Tracer measurements in growing sea ice support convective gravity drainage parameterizations

International audienceGravity drainage is the dominant process redistributing solutes in growing sea ice. Modeling gravity drainage is therefore necessary to predict physical and biogeochemical variables in sea ice. We evaluate seven gravity drainage parameterizations, spanning the range of approaches in the literature, using tracer measurements in a sea ice growth experiment. Artificial sea ice is grown to around 17 cm thickness in a new experimental facility, the Roland von Glasow air-sea-ice chamber. We use NaCl (present in the water initially) and rhodamine (injected into the water after 10 cm of sea ice growth) as independent tracers of brine dynamics. We measure vertical profiles of bulk salinity in situ, as well as bulk salinity and rhodamine in discrete samples taken at the end of the experiment. Convective parameterizations that diagnose gravity drainage using Rayleigh numbers outperform a simpler convective parameterization and diffusive parameterizations when compared to observations. This study is the first to numerically model solutes decoupled from salinity using convective gravity drainage parameterizations. Our results show that (1) convective, Rayleigh number-based parameterizations are our most accurate and precise tool for predicting sea ice bulk salinity; and (2) these parameterizations can be generalized to brine dynamics parameterizations, and hence can predict the dynamics of any solute in growing sea ice. Plain Language Summary The cold atmosphere in the Arctic and Southern Oceans can cause sea water to freeze at the surface, forming sea ice. The salt present in the sea water is trapped in the newly formed sea ice, in pockets of very salty liquid, or brine. Brine is much denser than the ocean below, which causes the brine to drain into the ocean. This process is generally referred to as gravity drainage. It is critically important, affecting the ocean's circulation, the movement of greenhouse gases through sea ice, and the supply of nutrients to sea ice algae. Several authors have suggested ways to model gravity drainage. Broadly, these can be split into two approaches: convective and diffusive. Here, we use a laboratory sea ice growth experiment to test these models. We track the movement of brine using measurements of two chemicals in the sea ice and compare measured concentrations to those predicted by the models. Our results show that convective approaches are our most powerful tool for predicting the movement of brine by gravity drainage and do an excellent job