Role of thin clouds in modulating the cloud radiative effect of marine low clouds

The cloud radiative effect (CRE) of low-level marine clouds has traditionally been expressed primarily as a linear function of the cloud cover. However, recent studies have revealed a substantial change in CRE even at a constant cloud cover. This change is attributed to variations in cloud morphology governed by the horizontal and vertical distribution of cloud water. A unique feature of these morphologies, especially for low marine clouds, is the occurrence of distinct quantities of optically thin clouds. Understanding the impact of these thin clouds on low-level marine CRE is crucial for two reasons. First, spaceborne studies indicate a prevalent occurrence of thin clouds in areas characterized by peak low-level cloud cover. Second, the relationship between thin clouds and CRE may differ from that of their optically thicker counterparts due to their semitransparency to incoming shortwave and outgoing longwave radiations. This study investigates the influence of thin clouds on the CRE of low-level clouds over the Southeast Pacific Ocean using six years of concurrent measurements from MODIS and CERES spaceborne sensors. The results show a substantial influence of thin clouds on the shortwave and longwave components of CRE, as well as the balance them. The findings emphasize the need for a more comprehensive representation of thin clouds and, therefore, cloud morphology in climate models.