Analysis of Mesoscale Linear Features Observed in the Arctic Atmospheric Boundary Layer

Abstract

The existence of synoptically distributed, coherent, linear mesoscale features with wavelengths of 12?18 km in a shallow (zl ≈ 150 m) atmospheric boundary layer is documented. These were observed north of Alaska over the ice-covered Beaufort Sea during the LEADEX program in April 1992. These banded features appear both in satellite infrared (but not visible) images and in concomitant in situ aircraft measurements. Those measurements were of cloud condensation nuclei (CCN), potential temperature (?), and meridional velocity (v) and were taken within and above the arctic atmospheric boundary layer. These 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 slantwise 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 profile in the boundary layer. The strongest arguments for slantwise convection are as follows: 1) a significant meridional (cross-band) flux of beat 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, consistent with convection in the boundary layer; and 4) generally weak correlations between lead signals in a downward-looking radiometer and ?, v, and CCN. The data also suggest that at least the influence of the circulations can reach up beyond the well-mixed boundary layer into the stable, lower troposphere. (However, this signal cannot be dismissed as, nor definitely identified with, gravity waves.) It is noted that if slantwise convection is present as described, then it represents 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 heat, momentum, and particulates. This is in addition to large leads and shear-generated turbulence in the boundary layer, both of which create vertical mixing in the Arctic's lower atmosphere that is spatially and temporally intermittent.