Spectral Solar Irradiance and Its Entropic Effect on Earth's Climate

The high-resolution measurements of the spectral solar irradiance (SSI) at the top of the Earth’s atmosphere by the Solar Radiation and Climate Experiment (SORCE) satellite are used to examine the magnitude and spectral distribution of the Earth’s entropy flux from the TOA incident solar radiation. Two extreme cases are examined by using Planck expression: I. isotropic hemispheric incident solar radiation; II. (a) the specific energy intensity of solar radiation received at the top of the atmosphere (TOA) is the same as that radiated at the Sun’s surface, and (b) incident solar radiation is isotropic within the cone of the solid angle to the Sun subtended by any point at the TOA. The estimated Earth’s entropy flux from the observed SSI under the assumption I is shown about 4 times larger than that under the assumption II. The difference in magnitude is comparable to the typical value of the entropy production rate associated with atmospheric latent heat process. Further comparison with a conventional approach by using a blackbody Sun’s brightness temperature exhibits that the estimated Earth’s entropy flux from a blackbody Sun under the assumption II is about the same as that from the conventional approach. Furthermore, sensitivity study shows that the variability of TOA SSI could significantly impact the magnitude and spectral distribution of the estimated Earth’s entropy flux. Overall, this study suggests that the measurements of the TOA SSI are crucial for estimating the entropy production rate of the Earth’s climate.