A proper modeling of flow and turbulence within and over
urban canopies is key to properly predict air pollution
and dispersion in cities. Trees are an integral part of
the urban landscape. In many suburban neighborhoods, tree
cover is 10 to 30% and trees are often taller than
buildings. The effect of trees on drag, mean wind and
turbulence in cities is not accounted for in current
weather, air pollution and dispersion models. Our goal is
to use high-resolution Large Eddy Simulations (LES) over
a realistic urban canopy to inform about the effects of
trees drag, mean wind and turbulence in the urban
roughness sublayer (RSL). The simulated area is part of
the Sunset-Neighborhood in Vancouver, Canada. In this
area, long-term wind and turbulence measurements are
available from instruments on a 28m-tall tower. Further,
a high precision airborne Light Detection and Ranging
(LiDAR) point cloud provides data to represent both
buildings and trees at high spatial resolution in a
realistic configuration. Trees are described by
location-specific leaf area density (LAD) profiles. LES
simulations are performed over a 512 x 512m
characteristic subset of the city that contains the tower
location and source area. In the LES, buildings are
accounted for through an immersed boundary method,
adopting a zero level-set distance function to localize
the surface location, whereas drag forces from trees are
parametrized as a function of the height-dependent LAD.
Spectra of streamwise and vertical velocity components
compare well between tower data and the model data,
confirming the good performances of LES in simulations of
flow over fully rough surfaces. We show how the presence
of trees affects mean velocity and computed momentum flux
profiles, significantly decreasing dispersive terms in
the bulk of the flow, which are usually found to play a
role in pressure driven boundary layer flows. The impact
of trees on integral length scales in the flow is
discussed.