Land-Ocean Contrast in Low-Cloud Adiabatic Fraction Modulated by Cloud Thickness and Droplet Size

Cloud adiabatic fraction (f_ad) is crucial for understanding cloud microphysics and entrainment mixing. This study presents the first global satellite-based assessment of land-ocean contrasts in f_ad for low-level clouds. Results show that land clouds have a lower f_ad (0.22) than oceanic clouds (0.35), consistent across all seasons. This contrast arises from the combined effects of cloud macrophysical and microphysical differences. Continental clouds tend to be thicker, enhancing evaporative dilution due to a longer vertical development path. Moreover, under similar cloud thickness and temperature conditions, continental clouds exhibit smaller droplet sizes, which evaporate more efficiently, especially in shallow clouds. A factor-substitution analysis quantifies this interplay: cloud thickness provides the primary macrophysical control, droplet size delivers microphysical amplification, and colder land cloud bases provide a partial thermodynamic offset. This process-oriented understanding advances the mechanistic interpretation of cloud–environment coupling and constrains cloud parameterizations in climate models.