Improvement of the bulk formula in weak-wind conditions

The breakdown of surface-layer similarity theory in weak-wind conditions has been attributed to the effect of wave state and the importance of large boundary-layer eddies and mesoscale motions. We examine the influence of mesoscale motions on the turbulence fluxes for stable conditions. Analysis of CBLAST Weak Wind data indicates that the average velocity variance due to mesoscale motions at the CBLAST Weak Wind site is comparable to that over flat land surfaces.

Numerous problems compromising evaluation of the bulk formula for weak turbulence conditions are summarized. The mesoscale motions, mostly of unknown origin, lead to nonstationary distortion of the wind profiles in the surface layer and in the underlying wave boundary layer. Although the various interactions are too complex to physically isolate, it is possible to construct an improved formulation for surface turbulent fluxes. The importance of meandering motions on horizontal dispersion is also investigated for this site.