level clouds between 6 and 9 km in altitude were seen over central Mali to the south of the dust
369
layer (Fig. 9a and 9b).
370
20
371
5.3 The westward transport of the dust out of the continent
372
The role of this large scale dust-lofting event on the export of dust out of Africa has also been
373
examined in this study based on back-trajectory analyses (using the FLEXPART model) and
374
CALIPSO observations on 6 and 7 August 2006 near the West African coast, to the west of the area of
375
dust emission produced by the cold pool on 4 August 2006. Also, an attempt to estimate the dust mass
376
transported westward resulting from this event has been made based on the model results.
377
Although the development of new convection over the Hoggar may have scavenged a large amount of
378
the airborne dust, a westward transport over the Atlantic Ocean of the remaining dust was evident in
379
the CALIPSO observations shown in figures 10a and 10c. On 6 August 2006 at 0237 UTC, CALIPSO
380
observations showed a layer of dust over 18-27°N between 3 and 5 km in altitude (Fig. 10a). The
381
backtrajectory analysis starting on 6 August from the location and the height of this layer showed that
382
the dust originated two days before over the region bounded by 2°W-2°E and 19-22°N (Fig. 10b). This
383
corresponds to the dust produced by the cold pool over Algeria on 4 August as now discussed.
384
The transport of dust to the west was not favorable before the evening on 4 August. This is due to the
385
fact that the dust front was confined between the surface and 2 km in altitude as was evidenced by the
386
CALIPSO observations shown in Fig. 8b. The wind at these altitudes (i.e. below 2 km) is generally
387
northerly or southerly and so unfavorable for westward advection. In the daytime, the dust was
388
subject to vertical transport throughout the depth of the Saharan boundary layer and the height of the
389
dust layers was set by the daytime thickness of the Saharan boundary layer. Over Tamanrasset, the
390
height of the Saharan boundary layer was observed from soundings to reach roughly 4 km at 1200
391
UTC and 5 km at 1800 UTC (See Fig. 7a in Cuesta et al, 2008). At these altitudes (i.e. 3-5 km),
392
consistent easterly flow is present in the vicinity of the African Easterly Jet (AEJ) and westward
21
393
transport by these winds was responsible for carrying the dust from above the area of the cold pool to
394
the location observed by CALIPSO near the coast.
395
http://weather.uwyo.edu/cgi-
396
bin/sounding?region=africa&TYPE=GIF%3ASKEWT&YEAR=2006&MONTH=08&FROM=0400&
397
TO=0600&STNM=60680
398
399
On 7 August 2006, CALIPSO observations show a much more elevated dust layer (4-6 km) over 15-
400
25°N to be associated with much higher values of backscatter coefficient (> 3 x10-3 km-1 sr-1) than in
401
the previous day (Fig. 10c). The back-trajectory analysis initiated on 7 August at the location and the
22
402
height of this layer of dust showed that the dust was carried westward on 4 August 2006 from above
403
the area 0-5° E and 20-25° N (i.e. the location of the dust cloud caused by the cold pool) at 5-6 km in
404
altitude (i.e. Fig. 10d). This is also consistent with the fact that for a typical 8 m s-1 wind speed at 700
405
hPa, the transport of dust from its origin over the continent to the ocean (i.e. over ~2000 km) requires
406
~ 3 days, which corresponds to the time interval from the initial raising of dust on 4 August to the time
407
of the observed dusty layer on CALIPSO near the Atlantic coast on 7 August.
408
In order to estimate the westward transport of dust resulting from this event, a vertical cross section at
409
longitude 4°W of dust concentration simulated by MesoNH was made on 6 August at 1200 UTC i.e.
