Variation in zero plane displacement and roughness length for momentum revisited

Authors

Ashvath Singh Kunadi
Richard P. Silberstein, Edith Cowan UniversityFollow
Sally E. Thompson

Document Type

Journal Article

Publication Title

Boundary-Layer Meteorology

Volume

190

Issue

8

Publisher

Springer

School

School of Science

Funders

University of Western Australia

Comments

Kunadi, A. S., Silberstein, R. P., & Thompson, S. E. (2024). Variation in zero Plane displacement and roughness length for momentum revisited. Boundary-Layer Meteorology, 190(8), 36. https://doi.org/10.1007/s10546-024-00876-8

Abstract

Zero plane displacement height (d0) and momentum roughness length (z0m), describe the aerodynamic characteristics of a vegetated surface. Usually, d0 and z0m are assumed to be constant functions of the physical characteristics of the surface. Prior evidence collected from the literature and our examination of flux tower data show that d0 and z0m vary in time at sites with tree and shrub canopies, but not grasslands. The conventional explanations of these variations are based on linear functions of wind velocity and friction velocity, with little theoretical basis. This study explains the variation in aerodynamic parameters by matching four analytical canopy velocity models to a logarithmic above-canopy velocity profile at canopy height. d0 and z0m come out as functions of 2 non-dimensional terms, the canopy momentum absorption capacity (parameter) and a (measurable) Péclet number. To test the theories of variation, we analysed the velocity profiles from Ozflux and Ameriflux sites. None of the theories could recreate d0 and z0m at half-hourly intervals. However, the canopy velocity models were able better to recreate the distribution of the variations in d0 and z0m. Additionally, the estimates of canopy momentum absorption capacity varied consistently with phenological changes in the canopies, whereas, the fitting parameters of the linear regression of using wind speed and friction velocity did not exhibit physically interpretable variations. The canopy velocity models may offer better predictions with an accurate estimation of the canopy height, a horizontally homogeneous and rigid canopy, and incorporation of the roughness sublayer.

DOI

10.1007/s10546-024-00876-8

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