Turbulent surface fluxes and air–ice coupling in the Baltic Air–Sea–Ice Study (BASIS)

Author： Launiainen Jouko Cheng Bin Uotila Juha Vihma Timo

Publisher： International Glaciological Society

ISSN： 1727-5644

Source： Annals of Glaciology, Vol.33, Iss.1, 2001-01, pp. : 237-242

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Abstract

Turbulent surface fluxes were studied using observations taken over sea ice in the Baltic Sea in March 1998. The fluxes of momentum and sensible heat were measured by a sonic anemometer and compared with fluxes derived from wind velocity and air-temperature profiles. The neutral 10 m drag coefficient showed no apparent dependence on wind speed (in the range 2–20 m s⁻¹), resulting in a mean value of 1.0 × 10⁻³ for smooth snow-covered ice and 1.5 × 10⁻³ for deformed ice. The overall mean value was 1.28 × 10⁻³. The roughness length for temperature revealed a greater apparent dependence on wind speed and was slightly larger than the aerodynamic roughness for low wind speeds, and vice versa for moderate and high winds. We give an empirical expression that predicts how the scalar roughness depends on the aerodynamic roughness (drag coefficient) and wind speed. Agreement of the gradient-method results with the eddy-flux results supports the validity of the Monin–Obukhov similarity theory. Fluxes modelled by a coupled air–ice–sea model compared well with the eddy-flux and gradient methods. Surface temperature estimates by the three methods also agreed well. Tests and sensitivity analysis emphasize the need for especially accurate sensor calibration and strict information about the sensor heights for the gradient method.