Investigation of direct normal irradiance and total cloud cover connection over Nigeria using satellite data

This study assessed the connection between Direct Normal Irradiance (DNI) and Total Cloud Cover (TCC) throughout Nigeria, a region with considerable yet underexploited solar energy potential. Daily DNI data from Modern-Era Retrospective Analysis for Research and Applications Version 2 (MERRA-2) and TCC data from the ERA5 reanalysis of the European Centre for Medium-Range Weather Forecasts (ECMWF) were aggregated into monthly means and analyzed across four climatological seasons. Seasonal variability was evaluated using standard deviation, while long-term trends were examined through simple linear regression, with statistical significance appraised at p < 0.05. Pearson’s correlation coefficient was used to estimate DNI–TCC relationship. Results show that TCC peaked during JJA in the Af and Am climates, whereas DNI peaked in DJF and SON, particularly in the BSh and BWh regions. Variability in both DNI and TCC was highest in DJF and lowest in JJA. DNI variability was highest in the Aw zone and lowest in Am, while TCC variability peaked in the Aw and BWh zones and was lowest in Af and Am. Regression analysis revealed a strong inverse relationship between DNI and TCC in the Csb and BWh zones, while Af and Am exhibited complex interactions. Correlation analysis showed strongest negative relationship during DJF and JJA (mean r = –0.73), and weakest during MAM (r = –0.33). Trend analysis indicated a modest increase in DNI across all climate zones, with TCC decreasing except in Af. DNI values exceeding 400 W/m² were most likely in northern zones during DJF and SON but low in the south.