Thin Clouds Control the Cloud Radiative Effect Along the Sc‐Cu Transition.

In situ and spaceborne studies reveal the prevalence of thin clouds in the major Stratocumulus‐to‐Cumulus Transition (SCT) regions. Using instantaneous satellite and reanalysis data, this study investigates the properties of thin clouds in the Southeast Pacific Ocean and their impact on the cloud radiative effect (CRE). Our findings demonstrate that thin clouds are intrinsic to the SCT. The overcast stratocumulus‐dominated regime exhibits a minimal presence of thin clouds, which become notably prominent after the clouds breakup into the cumulus‐dominated regime. The regime dependence of the occurrence of thin clouds is also observed in terms of the marine cold‐air outbreak parameter and the sea surface temperature. Thin clouds at a given cloud cover significantly modulate the shortwave (SW) and longwave (LW) components of CRE. SW CRE decreases by 46 %–65 % with increasing thin cloud cover. They account for a larger variance in cloud albedo than the combined influence of the liquid water path and effective radius. Furthermore, LW CRE decreases by about 12 %–52 % with thin cloud cover. An increase in the fraction of thin clouds also leads to a larger fraction of negative SW CRE offset by positive LW CRE at a given cloud cover. This LW compensation ranges from approximately 8 % at overcast cloud cover to as much as 19 % at about 50 % cloud cover. These findings elucidate the crucial role of thin clouds, and thus cloud morphology, in modulating CRE and underscore the necessity of their accurate representation in climate models. Plain Language Summary: The eastern regions of major global oceans are primarily covered by low‐altitude clouds. These bright clouds overlay the dark ocean surface, reflecting incoming solar radiation and cooling the Earth. Concurrently, they act as a barrier to the energy emitted by the ocean surface, preventing it from escaping the Earth and thereby contributing to the Earth's warming. Satellite observations reveal the prevalence of thin clouds in these oceanic regions. In contrast to their thicker counterparts, thin clouds are less effective in reflecting solar radiation and blocking surface emissions. This study investigates the impact of thin clouds on radiation using data from various sources. We show that the presence of thin clouds significantly reduces cloud brightness by approximately 46 %–65 %, even when the cloud cover remains constant. Thin clouds have a more substantial effect on a cloud's ability to reflect solar radiation compared to the combined influence of cloud water and cloud droplet size. Furthermore, since thin clouds allow some surface emissions to pass through, their presence at a constant cloud cover reduces their warming effect by up to 52 %. These findings emphasize the crucial role of thin clouds in shaping the contribution of clouds to Earth's energy balance. Key Points: Thin clouds dim cloud brightness by up to 65 percent at a fixed cloud coverThin clouds exert a greater influence on cloud albedo than the combined effects of cloud water and droplet size under constant cloud coverThin clouds reduce longwave warming by up to 52 percent and modulate the balance between longwave warming and shortwave cooling [ABSTRACT FROM AUTHOR]