Interaction of a negatively buoyant line plume with a density interface

Laboratory experiments were carried out to investigate the interaction between turbulent line buoyant plumes and sharp density interfaces, with the aim of using the results to interpret oceanic observations pertinent to crack openings in the polar ice-cap (leads). These openings take the form of long narrow channels, and are often modeled as line bouyant plumes. The plumes descend as in a homogenoous fluid, impinge on the density interface, and then spread horizontally as gravity currents. Depending on the Richardson number Ri = Δ bl_d/q₀ ^{2 3}, where Δb is the buoyancy jump across the interface, l_D is the half-width of the plume before the impingement and q₀is the buoyancy flux per unit length of the source, different flow patterns were identified. When Ri < 0.5, the plumes penetrate deep into the bottom layer, deflect horizontally and then spread while showing little vertical rise. When 0.6 < Ri < 5, the penetration is significant, but the fluid bounces back after entraining heavy fluid from the lower layer and then spreads horizontally above the interface as a gravity current. Appreciable mixing between this current and the lower layer was detected when Ri <1. When Ri > 10, the penetration was small and a sharp-nosed gravity current emerged some time after the impact. Measurements were made on the penetration depth, the velocities of the gravity current and the subsurface flow towards the plume, the entrainment rate and other wave parameters. Possible implications of the results for oceanic cases are also discussed.