Near-Surface Vertical Flux Divergence in the Stable Boundary Layer

Abstract

Flow in the stable boundary layer is examined at four contrasting sites with greater upwind surface roughness. The surface heterogeneity is disorganized and in some cases weak as commonly occurs. With low wind speeds, the vertical divergence (or convergence) of the momentum and heat fluxes can be large near the surface in what is normally assumed to be the surface layer where such divergence is neglected. For the two most heterogeneous sites, a shallow “new” boundary layer is captured by the tower observations, analogous to an internal boundary layer but more complex. Above the new boundary layer, the magnitudes of the downward fluxes of heat and momentum increase with height in a transition layer, reach a maximum, and then decrease with height in an overlying regional boundary layer. Similar structure is observed at the site with rolling terrain where the shallow new boundary layer at the surface is identified as cold-air drainage generated by the local slope above which the flow undergoes transition to an overlying regional flow. Significant flux divergence near the surface is generated even over an ice floe for low wind speeds and in a shallow Ekman layer that forms during the polar night. For higher wind speeds, the magnitude of the downward fluxes decreases gradually with height at all levels as in a traditional boundary layer.