Interactions Between Clouds, Atmospheric Boundary-Layer, and Snow-Covered Sea Ice during the SHEBA Winter

Cloud liquid water (CLW) in wintertime Arctic mixed-phase clouds produces dramatic transitions in downwelling longwave radiation, and regulates responses in other surface energy budget (SEB) terms such as the turbulent sensible heat flux and the conductive heat flux through snow and ice. The modulation of the CLW is by synoptic and mesoscale forcing, while cloud-top longwave cooling of the primarily stratocumulus clouds leads to upside-down convective mixing and a descending mixed layer at the same time as the surface warms through longwave radiative fluxes thereby producing a surface-based mixed layer. This surface heating retards the upward conductive heat flux, producing distinct thermal waves penetrating at least one meter into the sea-ice below the approximately 20 cm of snow. This all occurs during the polar night with no solar radiation, a time which is generally considered to have a very stable boundary layer. All of these processes and process interactions are illustrated using observations from winter at SHEBA, while the magnitudes of the SEB responses to the CLW are also available from these observations. Details of the interactions are examined using a 12-day period in early January 1998. The frequency of these events and the magnitudes of the various transitions for the SHEBA winter will be presented.

In addition, these same processes and process relationships are be examined in simulations with the Weather Research and Forecasting Model (WRF) and in reanalysis data, which are primarily dependent on model output in the Arctic. It will be shown that the physical processes and process relationships in these models differ from observations because of their inability to reproduce the observed amount of CLW, though these erroneous processes sometimes produce temperatures very similar to those observed.