Stratospheric impacts on dust transport and air pollution in West Africa and the Eastern Mediterranean

Saharan dust intrusions strongly impact Atlantic and Mediterranean coastal regions. Today, most operational dust forecasts extend only 2–5 days. Here we show that on timescales of weeks to months, North African dust emission and transport are impacted by sudden stratospheric warmings (SSWs), which establish a negative North Atlantic Oscillation-like surface signal. Chemical transport models show a large-scale dipolar dust response to SSWs, with the burden in the Eastern Mediterranean enhanced up to 30% and a corresponding reduction in West Africa. Observations of inhalable particulate (PM(10)) concentrations and aerosol optical depth confirm this dipole. On average, a single SSW causes 680–2460 additional premature deaths in the Eastern Mediterranean and prevents 1180–2040 premature deaths in West Africa from exposure to dust-source fine particulate (PM(2.5)). Currently, SSWs are predictable 1–2 weeks in advance. Altogether, the stratosphere represents an important source of subseasonal predictability for air quality over West Africa and the Eastern Mediterranean

Retrieving the global distribution of the threshold of wind erosion from satellite data and implementing it into the Geophysical Fluid Dynamics Laboratory land–atmosphere model (GFDL AM4.0/LM4.0)

Dust emission is initiated when surface wind velocities exceed the threshold of wind erosion. Many dust models used constant threshold values globally. Here we use satellite products to characterize the frequency of dust events and land surface properties. By matching this frequency derived from Moderate Resolution Imaging Spectroradiometer (MODIS) Deep Blue aerosol products with surface winds, we are able to retrieve a climatological monthly global distribution of the wind erosion threshold (Vthreshold) over dry and sparsely vegetated surfaces. This monthly two-dimensional threshold velocity is then implemented into the Geophysical Fluid Dynamics Laboratory coupled land–atmosphere model (AM4.0/LM4.0). It is found that the climatology of dust optical depth (DOD) and total aerosol optical depth, surface PM10 dust concentrations, and the seasonal cycle of DOD are better captured over the “dust belt” (i.e., northern Africa and the Middle East) by simulations with the new wind erosion threshold than those using the default globally constant threshold. The most significant improvement is the frequency distribution of dust events, which is generally ignored in model evaluation. By using monthly rather than annual mean Vthreshold, all comparisons with observations are further improved. The monthly global threshold of wind erosion can be retrieved under different spatial resolutions to match the resolution of dust models and thus can help improve the simulations of dust climatology and seasonal cycles as well as dust forecasting

A tribute to Michael R. Raupach for contributions to aeolian fluid dynamics

Since the pioneering work of Bagnold in the 1940s, aeolian research has grown to become an integral part of earth-system science. Many individuals have contributed to this development, and Dr. Michael R. Raupach (1950–2015) has played a pivotal role. Raupach worked intensively on wind erosion problems for about a decade (1985–1995), during which time he applied his deep knowledge of turbulence to aeolian research problems and made profound contributions with far-reaching impact. The beauty of Raupach’s work lies in his clear conceptual thinking and his ability to reduce complex problems to their bare essentials. The results of his work are fundamentally important and have many practical applications. In this review we reflect on Raupach’s contribution to a number of important aspects of aeolian research, summarise developments since his inspirational work and place Raupach’s efforts in the context of aeolian science. We also demonstrate how Raupach’s work provided a foundation for new developments in aeolian research. In this tribute, we concentrate on five areas of research: (1) drag partition theory; (2) saltation roughness length; (3) saltation bombardment; (4) threshold friction velocity and (5) the carbon cycl

A clarion call for aeolian research to engage with global land degradation and climate change

This editorial represents a clarion call for the aeolian research community to provide increased scientific input to the Intergovernmental Panel on Climate Change (IPCC) and the United Nations Convention to Combat Desertification (UNCCD) and an invitation to apply for ISAR funding to organize a working group to support this engagement

The AMMA mulid network for aerosol characterization in West Africa

Three ground based portable low power consumption microlidars (MULID) have been built and deployed at three remote sites in Banizoumbou (Niger), Cinzana (Mali) and M'Bour (Senegal) in the framework of the African Monsoon Multidisciplinary Analyses (AMMA) project for the characterization of aerosols optical properties. A description of the instrument and a discussion of the data inversion method, including a careful analysis of measurement uncertainties (systematic and statistical errors) are presented. Some case studies of typical lidar profiles observed over the Banizoumbou site during 2006 are shown and discussed with respect to the AERONET 7-day back-trajectories and the biomass burning emissions from the Combustion Emission database for the AMMA campaign

