Investigation of the stable boundary layer over Greenland, results from the aircraft-based experiment IGLOS

Recent comparisons of current climate models have shown that an adequate representation of all atmospheric processes in high latitudes is still lacking. Among others, the processes in the stable boundary layer (SBL) still remain a challenge to numerical models. The deficits in existing parameterizations and available SBL data sets demand for further experimental studies on the SBL over snow surfaces including the overlying lower part of the free atmosphere.

In order to create a comprehensive dataset for validation and improvement of SBL parameterizations, the experiment IGLOS (Investigation of the Greenland boundary Layer Over Summit) was conducted in Summer 2002 over the Greenland Summit area. The aircraft-based measurements are evaluated in conjuction with other measurements made at Summit Camp, especially radiation fluxes and turbulent quantities measured at the 50m-tower of the ETH Zurich.

During IGLOS, six flight missions were carried out under quite different synoptic conditions. Well-developed stable boundary layers were found in all cases. The SBL heights were mostly below the values suggested from previous studies. The surface inversion thickness did not exceed roughly 100m, even under high wind conditions.

The vertical velocity spectra show a double-peak structure representing wave motions (and aircraft movements across strong gradients) at longer wavelengths and small-scale turbulence at shorter wavelengths. The frequency of the spectral gap matches Brunt-Vaisala frequency estimations from measured vertical gradients.

The measured values of the small-scale variances of temperature, humidity and vertical wind speed are rather small and correspond to only approximately 10% of the variances in the longwave part of the spectra.

The vertical mixing of the atmosphere was very weak, although a vertical wind shear of up to 0.15 1/s was present. Vertical turbulent transports did exhibit very small values of mostly less than 10 W/m². Significant turbulent fluxes were found only intermittently over flight sections of the order of a few kilometers. The turbulent height of the SBL turned out to be much smaller than the surface inversion thickness. Vertical profiles of radiation measurements suggest that longwave radiation divergence significantly contributed to the SBL energy budget.