Lagrangian time scales of the turbulence above two-dimensional canopies

Lagrangian statistics are obtained from a water-channel experiment of an idealized two-dimensional urban canopy flow in neutral conditions. The objective is to quantify both the streamwise and vertical components of the Lagrangian time scale of the turbulence (TuLand TwL, respectively) above the canopy as well as to investigate their dependence on the aspect ratio of the canopy, AR, as the latter is the ratio of the width (W) to the height (H) of the canyons constituting the canopy. It is found that TwLfollows Raupach’s linear law within the constant flux layer in both the canopies analysed (AR=1 and AR=2). The same holds true for TuLbut only when AR=1, while for AR=2 it follows Raupach’s law for z>2H (here z is the height). Below that level, TuLis nearly constant with height, showing at z=H a value approximately one order of magnitude greater than the one found for AR=1.