Horizontal Eddy Diffusion Coefficients Kxx and Kyy

We have seen that when a- varies as t the cro.sswind eddy diffusion coefficient is related to the variance of plume spread by 

Measurements of T/ in the atmosphere are extremely difficult to perform and it is difficult to establish whether the condition t Ti holds for urban scale flows. Csanady (1973) indicates that a typical eddy that is generated by shear flow near the ground has a Lagrangian timescale on the order of 100 s. Lamb and Neiburger (1971), in a series of measurements in the Los Angeles Basin, estimated the Eulerian timescale T[, to be about 50 s. In a discussion of some field experiments, Lumley and Panofsky (1964) suggested that Ti, ATf. . If the averaging interval is selected to be equal to the travel time, then an approximate value for A, can be deduced from the measurements of Willis and Deardorff (1976). Their data indicate that for unstable conditions (L 0) and a travel time t = 

Employing the previous travel time estimate and combining this result with (18.61) gives 

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