The Net Radiative Effect of the Ill-Defined Clear-Sky in the Vicinity of Clouds

Abstract

Clouds play a crucial role in Earth's energy budget resulting in significant deviations from clear-sky fluxes. This important role, combined with their complexity, makes them a major source of uncertainty in climate projections. Over the past two decades, the radiative signatures of clouds in their surroundings have been extensively studied, highlighting the challenges of defining clouds and distinguishing them from the aerosol embedded in clear-sky. Accurately defining clouds and aerosol is essential for quantifying their radiative effects and for remote sensing of the surface and atmosphere under clear-sky conditions. In this study, we combine two of the most mature and sophisticated space-borne instruments: the MODerate resolution Imaging Spectroradiometer with its high-resolution multispectral images and the Clouds and the Earth's Radiant Energy System broadband top-of-the-atmosphere radiative flux data. This0 allows us to quantify the net RE of clouds within the clear-sky around them for the first time. Our findings show that the local RE of clouds over the ocean around noon ranges from −7 to −10 W $m^{-2}$ in the solar band and 1–1.5 W $m^{-2}$ in the longwave infrared. These results suggest that clouds drive a significant portion of the aerosol direct RE. Additionally, the near-cloud longwave infrared effect is equivalent to an effect of $\sim$90 ppm of ${\text{CO}}_2$. We propose to categorize the sky into three regimes—cloudy, cloud-influenced-clear-sky, and far-field clear-sky and suggest a path to move beyond a discrete definition of the sky. This will improve our understanding of the role of clouds and aerosol in the climate system, ultimately reducing uncertainties in climate projections.