Capacitance probe measurements of brine volume and bulk salinity in first-year sea ice

摘要

First-year sea ice plays an important role in the global climate system. It changes the physical properties of the surface of the polar oceans, and modifies the energy and mass transfer between the ocean and the atmosphere. An understanding of the way sea ice affects ocean-atmosphere exchange requires detailed knowledge of the evolution of ice physical properties, which are governed by its temperature and bulk salinity. To this effect, we assessed the utility of commercially available capacitance probes in determining the salinity evolution of first-year sea ice. Measurements of the complex dielectric permittivity, ε = ε′ - iε″, at 50 MHz were carried out in land-fast ice in McMurdo Sound, Antarctica, and in the Chukchi Sea near Barrow, Alaska. For comparison, we also deployed the probes in artificial, young sea ice in an outdoor tank experiment in Fairbanks, AK. The dielectric permittivity data compare well with predictions from a dielectric mixture model. We have derived a simple relation that allows for the derivation of brine volume fraction and bulk salinity in columnar first-year sea ice from the real part of the complex dielectric permittivity. For ice at temperatures below the percolation threshold, the error in the derived bulk salinity is less than 15%. The dependence of dielectric permittivity on brine inclusion morphology needs to be taken into consideration, and measurements indicate that changes in pore morphology are recorded in the capacitance measurements. In this paper we use the real part, ε′, of the complex dielectric permittivity to study the bulk salinity of bubble-free columnar ice. Further investigations, using the imaginary part of the complex dielectric permittivity, ε″, will make it possible to use the same probes to measure the bulk salinity and pore morphology of other types of ice, e.g., frazil, platelet, and multi-year ice.