410
after the emission processus has ended. The longitude 4°W was chosen because it is far enough from
411
the original area of dust lofting and represents a reasonable location for the dust to exit the continent
412
(i.e. Fig 11a). The model-simulated dust concentrations range between 300 and 400 µg.m-3 over 2.5 to
413
4.5 km in altitude near 22 °N (Fig. 11b). Along the same vertical cross section, the AEJ stands out
414
clearly between 2.5 and 6 km in altitude with a core of high wind (15 m s-1 )between 3 and 6 km in
415
altitude and 12-22°N (Fig. 11c). From Figures 11c and 11b, we can estimate the mass flux of dust
416
transported westward by the AEJ. To do so, we derive the mass flux of dust from the product of the
417
mean wind speed (15 m s-1) and the mean dust concentration (350 µg m-3) over the atmospheric
418
column 2-5 km and latitude range 20° N–22° N. Therefore, the result represents the net westward
419
export of dust mass from this area toward the Atlantic. The westward dust flux leaving the region at 4
420
W is 0.2 Tg per day. If we consider that the dust transport, due only to this event, occurred over 2 days
421
(from 5 to 6 August); then the contribution of this event to the total dust export from the Sahara toward
422
the Atlantic is about 0.4 Tg.
23
423
Todd et al., [2008], using the same approach, have estimated the dust emissions downstream of the
424
Bodélé Depression to be between 0.9 and 2.9 Tg per day. This indicates that the dust transported
425
westward during a single event of the like is significant comparing to the dust emitted daily from the
426
Bodélé Depression for example.
427
24
428
6. Discussion and conclusions
429
The evolution and the characteristics of an intense dust event that occurred over the Sahara desert on
430
August 3-5, 2006 have been documented using a combination of satellite observations and mesoscale
431
numerical modelling.The mesoscale model MesoNH was able to reproduce quite well the spatio-
432
temporal distribution of dust and clouds associated with this event. The dusty cold pool originated
433
from a large MCS that occurred over West Africa on 3 August 2006 propagated northward greater
434
than 1000 km over 3 days with an initial speed of 18 m s-1. The cold pool that expanded initially as a
435
density current raised a large quantity of dust (~ 2 Tg) and advected abundant moisture (with water
436
vapor mixing ratio 12 g kg -1) into the Saharan desert. The arc-shaped plume of dust extended over
437
2000 km in length and was associated with AOD values on the order of 1.5. Mass flux of dust in the
438
order of 0.2 Tg per day was transported out of the Sahara toward the Atlantic Ocean by the AEJ after
439
being mixed within the Saharan boundary layer up to altitudes of 3-5 km. The general synoptic scale
440
wind conditions and position of the AEJ in the period of the observations are rather common
441
conditions over West Africa. As a consequence, any dust raised to the level of the AEJ by deep dry
442
boundary layer convection will be advected westward over the Atlantic.
443
On August 4-5 moisture advection associated with the cold pool together with topographic lifting
444
associated with the Hoggar Mountains were responsible for the development of new convection over
445
the Sahara. The new convection resulted in weak precipitation over the Sahara (rainfall rate of 3 mm
446
hr-1 or less). This could be due to the fact that in arid regions most of the precipitation evaporates
447
before reaching the ground [e.g. Srivastava, 1985; Knippertz et al., 2009]. This study has
448
demonstrated for the continent of Africa that cold pool dynamics can link a single thunderstorm to
449
vigorous dust storms thousands of kilometers away. A similar expansion of a cold pool in which new
25
450
convection feeds cold air into the cold pool as it propagates, have been documented in North America
451
[e.g., Carbone et al., 2002; Tuttle et al., 2004].
452
The present study also suggests that the occurrence of MCSs at high latitudes (i.e. > 18°N) during the
453
monsoon season has an important contribution in terms of dust emissions and transport towards the
454
Americas. Furthermore, such events may contribute importantly to the high dust activity observed
455
annually over West and North Africa in summertime. In 2006 for example, at least four events of the
456
like occurred over the Sahara, some of which have been explored in other studies [Cuesta et al., 2010;
457
Knippertz and Todd, 2010]. All these events are characterized by extraordinary latitudinal extensions
458
of the West African monsoon known as the monsoon surges. The latitudinal extents of these dust pools
459
(900 km for Cuesta et al. [2010]; 2000 km in this study and in Knippertz and Todd [2010]) exceed by
460
a substantial margin the mean value of 450 km for 30 MCS-related gust front events in the same
461
season that occurred over the Sahel [McGraw-Herdeg, 2010]. Climatologically speaking, the monsoon
462
flow is bounded by the Inter-Tropical Front (ITF) to the north, and the southerly flow stops well short
463
of the Saharan heat low. But in these exceptional events, the expanding dust pool pushes the ITD
464
northward strongly deforming it (Figure 2c and 2d), and brings sufficient moisture into the dry Sahara
465
Desert for weak precipitation to form there (Figure 6), both in this study and in Cuesta et al. [2010] for
466
another monsoon surge event in July 2006.
