Roll convection within an Arctic cold-air outbreak: Interpretation of in situ aircraft measurements and spaceborne SAR imagery by a three-dimensionalatmospheric model

Atmospheric roll convection within an Arctic cold-air outbreak was observed over the Greenland Sea during the ARKTIS 1993 experiment on 24 March 1993 by in situ aircraft measurements and synthetic aperture radar (SAR) imagery from the first European Remote Sensing satellite (ERS-1). Inside a boundar y layer heated from below, two kinds of rolls were observed, one aligned pa rallel and the other perpendicular to the mean wind direction. The wind-par allel rolls occupied the entire boundary layer, whereas the wind-perpendicu lar rolls were confined to a region around the top of the boundary layer, w here a strong vertical shear in the downstream wind component was observed. A three-dimensional numerical model has been applied to simulate the observ ed convective pattern. It is shown that the model does not reproduce the ob served pattern when using a height-constant geostrophic wind profile. Howev er, when adjusting the vertical wind profile to the one measured from the a ircraft, the model reproduces buoyancy-driven wind-parallel boundary layer rolls whose aspect ratio, orientation, and circulation velocity agree well with the corresponding characteristics of the observed rolls. The model cal culations show further that shear-driven rolls aligned perpendicular to the buoyancy-driven rolls are generated at the top of the boundary layer. Insi de the boundary layer, circulations associated with the shear-driven rolls are suppressed by the buoyancy-driven rolls. The near-surface wind field de rived from the ERS-1 SAR image agrees well with the one derived from the mo del.