Evaluation and Calibration of Downward Longwave Radiation Models under Clear Sky Condition across the Sahelian Region of Nigeria

Abstract

Downward longwave radiation (DLR) is a major component of the surface energy budget, yet the transferability of empirical clear-sky DLR models across Nigeria’s Sahelian climatic zone remains uncertain. This study evaluated eleven existing empirical DLR models and five locally calibrated modifications for Kano and Sokoto. Monthly mean relative humidity, air temperature, and DLR data covering 1984–2023 were used, with 1984–2015 allocated to model development and 2016–2023 reserved for validation. Model performance was assessed using Mean Bias Error, Root Mean Square Error, Mean Percentage Error, t-test, Index of Agreement, correlation coefficient for the modified models, and an overall ranking procedure. Among the existing models, the Idso formulation ranked first at both locations. For Kano, the modified Guest model was the best-performing calibrated model and also ranked ahead of the existing Idso model in the direct comparison. For Sokoto, the modified Idso model ranked first among the calibrated models; however, the existing Idso model retained the better overall rank in the direct comparison. Seasonal analysis showed higher DLR during the rainy season and lower values during the dry Harmattan season, with relative humidity and atmospheric moisture varying concurrently with DLR. The results indicate that local calibration can improve empirical DLR estimation, although the benefit depends on location and model formulation.