Salinity and solid fraction of frazil and grease ice

Abstract

AbstractUnder turbulent conditions ice growth in sea water often occurs as tiny suspended frazil ice crystals. When the turbulence is insufficient to keep the crystals in suspension, they may accumulate in a surface grease layer of pure ice and sea water. Here we give an account of this grease of low solid fraction and high salinity prior to its freeze-up into a solid ice cover. We provide equations for determining the bulk salinity, Sg, and solid ice volume fraction, ϕs, of the grease layer by indirect and direct methods, review previous observations, and present new data. For the evolution of the solid fraction of accumulating grease ice we find (I) an early mode 0.08 < ϕs < 0.12 and (II) a long-term packing limit of 0.28 < ϕs < 0.31, which for sea water of salinity 34 corresponds to 29.5 < Sg <31.5 and 24 < Sg < 26, respectively. We associate (I) with a mechanical limit related to initial random packing of frazil crystals and (II) with a thermodynamic limit, beyond which the grease will freeze-up into a solid ice cover. By comparing the results with a simple model of random close packing of anisotropic particles, we find that the results are consistent with frazil flocs having an aspect ratio of ˜10, much smaller than values assumed in most model applications that include sea-water frazil ice processes.