Spectral solar irradiance and its entropic effect on Earth's climate

Abstract. The high-resolution measurements of the spectral solar irradiance at the top of the Earth's atmosphere by the Solar Radiation and Climate Experiment (SORCE) satellite suggest significant deviation of solar radiation from the commonly assumed blackbody radiation. Here, we use these spectral irradiance measurements to estimate the Earth's incident solar radiation entropy flux, and examine the importance of a proper estimation approach. The Earth's incident solar radiation entropy flux estimated by directly applying the observed spectral solar irradiance into the most accurate Planck expression is compared with that estimated with a conventional approach that uses the Sun's brightness temperature under the assumption of a blackbody Sun. The globally averaged non-blackbody incident solar radiation entropy flux at the top of the Earth's atmosphere equals 0.31 W m⁻² K⁻¹. This value is about 4 times larger than that estimated from the conventional blackbody approach, with the difference comparable to the typical value of the entropy production rate associated with atmospheric latent heat process. Further analysis reveals that the decrease of spectral solar radiation entropy flux with radiation traveling distance, unlike the decrease of spectral solar radiation energy flux with radiation traveling distance, is wavelength dependent, and that the difference between the two estimates can be attributed to the fact that the conventional approach ignores the influence of radiation traveling distance on the spectral solar radiation entropy flux. Moreover, sensitivity study further shows that the distribution of top-of-atmosphere spectral solar irradiance could significantly impact the magnitude of the estimated Earth's incident solar radiation entropy flux. These results together suggest that the spectral distribution of incident solar radiation is critical for determining the Earth's incident solar radiation entropy flux, and thus the Earth's climate.