LAUSR

Downward longwave radiation flux is an important variable for estimating net radiation, and investigating the surface energy budget and climatic studies. The flux is usually estimated using empirical models based on the information of meteorological parameters such as relative humidity, surface and air temperature and water vapour pressure. This paper presents the assessment of eleven widely used empirical models for estimating downward longwave radiation using ground-based dataset acquired from January 2016 to December 2017 at Ile-Ife, a tropical city in Nigeria. The original Idso (1981) and Niemela, Raisanen and Savijarvi (2001) models performed better than the other models with errors less than 5.0% when compared with the measured values. The performances of all the models improved greatly after calibration. Guest (1998) model which gave low errors (MBE = 0.65 Wm-2, RMBE = 0.15 %, RMSE = 9.38 Wm-2, RRMSE = 2.14 %, MAE = 7.84 Wm-2, RMAE = 1.79 %) performed best followed by Dilley and O’Brien (1998), Idso (1981), Prata (1996), Brutsaert (1975), Garratt (1992), Niemela, Raisanen and Savijarvi (2001) and Angstrom (1929) models. The calibrated models presented in this study can be used to estimate the flux under cloudless sky conditions at Ile-Ife and at other places with similar meteorological condition as Ile-Ife where this flux is not measured due to technological problems and high cost of purchasing and maintaining the needed sensors. The model for estimating the flux proposed showed better performance with low statistical errors than all the existing empirical models tested, and conform greatly (R2 = 0.88) with the measured data.