ROLL VORTICES AND BOUNDARY-LAYER DEVELOPMENT DURING A COLD-AIR OUTBREAK

The development of the boundary layer during a cold air outbreak in th e Fram Strait is documented by aircraft measurements. The convection w as organised into roll vortices with aspect ratios increasing from 2.9 near the ice edge to more than 6 at 100 km further downstream. This i ncrease coincides with an increase of the latent heat release in the c loud layer. The stability parameter -z(i)/L varies from about zero at the ice edge to 30 at a distance of 200 km downstream over open water where the satellite picture still shows cloud streets. The increase is mainly due to the deepening of the boundary layer. The turbulent vert ical sensible and latent heat fluxes near the surface amount to 400 W m(-2) within a 300 km off-ice zone. 25% of the upward heat flux in the subcloud layer is carried out by organised roll motions. Experiments with a 2-dimensional non-hydrostatic model show a similar roll aspect ratio in the first 50 km, but further downstream where condensational heating is more important the modelled roll wavelengths are distinctly smaller than the observed ones.