Premium
The behaviour of the roughness length for temperature over heterogeneous surfaces
Author(s) -
Malhi Yadvinder
Publication year - 1996
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.49712253305
Subject(s) - surface roughness , roughness length , surface finish , materials science , wind speed , heat transfer , displacement (psychology) , convection , diffusion , mechanics , optics , meteorology , thermodynamics , physics , composite material , wind profile power law , psychology , psychotherapist
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 0T 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 0T are presented from two field experiments over sparsely vegetated surfaces; typical values of z 0T 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 0T may be the use of radiometric surface temperature as a representative surface temperature.