On the development of a simple downwelling longwave radiation scheme

This paper addresses the refinement and testing of a simple downwelling longwave radiation model. Oklahoma atmospheric surface-layer instrumentation system (OASIS) radiation data in combination with Oklahoma Mesonet weather data were used to evaluate various techniques for estimating downwelling longwave radiation, for daytime and nighttime as well as clear and cloudy skies. The Brutsaert [Water Resour. Res. 11 (5) (1975) 742] equation, which requires near-surface temperature and vapor pressure data, was chosen for further investigation. A simple regression calibration was performed for Brutsaert’s leading coefficient using hourly data from four OASIS sites. The calibrated equation was applied to five independent OASIS sites and the hourly predictions of downwelling longwave radiation showed good agreement with the measurements. The mean bias error (MBE) ranged between −3.95 and 4.24 W m−2, and the root mean square error (RMSE) was approximately 20 W m−2 in all five cases. Comparisons to a more complex longwave radiation formulation that explicitly considers cloudiness were also quite favorable.