The behaviour of the roughness length for temperature over heterogeneous surfaces

The definitions of roughness length and zero-plane displacement are examined, as well as the relationship of these quantities to the resistance of transfer processes close to a surface. It is demonstrated that, if molecular diffusion or free convection processes are dominant within the interfacial sub-layer, then the roughness length for temperature z(OT) can be expected to decrease with increasing wind speed. Over homogeneous surfaces this sensitivity to wind speed is reduced by the relative efficiency of heat transfer, but over sparsely vegetated surfaces the sensitivity is greatly increased. The problem of correct measurement of surface temperature over a heterogeneous surface is outlined and examined; it is demonstrated that use of the area-averaged radiometric temperature as a representative surface temperature can lead to consequences such as zero, negative or infinite apparent transfer resistances. Measurements of z(OT) are presented from two field experiments over sparsely vegetated surfaces; typical values of z(OT) are around 10(-10) m, and vary by orders of magnitude over a quite moderate range of wind speed. These measurements are interpreted in terms of the behaviour of interfacial sub-layer transfer processes; it is shown that a significant cause of the low measured value of z(OT) may be the use of radiometric surface temperature as a representative surface temperature.