A Convective Transport Theory for Surface Fluxes

Abstract

For a boundary layer in free convection where turbulent thermal structures communicate information between the surface and the interior of the mixed layer, it is hypothesized that the surface momentum flux can be parameterized by u*2 = bDwBMML, the heat flux by w???s = bHwB(?skin??ML), and the moisture flux by w?r?s = bHwB(rskin?rML). In these expressions u* is the friction velocity, M is mean wind speed, ? is potential temperature, r is mixing ratio, subscript ML denotes the interior of the mixed layer, and subscript skin denotes the characteristics of the underlying solid or liquid surface. A buoyancy velocity scale is defined by wB≈[(g/?v)zi(?vskin??vML)]½, where zi is mixed-layer depth, ?v is virtual potential temperature, and g is gravitational acceleration. Using data from the BLX83 field experiment in Oklahoma (roughness length: 0.05 m, latitude: 35.03°N, vegetation: mixed pasture and crops, season: spring), the convective transport coefficients are empirically found to be bH = 5.0?10?4 for heat and moisture, and bD=1.83 ? 10?3 for momentum. These parameters worked well when tested against independent data from the Australian Koorin field experiment (roughness length: 0.4 m, latitude: 16.27°S, vegetation: uniform sparse trees, season: winter). If these parameterizations and coefficient values are validated for other sites, then convective transport theory could be considered as a candidate to replace the resistance law similarity theory based on profile matching, for conditions of free convection. The theory is extended to include near-free convective conditions in which mechanical transport associated with. mean wind shear contributes to the still dominant buoyant transport. Scaling variables such as Obukhov length are rewritten using the convective transport relationships, which could potentially be used in similarity theories to compute other surface-layer and mixed-layer quantities.