DETERMINATION OF RADIO REFRACTIVITY GRADIENT OVER NIGERIA USING NOAA-15, 16, AND 18 SATELLITE DATA

Abstract

The seasonal variation of tropospheric radio refractivity up till 250 mb (about 10.36 km) above the ground has been investigated. Also investigated is the refractivity gradient in the approximately 1 km over twenty four (24) stations spread across the four climatic zones of Nigeria: Coastal, Derived savannah, Guinea savannah and Sudano-sahelian region. The data used were monthly pressure, temperature and relative humidity data for the period 2004-2007 obtained from the Department of Satellite Applications Facility on Climate Monitoring (CM-SAF), Germany. The data was retrieved from Advanced Tiros Operational Vertical Sounder (ATOVS) observations onboard polar orbiting NOAA-15, 16 and 18. The results obtained showed that while the wet term of refractivity displayed a regular variation at all the tropospheric levels, the dry term displayed irregular variations at levels beyond 925mb. This irregular feature may be attributed to fluctuations in air temperature due to decrease in water vapour content with height. The refractivity is observed to demonstrate seasonal, regional and height variations. The wet and dry season surface refractivity differences at the coastal and derived savannah region were found to be 5.34N-units and 18.92N units respectively. At the guinea savannah and sudano-sahelian region the differences were found to be 34.94N-units and 57.23N-units respectively. An empirical exponential model of the form N = Ns exp (-0.137h), N = Ns exp (-0135h), N = Ns exp (-0.132), and N = Ns exp (-0.127h) developed for predicting vertical refractivity in the coastal, derived savannah, guinea savannah and sudano sahelian region respectively were found to perform fairly well in each of the regions. Average wet season refractivity gradient for the coastal and derived savannah region were found to be -44.24Nunits/km and -43.91N-units/km, respectively while their dry season values were -43.84N-units/km and -42.10N-units/km, respectively. The wet season gradient for the guinea savannah and sudanosahelian region was found to -43.32N-units/km and -41.62N-units/km respectively while their dry season values are -39.87N-units/km and -35N-units respectively. These results showed that the wet season favour super-refraction in all the regions while the dry season was found to favour sub refraction in the midland and sahelian region. The results also showed that super-refractive condition decreases from the coastal region to the sudano-sahelian region. An empirical exponential model of the form G = βеσΝѕ developed for predicting refractivity gradient was found to perform fairly well in all the climatic regions. In general, with the results obtained from this study we have able to establish the fact that radio refractivity does not display seasonal variation at the entire climatic zone and tropospheric level studied using satellite data. Also the different climatic zones have different refractivity gradient structure.