Influence of Topography on Surface Radio Refractivity Patterns over North Central, Nigeria

This paper investigates the interaction between terrain features and surface refractivity during the past decades in the North Central, Nigeria. Some issues were addressed in the study namely seasonal variation of surface refractivity over a period of 25 years (1983 – 2007), spatial distribution of surface refractivity covering 287 stations spreading across the Nigerian middle belt and relationship between terrain features and spatial variation of surface refractivity. Satellite-measured meteorological parameters comprising air temperature, relative humidity and pressure at 2 m height (relative to the surface) were obtained from National Aeronautic Space Administration (NASA) and used to compute annual, January and July averages of surface refractivity over the period. The results of surface refractivity were interpolated and compared with North Central terrain features to establish a correlation. Surface refractivity reduced with increasing altitude across the North Central, Nigeria; the reduction at interval of 100 m height was at an average of 1.27 N-Units. On the average, refractivity gradient varied at the rate of 7.87 N-Units/km. The least altitudinal variation occurred at the boundary layer while the highest variation occurred at 800 m and above. Surface refractivity and refractivity gradients at North Central, Nigeria were influenced by topographical features and the prevailing atmospheric conditions which are dependent on the seasonal rainfall regimes.