Climatology of radar anomalous propagation over West Africa. (February 2015)

• Record Type: Journal Article
• Authors: Kaissassou, Samuel; Lenouo, André; Tchawoua, Clément; Lopez, Philippe; Gaye, Amadou Thierno
• Is Part Of: Journal of atmospheric and solar-terrestrial physics. Volume 123(2015:Feb.)
• Page Start: 1
• Page End: 12
• Publication Date: 2015-02
• Subjects: Anaprop -- West Africa -- Duct -- Radar; Geophysics -- Periodicals; Atmospheric physics -- Periodicals; Géophysique -- Périodiques; Météorologie physique -- Périodiques; Electronic journals; 551.51
• DOI: 10.1016/j.jastp.2014.11.009
• Languages: English
• ISSNs: 1364-6826

Abstract

Abstract: A comprehensive examination of 5 years of European Centre for Medium-Range Weather Forecasts (ECMWF) data from T511 L60 version of the ECMWF model to determine ducting conditions over West Africa and the computation of statistical distributions of the vertical gradient of refractivity determined from 2 years of radiosonde data over Dakar (14.41°N, 17.26°W), Douala (4°N, 9.7°E) and Niamey (13.35°N, 2.03°E) were carried out. It is found that diurnal and seasonal variations of the refractivity of the atmosphere are influenced by air temperature and water vapor pressure fluctuation. Refractivity gradients lower than −157 km −1 often results in spurious returned echoes and misinterpretation of radar images such as erroneous precipitation detection. The results obtained show that the local climate has an appreciable influence on the vertical profile of refractivity, especially the seasonal north–south movement of the Inter Tropical Discontinuity which is associated with the alternance of wet and dry seasons over the region. It is found that most of ducts occur in the night, morning (0000, 0600 UTC) and late afternoon (1800 UTC). The occurrence probability of abnormal propagation events, such as ducts, can provide some valuable information about the propagation of electromagnetic waves over West Africa. Highlights: Climatological maps of occurrence of super-refraction and ducting layers have been constructed. The midday is seldom affected by ducting as a result of the ceaseless and intense storm activity. The refractivity index was obtained from in-situ measurements performed at the three stations. Sub-refractive prevails at 1200 UTC whereas super-refractive and ducting are observed at 0000 UTC. The convective motions resulting from the daytime heating act to increase the wind speed.