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<br />~~ <br />Turbulence is characterized by non regular motions which have not yet <br />been defined by numerical models with complete fluid and thermodynamic equations. <br />For the purposes of selecting appropriate numerical quantities which describe <br />the mixing characteristics of the atmosphere over flat terrain, the classifica- <br />tion scheme developed by Pasquill is generally applied. (see Table 2.33 ) <br />However, referring to Table 2.3- 2 field observations of diffusion over complex <br />terrain have demonstrated that turbulence, and therefore turbulent diffusion is <br />enhanced significantly above those values given in Table 2.33 for conditions <br />over flat terrain. <br />Recent attempts to quantify the airflow patterns over complex terrain, <br />and field validation of these numerical models have resulted in verification <br />of enhanced mixing resulting from the dominating effects of terrain. The <br />conclusions drawn from a field validation study using a numerical model <br />developed at the Rocky Mountain Forest and Range Experiment Station, and <br />evaluating flow characteristics with a north-south valley axis are given below. <br />"Local thermally driven flows such as mountain and valley <br />winds are simulated reasonably well .. These thermally <br />driven flows are determined properly primarily because the <br />thermal forcing processes are stationary and locked to the <br />terrain features Thermal modification of potential <br />flow across the terrain features dominates the driving forces." <br /> <br />2-19 <br />