The traditional way to account for tall canopies in surface layer meteorology is to include a displacement height, and a roughness length. Canopies are porous volumes with undulating tops that contain a variety of sources and sinks. Understanding the vertical and horizontal structure in canopies may also be important.
Additional measures of canopy structure are now becoming more readily available: Canopy cranes, used as observation platforms from which to undertake in situ canopy structure sensing; remote sensing instruments, such as SLICER, to obtain information about canopy structure. Potentially important structural properties include: a) Better estimates of the vertical plant area index profile (PAI);
b) Improved measurements of canopy top topography; and c) Better estimation of spacing and sizes of gaps. We address a) and b) in this presentation.
In canopies, local sources and sinks of momentum and scalars have an important influence on the mean state. Transport terms, both mean and turbulent, also significantly alter the mixing environment. We use vertical profiles of turbulence and mean state data from several forests and canopy structure data obtained remotely and in situ to examine what effects canopy structure may have on mixing characteristics above and within forests. Two idealizations are considered:
a). Approximate horizontal homogeneity. The dominant structural characteristic is PAI(z). We present profiles of light extinction and corresponding PAI(z). Properties of the turbulence are related to the structural properties of the canopy through the turbulent kinetic energy (TKE) equation. Light interception alters the mean temperature profile and hence the buoyant production term. Presence of leaves enhances turbulent dissipation. We present data to illustrate how seasonally changing PAI(z) alters the mean temperature profile, the turbulent spectra and the TKE budget inside midlatitude deciduous forests.
b) Horizontal inhomogeneity of canopy top also affects the vertical transports. It is known that flux-gradient relationships above forests are different from those appropriate to flat terrain within a "roughness sublayer ", extending to 2-3 canopy heights. In this layer, turbulent advection of turbulence is important in the turbulent kinetic energy (TKE) balances. We present measurements of these terms above several forests.
We then apply concepts developed to describe flow over hills Here, mean horizontal flow advects turbulence over high spots in the canopy, affecting both mean quantities and their higher moments. We estimate the likely range of variability in mean and turbulent flux measurements that may be due largely to the directly-measured "rugosity" of canopy top.