|
Turbulence Under Thin Ice at SHEBA |
McPhee Research Company
miles@wolfenet.com miles@wolfenet.com
Several previous studies of heat flux under sea ice have shown that heat and salt transfer at the ice/ocean interface are rate limited by molecular effects in thin sublayers near the interface. Under rapidly melting conditions, there is evidence that the respective transfer coefficients for heat and salt depend on the ratios of molecular diffusivity to molecular viscosity, i.e, the Prandtl and Schmidt numbers. Since the Schmidt number is much smaller, salt diffusivity ultimately controls the melt rate. The same principle, carried over to rapid freezing, implies supercooling and enhanced heat flux under thin, growing ice. During SHEBA an intensive observation period experiment was carried out during October, 1997, on half-meter thick ice in a recently frozen lead that served as the station runway. Turbulent heat and momentum flux were measured 2 and 4 m below the runway ice. In general, turbulence levels were much lower at the shallower level. Temperature gradients in ice of similar age and thickness at site Baltimore implied an appreciable conductive upward flux in the ice, yet no enhancement of ocean heat flux was observed relative to similar measurements under rough ice at the main SHEBA turbulence mast. The results suggest that any tendency for supercooling by double diffusive effects is relieved very close to the ice, with little impact on overall ice/ocean heat exchange.
