Agricultural and Forest Meteorology 106 (2001) 147–152 Dependence of kB −1 factor on roughness Reynolds number for barley and pasture Meelis Mölder ∗ , Anders Lindroth Department of Physical Geography, Lund University, P.O. Box 118, SE-221 00 Lund, Sweden Received 28 February 2000; received in revised form 28 June 2000; accepted 19 July 2000 Abstract One of the surface energy balance components, the sensible heat flux, can easily be described for modelling and remote sensing purposes by means of the thermal roughness length (z ot ). This roughness length is usually expressed relative to the roughness length for wind speed (z ou ): kB −1 = ln(z ou /z ot ). In the past, the kB −1 factor has been taken as a constant having a value of ca. 2 in the case of homogeneous canopies. There is some theoretical evidence that kB −1 should depend on friction velocity (u ∗ ) or roughness Reynolds number (Re o ). In this study, previously published barley and pasture data were re-evaluated. It was shown that the reciprocal of Stanton number, which is a part of kB −1 , could be expressed commonly for both surfaces. The re-evaluated kB −1 factor took the form: kB −1 = 0.37Re 0.3 o −(1.2 or 1.9) (for barley and pasture, respectively). © 2001 Elsevier Science B.V. All rights reserved. Keywords: Sensible heat flux; Roughness length; Stanton number; Surface radiation temperature; Field experiment 1. Introduction One of the surface energy balance components, the sensible heat flux, can easily be described by means of the roughness length concept. The evaporation term can then often be found as a residual. This is a simple one-layer canopy approach that contains the basic physics but keeps the number of needed parameters low. The approach is well justified for closed canopies where the soil contribution to fluxes is small; and it is particularly useful or unavoidable in atmospheric boundary layer studies where the canopy is merely a lower boundary, in regional and global modelling where only processes at large scales are of interest and resource-demanding computations are ∗ Corresponding author. Fax: +46-46-2224011. E-mail address: meelis.molder@natgeo.lu.se (M. Mölder). required and also in remote sensing where airborne or satellite measured surface temperature provides a direct link to the sensible heat flux. Since the momentum exchange is dominated by form drag but the heat transfer takes place by molec- ular diffusion then normally there is higher resistance to the heat exchange than to that for momentum, the difference being expressed via the kB −1 factor or alternatively by using different roughness lengths for wind speed (z ou ) and temperature (z ot ). The latter usually being smaller. Theory predicts that the loga- rithmic ratio of the two roughness lengths, ln(z ou /z ot ) or kB −1 , depends on the roughness Reynolds number, Re o (Zilitinkevich, 1970). The roughness length z ou is used as a characteristic length, and friction velocity u ∗ as a characteristic velocity to form Re o as u ∗ z ou /ν (ν is the viscosity). The relationship is given in the form of a power function: kB −1 ∼ Re n o , which can be 0168-1923/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0168-1923(00)00200-8