SPATIO-TEMPORAL DISTRIBUTION OF NET RADIATION AND ITS EFFECTS ON CLIMATE CHANGE OVER WEST AFRICA

Abstract

Net Radiation is an important parameter for land-surface model at the surface as it drives the process of evaporation, photosynthesis and heating of soil and atmosphere. It is also a total available energy on earth’s surface required for studying land-surface interactions which is the key component of surface radiation budget. In this research, thirty-six years’ atmospheric data between 1980 and 2015 were obtained from the Archives of the Modern Era Retrospective Analysis for Research and Application, Version 2 (MERRA-2) database. The aim of the research is to assess the spatio-temporal distribution of net radiation and to investigate its contribution to climate change in the Arid and Humid climatic zones over West Africa. The spatial-temporal distributions of net radiation and its components were observed. The effect of net radiation on climate change was investigated using the climate sensitivity to determine global warming potential, trends and Granger Causality analyses. The net radiation was further evaluated using the multivariate linear regression (MLR) and nonlinear autoregressive artificial neural network (NARX) models. The spatial distribution of net radiations showed that net radiation decreased from the humid zones having maximum value of 317.20 ± 13.88 W/m2 to the Arid zones of West Africa having maximum value of 184.50 ± 2.37 W/m2. These have been attributed to the changes in the surface albedo during the daytime and the surface emissivity during the nighttime. Temporal distribution of net radiation showed that the millennial years experienced deficit net energy for atmospheric processes being that the net radiation in the pre-millennial years has higher magnitudes than that of the millennial years. The urbanization, land use and land cover may be responsible for this deficit across the zones in West Africa. The effect of these observations on the climate change revealed that the warm nights, warm days and extreme wet days’ precipitation showed significant increasing trends at p < 0.05 alpha level across the climate zones of West Africa. Analyses of the climate sensitivity showed that it ranges from 2.10 – 3.92 ± 0.21 in the Arid zones and 1.73 – 2.25 ± 0.09 in the Humid zones. These values are within the global warming potential values of 1.5 – 4.5 proposed by the Inter Governmental Panel on Climate Change (IPCC). The implication of this observation is that there is prevalence of surface warming in the millennial years. The net radiation was also found to have significant causal relationship with the increasing trends in warm nights, warm days and extreme wet days’ precipitation. Finally, the developed models in this research, the Multivariate linear regression models and the Artificial neural network models, were found suitable for evaluation of net radiation over West African climatic zones and their respective countries based on the results of the quality assurance test. The models were found to perform better in the Arid zones than in the Humid zones due probably to error sources from the cloud and humidity that characterize the Humid zones. In conclusion, it was observed that the Arid zones is becoming wetter and the Humid is becoming drier in the millennium years than their usual dry and wet conditions.