Flows over Idealized Vegetation Canopy with Roughness Sublayer Correction: a Laboratory Experiment on the Turbulent Transport Mechanism

Abstract

Roughness sublayer (RSL) is the interface in-between roughness elements at the bottom and the inertial sublayer (ISL) aloft. Its dynamics therefore tightly influences the transport processes for atmospheric constituents from the ground level to the core of atmospheric boundary layer (ABL), which, however, has not been fully addressed yet. In this paper, analytical solution to the RSL mean wind profile, which is applicable to both RSL and ISL, is derived based on the theory of rough-surface flows. A series of wind tunnel experiments for flows over idealized vegetation canopy are then adopted to test the functionality of the newly derived analytical mean-wind profile. Hot-wire anemometry (HWA) are used to sample the velocities over arrays of reduced-scale plastic trees of different spatial densities. The analytical solution agrees well with the wind tunnel results that is extended from mean flows to turbulent momentum flux to elucidate the transport mechanism over vegetation canopies. While the thickness of turbulent boundary layer (TSL) is about the same, it is found that the thickness of RSL and ISL is a function of the aerodynamic resistance of vegetation canopies. Examination the dynamics in details reveals that the flows over vegetation canopies exhibit turbulent motions of two different length scales (in the wall-normal direction) which are functions of the RSL and ISL structure. Generally, the denser the vegetation canopy, the higher is the RSL intruding upward into the ISL together with enhanced vertical transport. The conventional log-law approximation to the flows in the atmospheric surface layer should be applied cautiously over (rough) urban canopies.