Solidification of leads: Theory, experiment, and field observations

Thin sea ice plays a central role in the surface heat and mass balance of t he Arctic Ocean. In order to develop understanding of these-balances we des cribe and analyze highly resolved temperature data taken through the air/se a/ice interface during the transition from an ice-free to an ice-covered Ar ctic Ocean surface. The data were taken to observe the thermodynamic evolut ion of a lead, a process that has previously only been accessible to measur ement techniques confined to the lead edge. Our detailed analysis of the fi eld data is guided by recent theoretical and experimental advances in under standing the phase dynamics of directionally solidified alloys. Because of the dearth of direct observations we also present time series of the releva nt heat fluxes inferred from our data and demonstrate the controlling influ ence that the internal phase evolution has on these quantities, We have pre viously examined the stability of the brine trapped in a growing sea ice ma trix both theoretically and experimentally and now find that haline convect ion, driven from within the growing layer, is consistent with this previous work and with the nature of direct turbulence measurements. The importance of this process is that although ice growth is continuous, the local brine flux commences abruptly, only after some time, in contrast. to what had pr eviously been supposed. Hence the ice growth process itself is a source of intermittency in oceanic boundary layer turbulence. Furthermore, we find th at in this particular situation the sea ice growth is not simply a square r oot function of time, in contrast to the model typically used in numerical simulations. By far the most practical methods of studying lead convection are numerical simulations and laboratory models, and a strong conclusion of this study is the importance of the proper treatment of the boundary condi tions describing the buoyancy flux.