Thermodynamic investigation of an evolving grease to pancake ice field

Abstract

We present results from a laboratory tank study of ice growing from saline water in a wave field, focusing on the transition from a predominantly frazil/grease-ice cover to a pancake-ice cover. Combining surface temperature observations with direct and indirect determinations of ice salinity and solid fraction, we describe the evolution of frazil- and pancake-ice area fraction, salinity and solid-ice volume fraction over the course of 1 day. In the investigated stage of transition, frazil ice surrounding the pancakes was found to have rather constant properties: a surface temperature 0.4-0.6 K below the freezing point of the underlying sea water, a salinity of 24-26 g kg -1 and a solid volume fraction of 0.25-0.29. The average salinity of young pancake ice, estimated from heat and salt budgets, decreased from 18 to 15 g kg-1, while average solid volume fractions increased from 0.6 to 0.7. The transition from frazil to pancake is estimated to take place when solid fractions reach 0.37-0.40 and surface temperatures are 0.7-0.9 K below the freezing point. We find that, for proper modelling of the greasepancake system, it is important to distinguish between a surface and volumetric fraction of pancakes and grease ice.