Evaluation and Calibration of Downward Longwave Radiation Models under Clear Sky Condition across the Coastal Climatic Zone of Nigeria

Abstract

This study evaluated clear-sky downward longwave radiation (DLR) at Ikeja and Warri, two stations in the coastal climatic zone of Nigeria. It assessed the relationship between DLR with relative humidity and air temperature, examined the performance of eleven existing empirical models, developed five locally calibrated regression models, ranked all sixteen models according to statistical reliability, and compared model outputs using validation indicators. Monthly average daily records of relative humidity, air temperature and DLR for 1984–2023 were obtained from the National Aeronautics and Space Administration (NASA). The data were divided into a 32-year calibration period (1984–2015) and an eight-year validation period (2016–2023). Model performance was evaluated using mean bias error (MBE), root mean square error (RMSE), mean percentage error (MPE), the t-test and the index of agreement (IA), with the correlation coefficient (R) also used for the modified models. At Ikeja, DLR ranged from 404.3542 Wm-2 in January to 423.3229 Wm-2 in April. At Warri, values ranged from 405.2708 Wm-2 in January to 424.0521 Wm-2 in April. Among the existing models, Kruk et al. model performed best at both stations, whereas Dilley and O’Brien performed least accurately. Among the modified models, the modified Dilley and O’Brien model was most suitable for Ikeja, while the modified Guest model was most suitable for Warri. The locally calibrated models provided improved estimates and may support DLR estimation in this coastal zone where pyrgeometer measurements are unavailable.