Dependence of a Plume Heat Budget upon Lateral Advection

Abstract

The structure and heat budget of a plume created by a linear heat source is investigated, demonstrating their dependence upon the larger-scale cross-source flow. A large-eddy model simulates individual thermal eddies produced by a source 200 m wide having an average surface temperature flux of 0.20 K m s−1, parameters appropriate to an Arctic lead—that is, to a break in the polar ice field. When the large-scale cross-flow is relatively weak, maximum turbulent development occurs over the source itself and the mean updraft is relatively strong. Thermally induced lateral inflow then concentrates the vertical heat transport over the source, and vertical advection accounts for over half the total heat transfer. Horizontal turbulent heat transport is important throughout the mean updraft. Turbulent vertical heat flux dominates the heat budget only in the near-surface region. With increasing externally forced cross-flow, mean and turbulent vertical velocities are weakened. Transport mechanisms are significantly altered when the cross-flow is sufficiently strong to move the region of maximum turbulence beyond the source. Turbulent vertical heat flux then becomes important through the entire plume depth, and its heat transfer becomes larger than that due to mean vertical transport. Maximum turbulent development occurs horizontally where the parcel transit time from the source centerline is approximately half the Brunt–Väisälä period of the surrounding environment. When this occurs over the source, its position coincides with that of the mean updraft. A criterion for strong updraft development, and consequent large vertical advective transport, is for the transit time of a parcel over the source to be less than the Brunt–Väisälä period.