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Variabilité de la dynamique éolienne au sol (direction et vitesse du vent) et de ses conséquences (visibilité horizontale) au Sahel central et Sahara méridional entre 1950 et 2009 (Exemple de quelques stations synoptiques au Niger)

Abstract : In the Sahel, climate variability was firstly analyzed using rainfall. However, this analysis has interested in a time less than ten day during the year, when the frequency of wind is not significant. Then, the climate variability was studied from the analysis of wind dynamic surfaces and their consequences related to environmental degradation between 1950 and 2009. Investigating climate variability from the wind speed and direction measured at synoptic stations (MWT). The daily variations explains the role of thermal turbulence. The seasonal cycles are distinguished by the wind speed and direction in connection with the ITCZ and ITD movements. Periods with strong aridity are highlighted by significant decrease of speed average. Surface winds show different types of droughts which not provided by rainfall data. Thus, the wind measurements provided by synoptic stations illustrate consistent results in accordance with the previous information extracted from the analysis of other parameters. These meteorological measurements are reliable and of some interest. Comparing the meteorological wind observations to 20CR, NNR-1, ERA 40 and ERA-Interim reanalysis. The monthly series show good correlation at Niamey and Maïné-Soroa, which is not the case at Bilma. The analysis of the annual cycle shows strong reanalysis winds over/underestimations. The winds interannual evolution in summer (JJA) and winter (DJF) shows that these discrepancies vary with wind station, periods and reanalysis. Therefore, ERA 40 and NNR-1 those are the most correlated to observations while 20CR shows strong differences. The comparison of meteorological measurements with ones assimilated by NCAR (SYNOP) indicates that low correlations reanalysis with the TCM measurements should be related to SYNOP files because of the importance of missing or erroneous data. Analyzing the variations of the consequence of combined rainfall and winds variations through the horizontal visibility and meteorological types associated with low visibility (blowing sand, sandy haze and dust haze). The interannual variations indicate significant visibility decreasing in relation with the rainfall variability. However, the increase of meteorological phenomena associated to the low visibility indicates also a climate change earlier than what is generally admitted on the basis of the only rainfall evolution. These changes concern the Saharan zone, more sensitive to wind erosion in the end 1950s. For Sahelian stations, a first degradation is visible around 1970, then a second at the beginning of 1980s. It is possible fora new observable degradation only in Niamey in the 1990s and 2000 to be of anthropological origin. Analyzing the relationship wind speed-horizontal visibility during the annual cycle. Given the rainfall gradient and the position upstream and downstream of stations on the continental scale wind trajectory NE-SW, we observe a trend suggesting other factors bring in. Indeed, gradually a real hysteresis cycle at Maïné-Soroa changes from wet period to a strong binary relation that has just disrupt a short rainy season at Nguigmi during dry period. Presumably, therefore, a third factor contributes in this relationship which transforms a hysteresis cycle to a binary relation: is it vegetation or anthropogenic factor?
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Submitted on : Thursday, November 9, 2017 - 10:11:31 AM
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Hassane Bouba. Variabilité de la dynamique éolienne au sol (direction et vitesse du vent) et de ses conséquences (visibilité horizontale) au Sahel central et Sahara méridional entre 1950 et 2009 (Exemple de quelques stations synoptiques au Niger). Climatologie. Université de Rouen; Université Abdou Moumouni, Niamey, 2013. Français. ⟨tel-01631374⟩



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