| OCR Text |
Show 4 of latent heat by condensation, the topographic conditions, and fric-tional effects. Since the main interest here is the orographically induced precipitation, only vertical motion due to topographic influences is considered in this report. It is known that the majority of the precipitation which falls at Alta is orographically induced. The vertical velocity for air flowing up a slope can be computed by the well- known expression w (?) 2.5 m 39r< P? o Voo " Mr- l (') where wm is expressed in millibars per 3 hours, g is the force of gravity pQ is the density of the air at ground level, PQ is the pressure at ground level, and VH is the slope of the ground. For the elevation of Alta, the term 700/ PQ is nearly unity and is neglected. Since little is known about the windflow in and over mountainous regions, a modified value of the 700 mb wind is used for V . An advan-o tage of using the wind at this level is that current and prognosticated data are readily available at this level. It was found that when the wind direction was from the 240° to 300 inverval, the 700 mb wind speed could be used directly for VQ. For other westerly directions, it was found that it was necessary to modify the 700 mb surface wind velocity as vo= v700 c o s 1 e ( 2) where 0 = ( 240° - fy0Q) and 0- = (< p7QQ - 300°) for southwesterly and northwesterly flow respectively, where f is the 700 mb wind direction. |
