The impact of Arctic cloud water and ice on cloud radiative forcing during the Arctic Summer Cloud-Ocean Study in August 2008

Abstract

The Arctic atmosphere is especially sensitive to changes in climate forcing; however, Arctic processes and feedbacks are not understood well enough to accurately predict how the Arctic environment might change under anthropogenic forcing. Further study of the basic atmospheric processes is needed, especially due to uncertainties in modeling cloud feedbacks. August and September are the months when the Arctic sea surfaces begin to freeze; clouds play an important role in determining when this process begins. In this study, the radiative properties of Arctic stratocumulus are studied by comparing measurements for two days in August 2008 during the Arctic Surface Cloud Ocean Study (ASCOS) with simulations using the Rapid Radiative Transfer Model (RRTM). Cloud radiative forcing for both days is examined, and the modeled radiative fluxes were found to compare well to observations. Sensitivity studies are conducted on single and multi-level stratocumulus clouds to study their radiative interactions with each other. Cloud-top cooling in upper clouds is found to radiatively turn off cloud-top cooling in clouds below it. The RRTM and the surface radiative observations are used together to constrain estimates of liquid droplet radius; constraining these radii shows the sensitivity of shortwave cloud radiative forcing and the insensitivity of long wave cloud forcing to changes in drop size.