Momentum transfer by a mountain meadow canopy: a simulation analysis based on Massman's (1997) model

摘要

Using a mountain meadow as a case study it is the objective of the present paper to develop a simple parameterisation for the within-canopy variation of the phytoelement drag (C_d) and sheltering (P_m) coefficients required for Massman's model of momentum transfer by vegetation. A constant ratio between C_d and P_m is found to overestimate wind speed in the upper canopy and underestimate it in the lower canopy. Two simple parameterisations of C_d/P_m as a function of the plant area density and thecumulative plant area index are developed, using values optimised by least-squares regression between measured and predicted within-canopy wind speeds. A validation with independently measured data indicates that both parameterisations work reliably forsimulating wind speed in the investigated meadow. Model predictions of the normalised zero-plane displacement height and the momentum roughness length fall only partly within the range of values given in literature, which may be explained by the accumulation of plant matter close to the soil surface specific for the investigated canopies. The seasonal course of the normalised zero-plane displacement height and the momentum roughness length are discussed in terms of the seasonal variation of the amount and density of plant matter.