MODELING THE EFFECTS OF FRAZIL ICE CRYSTALS ON THE DYNAMICS AND THERMODYNAMICS OF ICE SHELF WATER PLUMES

The seawater that comes into contact with the base of a floating ice shelf is modified as a result of the phase changes that occur. Melting is prevalent in the deepest parts of the subice cavity, and this drives a buoyant flow of Ice Shelf Water (ISW) along the sloping ice shelf base, The ascent of the ISW toward the surface of the ocean causes supercooling, because the freezing point rises with the falling pressure, and this induces a change from melting to freezing, Assuming that seed crystals exist, the ISW now fulfills the condition for the rapid growth of disc-shaped frazil ice crystals, which may subsequently settle (upward) out of suspension under the action of gravity. A simple numerical model of these processes has been developed, based on the theory of inclined plumes. The ISW is treated as a turbulent, particle-laden gravity current ascending a reactive boundary and containing a suspended crystal load which evolves in response to the supercooling of the water and the inverted sedimentation of the crystals. The frazil ice has two important effects on the behavior of the ISW plume. Because the generation of crystals throughout the plume provides such a large area over which phase changes can occur, the conversion of supercooling into ice happens much more readily than is possible through turbulent transfer of heat and mass at the ice shelf base. A suspended crystal load reduces the bulk density of the ISW, so that a growing suspension causes the plume to accelerate, while deposition of crystals onto the ice shelf effects a deceleration. There are positive feedbacks in that an acceleration of the plume induces more rapid crystal growth, while a deceleration allows suspended crystals to settle out more easily.