OCR Text |
Show Figures 20 and 21 display the vertical velocity fields for the two cases using a mountain height of 11 00m and a half width at half of the height of the mountain of 11000m. The contour interval is 0. 05m sec-1. The positive vertical velocities over the windward slope of the mountain are 3 times greater at 1935 MST than at 2250 MST. Also, the vertical velocities at higher elevations in the lee of the mountain are stronger. The predicted three hour precipitation totals are indicated for each horizontal grid point along the abscissa. At 1935 MST the peak total is 0. 586 cm; and the peak is just short of the ridge crest. At 2250 MST the precipitation peaks at the ridge crest at a value of 0. 134 cm; but relatively higher values are spread much more uniformly over the windward slope. The flow over a barrier is very sensitive to the flow velocities at lower levels. Considerable work is needed with varying input parameters to calibrate the mountain profiles to be used under actual conditions. The inversion structure at low levels has a profound effect on the shape of barrier encountered by the main flow stream in the layers above. Any cold air trapped up against the mountain will probably decrease the total lift, but start it further upwind of the mountain. Thus, the mountain profile used should be modified for inversion cases. It is notable that the upw ard motion which creates the orographic precipitation component typically occurs fairly close to the ridge crest ·and at fairly low levels. This agrees with a phenomenon in the Park Range area which is observed quite frequently. That is that the ceiling often drops from above 11, 000 ft. MSL to the surface of the valley at about 7, 000 ft. MSL in about five minutes. It apparently drops with no advection of any cloud from an upwind direction. This drop of over 1000m in 5-10 minutes is much faster than the fall velocity of even fairly large snow crystals. A logical explanation is provided by the model, in that this is the vertical velocity field acting on an input of air with higher moisture content; what is observed may be the formation of new cloud and precipitation by the vertical velocity field right on the windward slope of the ridge. The vertical velocity output of the model for the input data for analysis periods 5 and 6 on 16 November 1968 are presented in Figures 22-25. These analysis periods were covered in detail in case study in Monthly Progress Report No. 42. The positive vertical velocity field is weaker at 0749 MST than at the three subsequent times due primarily to weaker wind speeds in the lower layers. The orographic precipitation component across the barrier is plotted in Figure 26. These values are compared to the values observed at selected stations in Table 6. In some cases the model predictions look quite reasonable and agree well with the observed precipitation. In other cases they are in error, 45 |