Laserfiche WebLink
<br />~ <br /> <br />D Coriolis forces - The effects of coriolis acceleration on air <br />motion were neglected on the basis that the field or prototype regions <br />were relatively small (L-40 to 50 km). In this case the inertial <br />effects of the air motion were expected to predominate over coriolis <br />effects. Local terrain effects were expected to contribute in decreasing <br />the effect of the earth's rotation. <br />This assumption was not strictly valid for the San Juan Mountain <br />area which covered a larger areal extent. <br />2) Steady-state conditions - In the physical models it was <br />assumed that velocity, temperature or density fields were in steady- <br />state. This was a good approximation for the neutral airflow but in <br />the case of the barostromatic airflow the temperature and velocity <br />fields over the topographic models were unsteady with time. <br />3) Uni-directional upper-level flow - In the physical models the <br />upper-level flow above the terrain was considered uni-directional or <br />that little directional wind shear occurs due to a thermal wind or <br />horizontal temperature gradients. However, in the barostromatic air- <br />flow this assumption was not totally valid because of horizontal and <br />vertical temperature gradients. In addition, directional wind shear <br />could occur due to irregular terrain. <br />4) Radiation and cloud system effects - The weather situation of <br />interest is storm periods with fairly extensive cloud cover over the <br />region. Therefore, the various heat fluxes due to the sun, atmosphere <br />and earth were assumed negligible. However, in the case of the <br />barostromatic airflow the turbulent transfer of sensible heat was of <br />some importance. Thermodynamic and compressibility effects due to <br />cloud systems could not be simulated. <br />In the Elk Mountain stud~ field data were generally obtained <br />under cloudless sky conditions but there was no systematic attempt <br />to model the various heat fluxes in the laboratory airflow of this <br />study. <br />5) Source characteristics -'A particulate plume, e.g., silver- <br />iodide particles, quickly attains the wind speed in the horizontal <br />plane, while its rise is determined by its vertical momentum and <br />buoyancy due to heat and molecular-weight difference. Rise of the <br />plume is impeded by entrainment with air, which at first is due to <br />turbulence generated by the relative motion of the plume. As this <br />dies out, atmospheric turbulence dominates the mixing. Buoyancy <br />of the plume may be altered by the atmospheric stability. Stable <br />air acts as a restoring force on the plume, but in unstable air the <br />plume may rise to large heights. <br />The sources in the field are silver-iodide <br />approximately 20 gms of silver-iodide per hour <br />temperature (Ts) around l2000C. The material <br />per sec (ws) from the orifice. <br />For a wind-tunnel model of 1:9600 scale ratio it is not feasible to <br />scale or simulate Q, Ts ' hs and Ws for prototype silver-iodide <br />generators. The sources on the models correspond approximately to a <br />field virtual source elevated some 50-60 meters from the surface. For <br />the model airflows it was assumed that the effects of dissimilarity due <br />to Ts , Q , hs and Ws were quickly masked by the effects of turbu- <br />lent mixing as the material moves dO\;7Ustream after release. <br /> <br />generators which burn <br />(Q) at a flame <br />emits at a few meters <br /> <br />~ <br /> <br />19 <br />