Variabilité interannuelle et intra-saisonnière des précipitations en relation avec les deux régimes d’ondes d’Est en Afrique de l’Ouest

L’étude de la variabilité inter-annuelle et intra-saisonnière des  précipitations en relation avec les deux régimes d’ondes d’est est menée en Afrique de l’Ouest. La distribution spatiale des précipitations en relation avec ces ondes montre qu’elles contribuent à l’augmentation des  précipitations dans la bande de latitude 7°N – 15°N, vers la zone de convergence inter tropicale (ZCIT), avec deux maxima près du Fouta- Djalon en Guinée et vers le Lac Tchad. Les ondes sont très actives en juillet et août où les précipitations sont abondantes et faibles en juin et septembre marquant le début et la fin de la saison des pluies. Elles sont aussi très actives (faibles) pendant les étés à forte (faible) pluviométrie. Les deux régimes d’ondes d’Est sont différents et contribuent à accroître les précipitations de manières différentes. Pour l’onde de 6 – 9-jours, il existe une extension zonale des anomalies positives des précipitations dans la bande de latitude 7°N-15°N de l’Est à l’Ouest de la zone d’étude, avec deux maxima près du Fouta-Djalon en Guinée et vers le Lac Tchad. Dans ce régime, les maxima des anomalies positives des précipitations sont associés à de fortes anomalies de la composante zonale du vent, et  l’accroissement des précipitations est lié à d’importantes modulations du jet d’est africain (AEJ). Les deux maxima des anomalies positives des précipitations apparaissant vers le Fouta-Djalon et vers le Lac Tchad coïncident avec les maxima des composites des anomalies du flux zonal de vapeur d’eau associé à l’onde de 6 – 9-jours dans la couche 850–500 hPa (maxima de flux d’Ouest). Pour l’onde de 3 – 5-jours, il existe une forte extension  méridienne des anomalies des précipitations dans la bande de latitude 5°N-15°N en relation avec l’extension méridienne de la structure de l’onde. Dans ce régime, il n’y a qu’un seul maximum, vers la Guinée, des  composites des anomalies du flux zonal de vapeur d’eau (flux d’Ouest) dans la couche 850 – 500 hPa. Ainsi, les deux régimes d’ondes d’Est contribuent à l’accroissement des précipitations dans la région du Fouta-Djalon, tandis que le maximum des précipitations observé vers le Lac Tchad serait dû uniquement à l’onde de 6 – 9-jours.Mots-clés : onde de 6–9-jours, onde Africaine, structure spatiale, les anomalies des précipitations, flux de vapeur d’eau

La maladie de von-hippel lindau dans une famille togolaise

La maladie de Von Hippel Lindau(VHL) est une affection héréditaire autosomique dominante dont l’expression phénotypique est variable et multiviscérale. Le diagnostic nécessite des arguments cliniques et un plateau technique de pointe. nous rapportons les résultats d’une enquête au sein d’une famille togolaise à partir de deux observations cliniques. Ces observations mettent en exergue les difficultés de la pratique médicale en Afrique subsaharienne liées à un plateau technique inexistant

Frontiers of urban control: lawlessness on the city edge and forms of clientalist statecraft in Zimbabwe

This article develops the concept of ‘urban frontier’ to explore conflicts over state regularization of city edge informal settlements in Zimbabwe’s capital Harare. It conceptualises the presence of ‘lawless’ urban frontiers and ‘illegal’ territorial authorities in capital cities as expressions of a permissive form of central statecraft. In so doing, the article takes forward debates over the politics shaping the margins of Africa’s rapidly expanding cities, redressing scholars’ tendency to neglect central party-state strategic calculations and party politics in their analyses of unregulated settlements. Dominant interpretations generally hinge on state absence or weakness and emphasise localised influences. The case of Harare’s highly politicized city-edge informal settlements reveals the inadequacy of apolitical approaches particularly clearly, as all were controlled by the ruling ZANUPF party. The conflicts provoked by regularization provide a lens on disputes within the ruling party, which we interpret as disputes over different forms of clientalist statecraft. Analyses of urban frontiers can thus help move away from generic one-size-fits-all explanations of informality and patronage politics in Africa’s expanding cities

Classification of time series of Sentinel-2 images for large scale mapping in Cameroon

