Geostatistical Distribution of Vertical Refractivity Gradient Over Nigeria

Anomalous propagation conditions may cause multipath fading and strong signal enhancement on terrestrial line of sight links or interference on transhorizon paths. In detecting this conditions, spatial distribution of vertical refractivity gradient and ducting index were estimated across Nigeria using atmospheric data obtained from Era5 archive of European Centre for Medium‐Range Weather Forecasts (ECMWF). Refractivity and refractivity gradient were calculated from the obtained temperature, relative humidity, and pressure data using International Telecommunication Union (ITU) recommendation expression. Vertical refractivity gradient (VRG) and ducting index (Di) were deducted from refractivity gradient and modified the refractivity gradient. Correlation between vertical refractivity gradient, Di, and latitude were also computed. The result shows that the VRG values range between −5 and −65 N units/km in the Coastal and Guinea Savannah regions, and between −30 and −95 N units/km in the Midland and Sahelian regions. The Di values range between 10 and 60 in the Coastal and Guinea Savannah regions, between −10 and 110 in Midland and Sahelian region. Likewise, superrefraction and ducting are more prominent in the northern part, while superrefraction with minor subrefraction were noticed in the south. The Di performed better than VRG in detecting propagation conditions. Correlation between VRG and latitude ranges between −0.75 and −0.92. Correlation between Di and latitude is between 0.84 and 0.95. The values of VRG decrease northward, while those of the Di increase northward. Both VRG and Di revealed high variability in the dry months than in the wet months. Key Points The article investigated anomalous propagation conditions at 100 m above the ground via the vertical refractivity gradient and ducting index Ducting index performed better in detecting anomalous conditions over Nigeria than vertical refractivity gradient with high superrefraction