A comparison of the ECMWF forecast model with observations over the annual cycle at SHEBA

Abstract

A central objective of the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment was to provide a comprehensive observational test for single-column models of the atmosphere-sea ice-ocean system over the Arctic Ocean. For single-column modeling, one must specify the time-varying tendencies due to horizontal and vertical advection of air through the column. Due to the difficulty of directly measuring these tendencies, it was decided for SHEBA to obtain them from short-range forecasts of the European Centre for Medium-Range Weather Forecasts (ECMWF) global forecast model, into which SHEBA rawinsonde and surface synoptic observations were routinely assimilated. The quality of these forecasts directly affects the reliability of the derived advective tendencies. In addition, the ECMWF-forecast thermodynamic and cloud fields, and radiative and turbulent fluxes present an illuminating comparison of the SHEBA observations with a state-of-the-art global numerical model. The authors compare SHEBA soundings, cloud and boundary layer observations with the ECMWF model output throughout the SHEBA year. They find that above the boundary layer, the model was faithful to the SHEBA rawinsonde observations and maintained a proper long-term balance between advective and nonadvective tendencies of heat and moisture. This lends credence to use of the ECMWF-predicted advective tendencies for single-column modeling studies. The model-derived cloud properties and precipitation (which were not assimilated from observations) are compared with cloud radar, lidar, microwave radiometer, surface turbulent and radiative measurements, and basic surface meteorology. The model s slab sea-ice model led to large surface temperature errors and insufficient synoptic variability of temperature. The overall height distribution of cloud was fairly well simulated (though somewhat overestimated) in all seasons, as was precipitation. However, the model clouds typically had a much higher ratio of cloud ice to cloud water than suggested by lidar depolarization measurements, and a smaller optical depth, leading to monthly biases of up to 50 W m^(-2) in the monthly surface downwelling longwave and shortwave radiation. Further biases in net radiation were due to the inaccurate model assumption of constant surface albedo. Observed turbulent sensible and latent heat fluxes tended to be small throughout SHEBA. During high-wind periods during the winter, the ECMWF model predicted sustained downward heat fluxes of up to 60 W m^(-2), much higher than observed. A detailed comparison suggests that this error was due to both inadequate resolution of the 31-level model and a deficient parameterization of sea-ice thermodynamics.