Continuum or neighbourhood scale dispersion models

Typically these regions might include 103 individual obstacles. 

The key physical mechanisms of dispersion on this scale are the differences in mean wind speed and velocity (Uc), Uh within and above the canopy. In porous canopies, the mean wind speed normalised on Uh, i.e. (Uc/Uh) is greater than the turbulence intensities ctv/Uh, o-w/Uh 1/10, so that the cloud/plume is advected by the main wind within as well as above the canopy. In addition the topological and wake dispersion processes (described in Section 2.4.2) are as significant as turbulent eddying for dispersing matter both horizontally and vertically within the canopy. 

Note that the scales of the largest horizontal eddying motions of the turbulence within the canopy are of the scale of the obstacles ( d) and are much smaller than those in undisturbed boundary layers (which recent research show range up to 2 km, Hogstrom et al., 2002 [275]). 

The continuum scale is an intermediate range in terms of flow modelling, in that some methods are based on detailed or discrete modelling to predict average properties of dispersion, while others are based on continuum modelling approaches using aver-age/statistical descriptions of the aggregated properties of the canopy and the flow (see Table 2.7). 

If such an approach becomes widespread it will be even more necessary to calibrate and understand its merits and drawbacks by using detailed and accurate computational modelling techniques that have been thoroughly validated, such as Large Eddy Simulation methods (Rodi, 1997 [541]), stochastic simulation methods (Hort et al., 2002 [276] Turfus, 1988 [622]), and time-dependent Reynolds-averaged models. 

---