ESTIMATION OF HORIZONTAL VISIBILITY OVER SELECTED STATIONS IN NIGERIA

Abstract

This study aims at estimating horizontal visibility over selected stations within two main ecological zones; Savannah and Forrest of Nigeria. The specific objectives of this study were to establish empirical model equations for estimating horizontal visibility from more readily measured meteorological parameters. Consequently for stations where there are no visibility data, visibility could be estimated from meteorological parameters such as relative humidity, temperature and sunshine duration hour (s) which are more readily measured. A thirty-one-year (1980- 2010) record of daily horizontal visibility (0900 and 1500), sunshine duration hour(s), relative humidity (0900 and 1500) and Temperature (maximum and minimum) from the archives of Nigerian Meteorological Agency (NIMET) was used for this study. Nine regression models both linear and Multiple were used to estimate horizontal visibility. Regression analysis was carried on the visibility data with corresponding meteorological parameter (relative humidity, Temperature and sunshine duration hours) for each model at each station. The constants of regression for each model were then obtained and thus the regression line by plugging the meteorological parameter(s) for each model into each line. Consequently the estimate of horizontal visibility for each model at each station was obtained. The root-mean-square error (RMSE) and the mean-bias error (MBE) were used as a standard statistical metric to measure the models performance. The results show that the performance of the model(s) in estimating horizontal visibility vary from one region to the other. It was also established that model 7 and 2 performed better over Maiduguri and Kano in the Sahel Savannah region of the country with RMSE and MBE values of 1.11 and -15.02 and 1.35 and -13.40 respectively. It was also noticed that over Jos, model 7 with the RMSE value of 3.17E-14 and MBE value of -2.19E-14 performed equally well in estimating visibility. Generally the performance of all the models over Ikeja is quite poor, showing consistently a weak relationship between the observed and estimated visibility. The study concluded that in the absence of instruments for measuring horizontal visibility empirical model equations can be used to estimate visibility from more readily measured meteorological parameters