Two-year simulations of seasonal and interannual changes in Saharan dust emissions

International audienceUsing a soil‐derived dust emission scheme, the annual and monthly dust emissions in the western “dry” Sahara have been estimated for 1991 and 1992, respectively to 665 Mt and 586 Mt. Based on these simulations, an investigation of the seasonal and annual changes in dust emissions has been performed. The monthly dust emissions vary from 163 Mt in March 1991 to 8 Mt in December 1992. These seasonal variations are related to changes in the location and area of the source‐regions and in the frequency of dust events. On the opposite, the interannual variations of the dust emissions are due to changes in the occurrence of the most intense dust events

Comparison of Emission Models Used for Large Scale Simulation of the Mineral Dust Cycle

International audienceThree different emission models of minerai dust of increasing complexity have been compared to satel· lite observations of minerai dust Qvec the Western Sahara. in arder to evaJuate their capability to simulate the frequency of the dust events. The first model is a !inear function of the wind velocity. ft provides a very pocr agreement with the observations (8%). The second one is a function of the wind velocity raised to the third power with a unique threshold for the dust emission, weighted by a soil texture parameter. With regards to the limited number of input parameters required by such a model, the agreement with the observations is quite good (61 %) but the location and intensity of the dust sources are not accurately simulated. The third model is based on a physical description of the main processes of dust emission. Il alsa utilizes mapped inputs of parameters characterizing the surface features in the tested area. The agreement with the observations is very good (79%) and the model provides a fine description of the dust sources location and intensity. From camparison with observations, it is stated that a simple wind power model is not appropriate to simulate the minerai dust emissions. A single threshold source model provides simulation of the present dust emissions with a reasonable confidence level. Mareover, the input parameters required by such a model are available ta perfonn global simulations of the minerai dust cycle, even if its application may be limited to climatolagical studies for the present time. The best agreement with the satellite observations is obtained by using a physically based dust emission model. However th~ application of such models on large scale is limited by the availability of the input data characterizing the surface features of the desert aceas

Geomorphologic approach for modelling the surface features of arid environments in a model of dust emissions: application to the Sahara desert

International audienceMineral dust emissions from arid regions are influenced by the surface features encountered in the source regions. These surface features control both the erosion threshold and the intensity of the dust flux. Recently, a soil-derived dust emission scheme has been designed in order to provide an explicit representation of the mineral dust accounting for the influence of the surface features on the dust emissions. This physical scheme has been validated with micro-scale field measurements. Its large scale application has required the development of additional relations to estimate the input parameters from more accessible data: the mean height and the covering rate of the roughness elements and the mineralogical soil type. The determination of these surface data has been based on a geomorphologic approach which describes the surface features of arid areas in a 1 × 1° grid. Inside each square degree, up to five different areas characterised by different surface features have been distinguished. However, these areas have not been located inside the square degree. Each area can be constituted by several combined surface features, including roughness, vegetation, granulometry. Five main types of landscapes and eight main types of surface features have been distinguished. This approach is based on the combination of various data, mainly topographical, geological maps and climatological analysis. In addition to the problem of scale transfer, the main constraints to obtain a quantitative assessment are the confidence level of the existing data and the number of parameters to document. On the opposite, with this method, the fine scale required by the dust modelling can be separated from the scale accessible by the mapping approach, of the order of the square degree. This method can also be easily improved by aggregating new data and can be extended to other deserts. An example of application is given for the north-west of the Algerian Sahara where the method has been elaborated. The data provided by the modelling of the surface have been used to simulate dust emissions for 1990, 1991 and 1992 over the central and western Sahara. Over these three years, the mean annual dust emission is about 760 Mt·year–1 Although a significant interannual variability exists (mainly due to changes in the wind pattern), the most intensive emissions remain quite constant in terms of location. The percentage of agreement with satellite observations higher than 0.7 is 74 %, but only 32 % when using a model having a single threshold function for dust emission (i.e. the same surface feature for the whole Sahara) (cf. later Marticorena et al., 1997).Les émissions d’aérosols désertiques à partir des régions arides sont fortement influencées par les caractéristiques de la surface des sols en zone source, aussi bien en ce qui concerne les seuils d’érosion que l’intensité des flux de poussières émis. Récemment, un modèle physique a été développé permettant de rendre compte de l’influence des caractéristiques de surface sur les émissions de poussières. Ce modèle a été validé à partir de mesures de terrain à micro-échelle. Son application à plus grande échelle a nécessité le développement de relations supplémentaires permettant d’estimer les paramètres d’entrée du modèle à partir de données plus accessibles : la hauteur moyenne et le taux de couverture des obstacles présents à la surface et le type minéralogique de la couche superficielle du sol. Ces données de surface ont été déterminées au travers d’une modélisation, à partir d’une approche géomorphologique, de la surface des zones arides en secteurs d’un degré carré. Cinq types principaux de paysages et huit types principaux d’états de surface ont été distingués. L’approche est fondée sur l’utilisation de données variées, essentiellement les cartes topographiques, géologiques et les données climatologiques. L’utilisation de « points de calage » (observations de terrain, photographies aériennes, images satellites) permet une approche quantitative d’autant plus fiable que l’on est proche des points de calage. À l’intérieur d’un degré carré, il a été distingué jusqu’à cinq aires différentes, sans qu’elles soient spatialement localisée dans celui-ci. Chaque aire peut elle-même être constituée de plusieurs états de surface combinés, incluant rugosité, végétation et granulométrie. En dehors du transfert d’échelle, les contraintes principales sont la fiabilité des données et le nombre de points à documenter quantitativement. Les apports sont la différence de nature entre les échelles de dimension inférieures au degré carré – du domaine de la seule modélisation – et les échelles supérieures, plus proches de la cartographie, la facilité d’amélioration et de généralisation de la méthode à d’autres déserts. Un exemple de modélisation de la région où elle a été mise au point, au NW du Sahara algérien, est décrit. Les données issues de la modélisation des états de surface sur le Sahara ont été utilisées pour simuler les émissions d’aérosols désertiques pour 1990, 1991 et 1992. En moyenne, ces émissions sont de l’ordre de 760 Mt·an–1. Bien qu’elles présentent une forte variabilité interannuelle liée à des variations des régimes de vents, les zone d’émissions les plus intenses restent relativement constantes en terme de localisation