International audienceSentinel-2 satellites provide dense image time series exhibiting high spectral, spatial and temporal resolution. These images are in particular of utter interest to map Land-Cover (LC) at large scale. LC maps can now be computed on a yearly basis at the scale of a country with efficient supervised classifiers, assuming suitable training data are available. However, the efficient exploitation of large amount of Sentinel-2 imagery still remain challenging on unexplored areas where state-of-the-art classifiers are prone to fail. This paper focuses on Land-Cover mapping over Cameroon for the purpose of updating the national topographic geodatabase. The ι 2 framework is adopted and tested for the specificity of the country. Here, experiments focus on generic classes (five) which enables providing robust focusing masks for higher resolution classifications. Two strategies are compared: (i) a LC map is calculated out of a year long time series and (ii) monthly LC maps are generated and merged into a single yearly map. Satisfactory accuracy scores are obtained, allowing to provide a first step towards finer-grained map retrieval

CLASSIFICATION OF TIME SERIES OF SENTINEL-2 IMAGES FOR LARGE SCALE MAPPING IN CAMEROON

Abstract. Sentinel-2 satellites provide dense image time series exhibiting high spectral, spatial and temporal resolution. These images are in particular of utter interest to map Land-Cover (LC) at large scale. LC maps can now be computed on a yearly basis at the scale of a country with efficient supervised classifiers, assuming suitable training data are available. However, the efficient exploitation of large amount of Sentinel-2 imagery still remain challenging on unexplored areas where state-of-the-art classifiers are prone to fail. This paper focuses on Land-Cover mapping over Cameroon for the purpose of updating the national topographic geodatabase. The ι2 framework is adopted and tested for the specificity of the country. Here, experiments focus on generic classes (five) which enables providing robust focusing masks for higher resolution classifications. Two strategies are compared: (i) a LC map is calculated out of a year long time series and (ii) monthly LC maps are generated and merged into a single yearly map. Satisfactory accuracy scores are obtained, allowing to provide a first step towards finer-grained map retrieval. </jats:p

CLASSIFICATION OF VEGETATION CLASSES BY USING TIME SERIES OF SENTINEL-2 IMAGES FOR LARGE SCALE MAPPING IN CAMEROON

Abstract. Sentinel-2 satellites provide dense image time series exhibiting high spectral, spatial and temporal resolutions. These images are in particular of utter interest for Land-Cover (LC) mapping at large scales. LC maps can now be computed on a yearly basis at the scale of a country with efficient supervised classifiers, assuming suitable training data are available. However, the efficient exploitation of large amount of Sentinel-2 imagery still remain challenging on unexplored areas where state-of-the-art classifiers are prone to fail. This paper focuses on Land-Cover mapping over Cameroon for the purpose of updating the Very High Resolution national topographic geodatabase. The ι2 framework is adopted and tested for the specificity of the country. Here, experiments focus on generic vegetation classes (five) which enables providing robust focusing masks for higher resolution classifications. Two strategies are compared: (i) a LC map is calculated out of a year long time series and (ii) monthly LC maps are generated and merged into a single yearly map. Satisfactory accuracy scores are obtained (&gt;94% in Overall Accuracy), allowing to provide a first step towards finer-grained map retrieval. </jats:p

Convection activity over the Guinean coast and Central Africa during northern spring from synoptic to intra-seasonal timescales

International audienceThis study proposes an overview of the main synoptic, medium-range and intraseasonal modes of convection and precipitation in northern spring (March–June 1979–2010) over West and Central Africa, and to understand their atmospheric dynamics. It is based on daily National Oceanic and Atmospheric Administration outgoing longwave radiation and Cloud Archive User Service Tb convection data, daily TRMM and Global Precipitation Climatology Project rainfall products and daily ERA-Interim reanalysis atmospheric fields. It is first shown that mesoscale convective systems can be modulated in terms of occurrences number and intensity at such time scales. Based on empirical orthogonal function analyses on the 2–90-day filtered data it is shown that the main mode of convective and rainfall variability is located along the Guinean coast with a moderate to weak extension over Central Africa. Corresponding regressed deseasonalised atmospheric fields highlight an eastward propagation of patterns consistent with convectively coupled equatorial Kelvin wave dynamics. Then a singular spectrum analysis combined with a Hierarchical Ascendant Classification enable to define objectively the main spectral bands of variability within the 2–90-day band, and highlight three main bands, 2–8-, 8–22- and 20–90-day. Within these three bands, space–time spectral decomposition is used to identify the relative impacts of convectively coupled equatorial Kelvin, Rossby and inertia–gravity waves, as well as Madden–Julian Oscillation (MJO) signal. It confirms that eastward propagating signals (convectively coupled equatorial Kelvin wave and MJO) are highly dominant in these convection and precipitation variability modes over the Guinean coast during northern spring. So, while rain-producing individual systems are moving westward, their activity are highly modulated by sub-regional and regional scales envelops moving to the east. This is a burning issue for operational forecasting centers to be able to monitor and predict such eastward propagating envelops of convective activity