An Extended Comparison between LOWTRAN7 Computed and Observed Broadband Thermal Irradiances: Global Extreme and Intermediate Surface Conditions

Abstract

Differences between observed and LOWTRAN7-computed downward longwave irradiances were examined at each of four globally diverse locations for an entire year at each site. The final results are restricted to times determined to be completely or nearly cloud-free. The irradiances from 367 such times range from 60 to 435 W m⁻², and results indicate that the modeled irradiances and those measured directly using a pyrgeometer agree to within 5 W m⁻² at individual sites and to within lm than 0.2 W m⁻² when averaged over all four sites, neglecting any site-specific biases. The standard deviations and standard errors associated with these results are roughly 10 and 1 W m⁻², respectively. An unbiased estimate of the agreement between the model and observations results in a mean difference of 0.62 W m⁻² with standard deviation of 5 W m⁻² but an even larger 95% confidence interval because of the small sample size. The comparison variance can be logically ascribed to a number of different sources, including atmospheric variability and inhomogeneity, as well as to short-term instrument and LOWTRAN7 input variations. LOWTRAN7 and the observations agree better, in the mean, than the commonly accepted uncertainties for either would suggest. Maximum cloud radiative forcing at the surface for each site is quantified as a by-product of the comparison process.