Publication

The objective of this chapter is to examine the role of the horizontal eddy flux and its horizontal divergence in the surface-layer mass conservation. It is shown that the horizontal flux is generated by gradient diffusion and by a correlation between the vertical and horizontal velocities, the latter of which is the dominant mechanism in the roughness sublayer over a forest, where the shear stress is large. Thus, a horizontal flux divergence of the scalar of interest can arise from a streamwise gradient of its vertical flux and/or the momentum flux. An example is given for the temperature flux divergence over a forest on rolling terrain to illustrate that it is possible to measure experimentally the divergence in non-ideal conditions. Numerical simulations using an advection-diffusion model show that a compensating mechanism exists between horizontal advection and the horizontal flux divergence and that experimental attempts to correct the vertical eddy flux for either horizontal advection or the flux divergence alone do not necessarily lead to a complete surface-layer mass/energy-balance closure. In the appendix, an approximation equation is presented that allows the horizontal flux divergence to be calculated, with measurements made by a vertical array of sensors on a single tower.