The Impacts of Albedo, Solar Zenith Angle, and Clouds on the Transition from Melt to Freeze in the High-Latitude Arctic

Components of the snow surface and ice energy budgets were measured during the Arctic Summer Cloud Ocean Study (ASCOS) from 14 August–2 September 2008 near approximately 87.5N. The experiment was characterized by 4 distinct surface temperature regimes, each coincident with differing cloud and thermodynamic characteristics. Mean energy budget computations suggest that, at the location of ASCOS, the melt season experienced a transition to the onset of freeze-up near the midpoint of the experiment. Energy budgets were dominated by the radiative fluxes, and surface temperature was dictated by the downwelling longwave radiation due to the highly reflective surface and large solar zenith angles. The surface cloud radiative forcing was estimated by comparing the observed radiative fluxes with clear-sky fluxes computed from a radiative transfer model. Longwave and shortwave forcing generally ranged between 65-85 W m-2 and -50 to -5 W m-2, respectively, and the total forcing was positive for the entire experiment. The transition from melt to freeze-up appears to be a consequence of air-mass advection associated with tenuous low-level cloud cover and a lack of cloud liquid, essentially eliminating the warming associated with cloud longwave forcing. After this brief cold regime, low-level liquid-containing clouds returned and the progression to a widespread freeze-up was inhibited until the clouds disappeared once again. The influence of increasing surface albedo and solar zenith angles, as well as the effect of cloud cover on the downwelling radiative fluxes are identified as key components in the transition from melt to freeze-up in the Arctic.