Comparison of Emission Models Used for Large Scale Simulation of the Mineral Dust Cycle

International audienceThree different emission models of minerai dust of increasing complexity have been compared to satel· lite observations of minerai dust Qvec the Western Sahara. in arder to evaJuate their capability to simulate the frequency of the dust events. The first model is a !inear function of the wind velocity. ft provides a very pocr agreement with the observations (8%). The second one is a function of the wind velocity raised to the third power with a unique threshold for the dust emission, weighted by a soil texture parameter. With regards to the limited number of input parameters required by such a model, the agreement with the observations is quite good (61 %) but the location and intensity of the dust sources are not accurately simulated. The third model is based on a physical description of the main processes of dust emission. Il alsa utilizes mapped inputs of parameters characterizing the surface features in the tested area. The agreement with the observations is very good (79%) and the model provides a fine description of the dust sources location and intensity. From camparison with observations, it is stated that a simple wind power model is not appropriate to simulate the minerai dust emissions. A single threshold source model provides simulation of the present dust emissions with a reasonable confidence level. Mareover, the input parameters required by such a model are available ta perfonn global simulations of the minerai dust cycle, even if its application may be limited to climatolagical studies for the present time. The best agreement with the satellite observations is obtained by using a physically based dust emission model. However th~ application of such models on large scale is limited by the availability of the input data characterizing the surface features of the desert aceas

Limitation de l'érosion éolienne dans le cas de mélanges de grains du sol de tailles érodable et non érodable

International audienceCertains auteurs ont constaté lors d'expériences menées sur des surfaces érodables une réduction des flux de saltation mesurés après qu'un certain nombre d'événements d'érosion se sont produits. Selon ces auteurs, ceci pourrait résulter d'un stock en éléments érodables dans le sol limité, phénomène connu sous la dénomination de « supply limitation ». Pour essayer de comprendre ce que recouvre cette hypothèse, une série d'expérimentations ont été réalisées en soufflerie, expériences qui ont consisté à suivre l'évolution du flux d'érosion induit par un sol constitué d'un mélange de deux populations de grains de tailles différentes et soumis à une vitesse de friction seuil dont la valeur est intercalée entre les vitesses de friction seuil des deux populations de grains. En pratique, cela consiste à suivre l'évolution, au cours du temps, du flux d'érosion et de la couverture de surface pour chaque type de grains. Les résultats montrent que dans le cas où la fraction érodable de la surface du sol est composée par un mélange de particules érodables de différentes tailles (ce qui est le plus souvent le cas dans la nature), l'action érosive du vent provoque une dynamique très importante au niveau de la surface du sol. Cette dynamique qui est conditionnée par le mode d'agencement des particules n'affecte pas les caractéristiques générales de la surface du sol (rugosité aérodynamique et rugosité géométrique) par contre elle semble être très efficace pour affecter la susceptibilité du sol à l'érosion