The influence of atmospheric water content, temperature, and aerosol optical depth on downward longwave radiation in arid conditions

In this study, downward (LW) and outgoing longwave radiation measurements, air temperature (T), aerosol optical depth (AOD) at seven wavelengths, Angstrom exponent (α), and precipitable water vapor (PWV) data from Riyadh, an arid site in central Saudi Arabia, for the period between 2014 and 2016 were used to study their variations and to investigate the influence of the meteorological variables on the measured downward LW radiation under clear sky conditions. Downward LW radiation and the air temperature have the same distributions. While the outgoing LW radiation and the Angstrom exponent presented more than one peak in their distributions, the PWV was normally distributed with a mean value of about 11.9 ± 3.9 mm. Distribution of the AOD for all wavelengths has a log-normal shape. Theoretical simulations using SBDART code were conducted and showed that the downward LW radiative forcing increases by about 8% for every 1 mm increases in the water vapor, while it increases by about 4% in every 1 increase in the AOD at 500 nm value. Two variable models containing the PWV and T were developed to model the downward LW radiation. This model has a correlation coefficient of 0.91, MBE = − 0.004 W m−2, RMSE = 20.4 W m−2, and MPE = − 0.30%. Likewise, correlation analyses between the downward LW radiation and three independent variables (T, PWV, and AOD at 500 nm) were carried out. This model slightly improves the prediction of the LW radiation and has correlation coefficient of 0.93, MBE = 0.1 W m−2, RMSE = 17.3 W m−2, and MPE = − 0.20%.