Effect of sub-layer corrections on the roughness parameterization of a Douglas fir forest

This study investigated two aerodynamic methods to estimate the momentum roughness length and displacement height (z0 and d0 respectively) of a 32 m-tall Douglas fir forest using simultaneous micrometeorological and flux measurements. When the flux–gradient method was used to objectively determine z0 and d0, corrections for roughness sub-layer effects proved to be important. A new iterative method is employed to solve the set of equations when the corrections were made. The estimated average values of z0 and d0 with corrections vary from 1.2 m to 1.4 m and from 26.2 m to 28.4 m respectively depending on the correction method applied. In the absence of experimentally determined roughness sub-layer height, the corrections of Harman and Finnigan (2007) yielded the best overall estimates of aerodynamic parameters. Comparison with results of over 25 other studies has shown that the results obtained here fit the general trend rather well. The relationship between displacement height d0 and mean tree height hc is well described by the quadratic equation: View the MathML source. Similarly the quadratic equation: View the MathML source appears to be the best to illustrate the relation between aerodynamic canopy height ha and hc. These simple relationships can be easily incorporated to large scale land surface models, provided that spatially distributed canopy height information is available. Furthermore the alternative flux–variance technique cannot be objectively used to estimate z0 and d0 as no explicit method exists to select the exact value for coefficient C1, which mainly causes the variance method to compute systematically higher or lower sensible heat flux.