The influence of cloud cover on the surface energy budget during ASCOS

Surface radiative and turbulent flux measurements from the central Arctic, spanning a three-week (14 August – 1 September 2008) ice-based campaign during the Arctic Summer Cloud Ocean Study (ASCOS), are analyzed. The campaign was coincident with the transition from an ending melt season to the pre- or initial freeze-up. Net radiative flux often varies between 3 – 10 W/m2, although both positive and negative net fluxes were observed, the sign depending largely on the cloud cover present. Radiative fluxes show a clear and often non-linear relationship to cloud macrophysics (base and top height, cloud fraction) and basic microphysics (column LWP). The surface skin temperature, most often -3 – 0°C, is mainly dictated by the downward longwave radiative flux as opposed to the downward shortwave flux, at least directly. The magnitude of this signal may be enhanced by an increasing surface albedo during the campaign. Therefore an increase in cloud cover or cloud liquid water suggests a warming of the surface. Turbulent heat fluxes will then have a response to the surface-layer stability invoked by the cloud-radiative interaction. Case studies exemplifying the sensitivity of sensible and latent heat fluxes to changing cloud conditions (single low cloud layer, multiple cloud layers, precipitating clouds) are presented. Sensible heat fluxes changing sign (upward/downward directed) and becoming twice are large are shown to occur in response to changing cloud cover. A highly sensitive interplay of clouds, surface albedo and larger-scale advection contribute to the surface energy budget.