Near-Surface Vertical Flux Divergence in the Stable Boundary Layer
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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.
KeywordsInternal boundary layer Nocturnal boundary layer Roughness change Stable boundary layer Vertical flux divergence
We gratefully acknowledge the extensive comments of the reviewers that led to major improvements in the manuscript. Discussions with Ivana Stiperski significantly improved our perspective on the impact of sloped terrain on flux measurements. This project received support from Grant AGS-1614345 from the National Science Foundation. The Earth Observing Laboratory of the National Center for Atmospheric Research provided the measurements from the FLOSSII and SCP campaigns. We acknowledge the hard work by scientists and staff involved in collection of the SHEBA turbulence data, especially Christopher Fairall, Peter Guest, and the late Ed Andreas. The SHEBA data collection and analysis was supported by Grants OPP-97-01766 and OPP-00-84323 from the U. S. National Science Foundation. OP and AG were supported by funds from the National Oceanic and Atmospheric Administration/Earth System Research Laboratory/Physical Sciences Division during the preparation of this manuscript. Emily Andreas Moynihan prepared Fig. 5.
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