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Abstract
The Froude number (Fr) is a common flow parameter used to describe the behavior of the flow as it approaches a mountain range. In theory, a threshold Fr = 1 separates unblocked to blocked flow. In mountainous region, it tells if the flow will go over or around the relief, which is a fundamental to orographic precipitation distribution, as well as for cloud seeding and air pollution dispersion. The relationship between the Fr parameter and a non-idealized orographic flow is studied for the Wind River Range (WRR), Wyoming, using a Lagrangian approach to quantify blocking tendency. This study relates the upstream Fr to the actual flow behavior an inferred from high-resolution simulations. Air parcel trajectories, generated by HYSPLIT, are derived from 900 m resolution WRF simulations. Examination of the normalized horizontal deflection of the flow shows that the upwind Fr does not cleanly separate between blocked and unblocked flow behavior. For the WRR, the optimal Fr threshold separating between blocked and unblocked flows is less than 1 (Fr = 0.56 for the 8 cases examined herein). Only when the upstream Fr is really large (Fr > 2.0 to 2.5) is it virtually certain that parcels go over the WRR. Finally, we introduce a probabilistic method to define the best upstream location from which unblocked parcels arise, something of interest to the location of cloud seeding generators, for instance. In the case of the WRR, that is an area close the ridge line, in the center part of the WRR.
The method developed in this thesis can be applied to other mountain ranges in order to define the optimal Fr separating between stagnating and unblocked flow, e.g. for air quality prediction. It is recommended that a larger and more diverse set of synoptic cases be used than used in this study.