467
A second exceptional aspect of these monsoon surge events, in their exceptional latitudinal reach, they
468
often cause the collapse of the heat low to the north [Cuesta et al., 2010, and this study]. Though the
469
physical mechanism of this collapse has not been fully resolved in this study, observations here and
470
elsewhere provide additional constraints on suggested mechanisms. Lavaysse et al. [2010] discuss the
471
role of temperature decrease in contributing to heat low collapse, but it is important to note that
26
472
temperature drops in surface air at northernmost desert stations were difficult to discern (against 10-
473
15C diurnal variations) in both the present study and in Cuesta et al. [2010]. In this study, when the
474
dusty air pushed northward into the desert, a temperature drop is discernible at Tamanrasset (Figure
475
4a), but not further north at In Salah (Figure 4b). Likewise, Cuesta et al. [2010] identify no
476
temperature drop in dust arrival at Tamanrasset in their surge event, but only when rainfall events
477
occur. The absence of temperature drops points to the exhaustion of the density current and casts doubt
478
on the use of “cold pool” to characterize the event at this advanced stage of its evolution. The
479
continued northward progression of the dusty air, clearly evident in Figure 5, is pressure-gradient
480
driven (consistent with the findings in Knippertz and Todd [2010] for this event), and this conclusion
481
is supported in the present study by the model simulations.
482
Cuesta et al. [2010] present evidence that shading by cloud (and consequent temperature reduction) is
483
causing the heat low collapse in their event. The evidence presented here for the existence of
484
cloudiness in the SEVERI imagery (orange color in Figure 7) is consistent with that idea. However,
485
the area of lofted dust is substantially greater than water cloud at the higher latitude, and the abrupt
486
drops in surface visibility (Figures 4a, 4b) at all stations involved provide strong evidence that surface
487
shading by dust will occur and thereby suppress surface heating and sensible heat flux at high latitudes
488
where the heat low is found initially (Figure 2a). Future observations within the collapsing heat low
489
would shed important new light on this issue. Also, the radiative impact of the dust uplifted during
490
such events and its feedback on the atmospheric circulation and particularly on the Saharan Heat Low
491
dynamics are envisaged for future work via numerical simulations, necessary to sort out the radiative
492
effects of water cloud and dust. Finally, the evaluation of the frequency of the occurrence of these
493
events during a complete monsoon season and over many years is recommended for future studies.
27
494
Acknowledgement
495
"Based on a French initiative, AMMA was built by an international scientific group and is currently
496
funded by a large number of agencies, especially from France, UK, US and Africa. It has been the
497
beneficiary of a major financial contribution from the European Community's Sixth Framework
498
Research Programme. Detailed information on scientific coordination and funding is available on the
499
AMMA International web site http://www.amma-international.org".
500
MODIS observations used in this study were produced with the Giovanni online data system,
501
developed and maintained by the NASA Goddard Earth Sciences (GES) Data and Information
502
Services Center (DISC). The authors wish to thank the SATMOS, ICARE and Climserv for providing
503
SEVIRI and CALIPSO data. Some of the data used in this study were acquired as part of the Tropical
504
Rainfall Measuring Mission (TRMM). The data were processed by the TRMM Science Data and
505
Information System (TSDIS) and the TRMM Office; they are archived and distributed by the Goddard
506
Distributed Active Archive Center. D. Bou Karam’s work on this study was supported by the ‘Centre
507
National des Etudes Spatiales’ (CNES), and the authors wish to thank particularly Didier Renaut and
508
Carole Deniel. Earle Williams, Chris Thorncroft, Matthew Janiga and Michael McGraw-Herdeg
509
received partial support for this study from grant ATM0734806 from Atmospheric Dynamics (Jay
510
Fein) of the US National Science Foundation. The field work on gust fronts in Niger with the MIT
511
radar was supported by NASA Hydrology (Jared Entin).
512
513
28
514
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