ANALYSIS OF MESOSCALE LINEAR FEATURES OBSERVED IN THE ARCTIC ATMOSPHERIC BOUNDARY-LAYER

The existence of synoptically distributed, coherent, linear mesoscale features with wavelengths of 12-18 km in a shallow (z approximate to 1 50 m) atmospheric boundary layer is documented. These were observed no rth of Alaska over the ice-covered Beaufort Sea during the LEADEX prog ram in April 1992. These banded features appear both in satellite infr ared (but not visible) images and in concomitant in situ aircraft meas urements. Those measurements were of cloud condensation nuclei (CCN), potential temperature (theta), and meridional velocity (upsilon) and w ere taken within and above the acetic atmospheric boundary layer. Thes e aircraft data also exhibit smaller-scale circulations with scales of 3-8 times the boundary layer depth. Based on analysis of our dataset, we argue that the 12-18-km motions may be due to a hybrid form of sla ntwise convection within the boundary layer. The authors use the term hybrid because some of the energy, scale selection, and orientation of the linear features may be modulated by the nonlinear mean wind profi le in the boundary layer. The strongest arguments for slantwise convec tion are as follows: 1) a significant meridional (cross-band) flux of heat and CCN, 2) long-wavelength, two-dimensional circulation patterns aligned perpendicular to the strong, horizontal temperature gradient; 3) thin, warm bands parallel to thick, cold bands in the IR image, co nsistent with convection in the boundary layer and 4) generally weak c orrelations between lead signals in a downward-looking radiometer and theta, upsilon, and CCN. The data also suggest that at least the influ ence of the circulations can reach up beyond the well-mixed boundary l ayer into the stable, lower troposphere. (However, this signal cannot be dismissed as, nor definitely identified with, gravity waves.) It is noted that ii slantwise convection is present as described, then it r epresents another mechanism with mesoscale organization over synoptic- scale regions by which the Arctic's atmospheric boundary layer and the overlying, stably stratified lower troposphere may exchange hear, mom entum, and particulates. This is in addition to large leads and shear- generated turbulence in the boundary layer, both of which create: vert ical mixing in the Arctic's lower atmosphere that is spatially and tem porally intermittent.