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I I I I I I I I I I I I I I I I I I I - 63 - One difficulty which is encountered in a two dimensional model is that it is unable to handle the blocking of the air in a rigorous way. However, blocking can be simulated by either introducing a fluid sink on the upwind side of the mountain or by modifying the profile of the mountain. The latter approach has been adopted in this study, however, much more work remains to be , done in order to make the simulation as realistic as possible. Fortunately, the two dimensional model appears to be capable of predicting the depth of air which is affected by blocking in the following manner. When the critical ~ -1 wave number -- exceeds a certain value (generally of the order 1 km for u o the Cascade Mountains), the individual streamlines possess multiple solutions in the vertical. While this maybe a realistic result in the lee of a mountain, it is probably not realistic on the upwind side but instead indicates the presence of blocking. 3.2.4 Some Outputs from the Airflow Model The three outputs of the airflow model included here demonstrate the effects of varying wind direction and simulated blocking. All three outputs were run with the same upstream wind and temperature profiles , which, together with the critical wavenumbers, are shown in Fig. 3.1. The divisions between dry and cloudy air are based on the vertical distribution of relative humidity as measured by the sounding. Since the wind speed is very small at the surface, the critical wavenumber becomes very large and this produces undesirable effects. In fact it is likely that the air below 1.1 km never passes over the crest of the mountain. Only in rare cases is the wavenumber near the surface small enough to be used in the airflow model. Generally, if one does not wish to consider blocking, the critical wavenumber at some height above the surface