Estimation of Wintertime Downward Surface Long Wave Radiation for High- and Low-Altitude Sites in the South-Western Region of Saudi Arabia

This paper discusses the estimation of downward surface longwave radiation (DSLR) under all-sky conditions at two sites in south-western Saudi Arabia. In this work, Meteorological parameters of air temperature, near-surface water vapor pressure, and cloudiness at Abha (high altitude) and Jazan (Gizan) (low altitude) were measured to fit with thirteen clear-sky and six cloudy-sky empirical models for the years 1979 to 2008. The model-based DSLR data were compared with the MERRA dataset during the wintertime (December-February). The MERRA-estimated winter DSLR values ranged from 284 to 302Wm^−2 (290 to 318Wm^−2) at the high altitude site, and from 354 to 374Wm^−2 (356 to 380Wm^−2) at the low altitude site under clear-sky (cloudy-sky) conditions. The results show that, under clear-sky (cloudy-sky) conditions, the Ångström M1clr, model has relatively better performance compared to other models in estimating DSLR for both high- and low-altitude sites. As no single model was superior individually in estimating DSLR, ensemble means were obtained from the selected 8 (4) models at the high-altitude (low-altitude) site under clear-sky conditions, and 6 (3) models at the high-altitude (low-altitude) site under cloudy-sky conditions. The bias of DSLR ensemble mean is found to be within the ±5% range. Interannual variability of the model ensemble mean for DSLR is very closer to the MERRA data, both in terms of phase and amplitude. Furthermore, the ensemble means were able to determine the increasing trend in DSLR for both the high- and the low-altitude sites.