Assessment of the Impact of Solar Spectral Irradiance on Near Infrared Clear-sky Atmospheric Absorption and Heating Rates

In the near infrared (near-IR), differences in both the absolute level and spectral resolution of available observation-based solar spectral irradiances (SSI) are very significant. But very little attention has been given to the effect of these differences on radiative transfer calculations. Using SSI derived from a good number of measurement platforms, a very high-resolution line-by-line radiative transfer model was employed to investigate the effect of the differences in the absolute level of SSI on the clear-sky total absorbed solar irradiance and solar heating rates in the near-IR. These calculations were carried out from 4,000 – 10,000 cm^-1 (2.5 –1 μm), for a mid-latitude summer atmosphere and an overhead Sun. Absorption calculated using observed SSI from the surface, aircraft and satellite are between 2 – 8% lower than that for the widely-used ATLAS 3 spectrum. Tropospheric and stratospheric heating rates produce by these spectra are also lower than that produced by the ATLAS 3 spectrum by about 3 – 9% and 4 – 11% respectively. However, there appear to be a closer agreement between near-IR absorbed irradiances and heating rates computed using two most recent observed spectra (one ground-based and the other spaced-based). That notwithstanding, it is recommended that the uncertainty in the choice of the SSI in near-IR radiative transfer modeling should be quantified.