The relationship between observed Arctic cloud vertical structure and the recent sea ice thinning

The Arctic regions north of Alaska and Siberia have experienced much of the more dramatic sea ice thinning occurring within the last 5 years. A recent study finds downwelling longwave fluxes explain most of the variability in the location of the sea ice edge, with northward wind anomalies also helping to explain some of the variance in the regions north of Siberia (Francis et al., 2005). The physical process through which clouds may mediate the findings of Francis et al. (2005) is investigated further. Observations from SHEBA and FIRE/ACE suggest an important role for the liquid residing within a shallow surface-based boundary layer: even low liquid amounts emit strongly in the infrared region. During July, 47% of the cloud coverage had bases within 200 m of the surface, and clouds with optical depths > 1 were usually liquid. July temperatures at 100-200 m altitude exceeded that of the surface and are attributed to northward advection of warm air; this may help explain the presence of low-level liquid as well. The thin liquid cloud layers still allow for the transmission of some solar radiation, but the infrared radiation helps melt the upper sea ice layer more efficiently than does the transmitted solar radiation, which transmits more deeply into the sea ice. These results from the July SHEBA surface-based remote sensor observations over the Chukchi Sea will be developed further, building on findings for the May time period (Zuidema et al., 2005), and compared to and generalized with cloud properties from MODIS and surface radiative fluxes from CERES.