Microscale, spatial variation

Quantifying microscale spatial variability. A simple quantification of the degree of microscale variability observed for a given 2D microscale distribution was expressed as the ratio between maximum and minimum values of the distributions, considered as an estimate of the maximum variability (Seuront and Spilmont 2002), and the coefficient of variation CV (CV = SD/x, wherex and SD are the mean and the standard deviation, respectively) estimated for that range. 

Saffinan and Turner (43) considered collisions between droplets due to turbulence in rain clouds. Under turbulent conditions, droplet collision is governed by two different mechanisms isotropic turbulent shear and turbulent inertia. The choice of regime applicable to a droplet is determined by its size in relation to the Kolmogorov microscale denned earlier. Droplets of diameter d > t are subjected to die former of these processes (small-scale motion). Spatial variations in the flow give neighboring droplets different velocities and fliis result in collisions. Droplets of diameter d > T] are subjected to turbulent inertia. In this case, collisions result from the relative movement of droplets in the surrounding fluid. Droplets of different diameter will have different inertias and this results in collisions. Droplets of equal diameter, however, will not collide under this mechanism as fliey have the same inertia. 

In a study of horizontal and vertical microscale variations in PM,o and total suspended particulates in Taipei (22), particulate concentrations measured at open windows of a high-rise building decreased with height between the second and seventh floors but showed no consistent change between the seventh and fourteenth floors. At street level, no consistent patterns in spatial variation were found between the roadside, sidewalk, and covered walkway along a high-traffic main road, nor between main streets, side streets, and alleys. The absence of typical dispersion patterns could be due to emissions from sources such as motorcycles which were not confined to streets. 

In contrast to diffusion, mechanical dispersion is attributed to variations in advective velocities over a wide range of spatial scales. On the microscale, velocity... 

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