Evaluating the performance of some predictive models for estimating global solar radiation across varying climatic conditions in Nigeria

121-131 The performance of both temperature and sunshine dependent models were evaluated for different selected tropical sites, which are situated across four different climatic zones, viz. Sahelian, Guinea Savannah, Midland and Coastal area in Nigeria. The regression constants were obtained for the first, second and third order Angstrom type [Garcia J V, Principios F’isicos de la Climatolog’ia. Ediciones UNALM (Universidad Nacional Agraria La Molina: Lima, Peru), 1994; Hargreaves G &amp; Samani Z, Estimating potential evapotranspiration, J Irrigat Drainage Eng (USA), 108 (1982) pp 225-230] models for all the stations using the method of regression analysis. Model evaluation performance analyses were carried out to determine which model is more suitable for a given climatic condition. The results indicated that the third order Angstrom type correlations do not improve the accuracy of estimation of global radiation. In addition, the results also revealed that temperature and sunshine hour dependent models are more suitable for the simulation of global radiation in the Sahelian and Guinea Savannah climatic condition, respectively. All the models exhibited the tendency to perform suitably well in the Midland and Coastal areas. On the seasonal consideration, the sunshine hour dependent models were found more suitable for the estimation of solar global radiation across all the varying climatic conditions during the dry season. However, all the models were found suitable during the wet season except for the Sahelian where only the temperature dependent models have been found exclusively suitable. </smarttagtype

Estimation of global solar radiation in Nigeria using a modified Angstrom model and the trend analysis of the allied meteorological components

213-224The trend of some common and related atmospheric variables were investigated in the light of climate change on annual time scale and a suitable scheme was further developed for the simulation of annual global solar radiation in Nigeria. In this connection, annual trends of global solar radiation, air temperature, precipitation, relative humidity and sunshine hours was carried out, covering about 13 tropical stations during 1975-2006 in Nigeria using F-test as the significance test technique. It was found that nine stations exhibited an upward trend in global solar radiation series, of which 6 passed F-test at 1% significant level. At 11 stations, precipitation had shown an increasing trends but none passed F-test at 2.5% and hence, not significant. About 98.8% of the stations displayed an upward trend in sunshine hours of which 16% passed F-test at 1% significant level. On trend analysis for relative humidity series, eight stations exhibited a positive trend and only one station passed F-test at 1% significant level. The trend of temperature series in Nigeria under the period under investigation was found to be increasing at 12 stations and eight stations passed the F-test at 1% significant level. The other objective of this study was to determine a more suitable empirical equation by modifying Angstrom model for the estimation of global solar radiation using all data for all 14 stations pooled together to predict global solar radiation using linear and multiple linear regression. This was done to improve the low performance of the Angstrom model used for the annual estimation of global solar radiation. The model parameters ‘a’ and ‘b’ of Angstrom model were parameterized in terms of the geographical locations (latitude, longitude and elevation) and the meteorological variables (sunshine hour, precipitation, relative humidity and temperature), respectively. This scheme gave better simulation of the global solar radiation compared with other schemes and the original Angstrom models. In addition, the preferential consideration of relative humidity to precipitation as potent contributing factor in the estimation of global solar radiation was also established

Analysis of data on net longwave, shortwave and global radiation during transition period in a tropical station in Southwestern Nigeria

347-352The present study examines the distribution of surface radiation balance components measured during the Nigerian Micrometeorological Experiment (NIMEX-1). A field study was conducted to measure surface heat fluxes in the boundary layer during the transition period which marks the end of dry season and the onset of wet season (15 February and 10 March 2004) in Southwestern part of Nigeria. Regression equations were obtained using daily average values and hourly mean values between net shortwave and net longwave radiation, respectively with the global radiation. Higher correlations with low standard error of measurement were obtained for daily mean values than hourly mean values. It was also observed that the peak value of each radiation during the period occurred for two hours in the afternoon on an average as expected