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I I I I I I I I I I I I I I I I I I I - 65 - must be extrapolated down to the surface. In cases where the wind near the surface is reasonably high this will be a reasonable approximation. On , calmer days, however, the procedure is questionable. The actual critical wavenumbers used in the calculations which follow are shown as dashed lines in Fig. 3.1. Although the three layer model was originally devised to represent two dry layers and one cloudy layer, regions , of dry and cloudy air may have the same critical wavenumber and thus obey the seme differential equation. Therefore, it is often convenient to place the boundaries between the three layers at locations which differ from those wpich define the cloudy layer. The information shown in Fig. 3.1 was used as input data for the theoretical model, and the critical wavenumber at 1.1 km was extrapolated to the surface. The streamlines predicted by the model when no account is taken of blocking are shown in Fi~. 3.2. The sounding data shown in Fir. 3.l were obtained from a station with elevation 0.55 km above t1SL, located on the western slope of the Cascades. The wind and temperature profiles at x = - = where the elevation is zero are required for calculatin~ the critical wavenumbers for the model. The variation of ~he wind field above surface at x = - co is assumed to be the same as its variation above surface at the , sounding station. The temperature profile is obtained using the parcel method and assuming that the air at x = - co is adiabatically lifted 0.55 km to reach the sounding station. Calculation of the critical wavenumbers at x = - 00 simply results in a downward displacement by 0.55 km of the values shown in Fig. 3.1. The smoothed profile of the mountain shown in Fig. 3.2 correspond3 to that for a westerly airflow over the Cascade Mountains, The winds over the