Parameterizing daytime downward longwave radiation in two Korean regional flux monitoring network sites

Daytime downward longwave radiation (R_ld) in clear sky and all sky conditions was estimated by various well-known physical or empirical models at two Korean regional flux monitoring network (KoFlux) sites which have not been included in previously published studies for R_ld models' performance. The Brunt (1932) model performed the best clear sky R_ld estimations at the KoFlux sites. The Maykut and Church (1973) model performed the best all sky R_ld estimations at the KoFlux sites where the bias and root mean square error (RMSE) were 3.4 and 26.6Wm^-2 at the Gwangneung site and 0.3 and 32.4Wm^-2 at the Haenam site, respectively. Newly proposed global R_ld model by Abramowitz et al. (2012) showed reasonable R_ld estimations for the all sky conditions. While selected R_ld models showed the reliable estimations under clear and all sky conditions, the systematic and random errors were still observed in this study. These errors were mostly caused by relatively simple estimations of the cloud cover fraction. More validation and calibration efforts should be conducted in a range of field conditions including different climatic regions and land cover types for better understanding of the model performance with uncertainties.