Convective heat transfer over thin ice covered coastal polynyas

Polynyas play an important role in the regional meteorology and oceanography of the high latitudes and in the global ocean circulation. Unique low-level observations of an Antarctic coastal polynya, the Ronne Polynya, were conducted using an instrumented aircraft. At the time of the observations, the polynya was mostly covered with thin ice perforated with holes and was composed of two distinct regimes: an inner region of newly formed and thin ice and an outer region of thicker more consolidated ice. The sensible heat flux over the polynya was similar to 100 W m(-2) and decreased with fetch, primarily as a result of the thickening ice cover. The mean sensible heat transfer and drag coefficients over the polynya were C-HN10 = (0.7 +/- 0.1) x 10(-3) and C-DN10 = (1.1 +/- 0.2) x 10(-3), respectively. The heat transfer coefficient is similar to that found over heterogeneous sea ice and is significantly lower than has been used in previous studies of heat fluxes over polynyas, which are often assumed to be open water. The transfer coefficients were not found to be a function of fetch or ice conditions as represented by the surface temperature and albedo. The data were used in an investigation of the output of sensible heat flux, potential temperature, and boundary layer depth from a simple fetch-dependent model. For this case study, surface temperatures and transfer coefficients appropriate to an ice-covered surface were required for an accurate simulation.