TURBULENT TRANSPORT FROM AN ARCTIC LEAD - A LARGE-EDDY SIMULATION

The upward transfer of heat from ocean to atmosphere is examined for an Arctic "lead". a break in the Arctic ice which allows contact between the cold atmosphere and the relatively warm ocean. We employ a large-eddy model to compute explicitly the three-dimensional turbulent response of the atmosphere to a lead of 200 m width. The surface heat flux creates a turbulent "plume" of individual quasi-random eddies, not a continuous updraft. which penetrate into the stable atmosphere and transport heat upward. Maximum updraft velocities and turbulence occur downwind of the lead rather than over the lead itself, because the development time of an individual thermal eddy is longer than its transit time across the lead. The affected vertical region. while shallow over the lead itself, grows to a height of 65 m at 600 m downwind of the lead; beyond that. the depth of the turbulent region decreases as the eddies weaken. The maximum vertical turbulent heat flux occurs at the downwind edge of the lead. beyond which a relative maximum extends upward into the plume. Negative surface heat flux immediately downwind of the lead creates a growing stable layer. but above that internal boundary layer the turbulent heat flux is still positive. Updraft maxima are typically 28 cm/s, but compensating downdrafts result in time-averaged vertical velocities of less than 1 cm/s in the plume. Conditional sampling separates the updraft and downdraft contributions. Formulas for the horizontal eddy development distance and for the vertical plume penetration height are presented. The relative importance of mean and turbulent transport is compared for both vertical and horizontal heat transfer: turbulence dominates the vertical heat transport whereas mean advection dominates the horizontal transport, these offsetting transports producing a quasi-stationary state.