The Seasonal and Diurnal Cycles of Refractivity and Anomalous Propagation in the Sahelian Area from Microwave Radiometric Profiling

Abstract

AbstractThe Sahelian zone of West Africa is a semiarid area where strong amplitude of the seasonal and diurnal cycles of water vapor and temperature is observed. One year of continuous observation of vertical profiles of water vapor and temperature gathered from Niamey, Niger, with a profiling microwave radiometer is used to analyze the climatology of refractivity and microwave propagation regimes in the low troposphere. Seasonal and diurnal cycles of refractivity and ground-based radar anomalous propagation are emphasized. It is shown that the combined effect of water vapor and temperature vertical gradients is responsible for strong seasonal and diurnal cycles of the ducting propagation regime. Statistics of propagation regimes are given. The probability density functions of the refractivity gradient are found lognormally distributed. Three months of C-band radar data simultaneous with the profiling microwave radiometer observations have also been collected. Relations between the vertical refractivity gradient and the ground-based radar anomalous propagation echoes (APE) are illustrated and discussed. APE spatial distributions are found strongly related to the main features of the orography and topography inside the radar-observed area. Contingency tests show that the probability for APE to be linked to ducting is higher than 95%. In addition, this paper suggests that observing the refractivity vertical profiles from a microwave radiometer profiler located close to a meteorological radar provides information on whether anomalous propagation has to be considered as a potential cause of spurious signal in the measured reflectivity field.