Transitions in supercritical flows along mountainous coastlines

During the Coastal Waves 1996 experiment, the effect of coastline geometry on supercritical along-coast flow in the marine atmospheric boundary layer was studied. Simulations with a mesoscale model, using the field experiment data from one case at Cape Mendocino, California, with both real and idealized conditions led to several conclusions and further questions. Among the conclusions are that the local terrain at Cape Mendocino has a profound effect on the flow characteristics. It was also observed in the model that the flow upstream of a change in coastline orientation was sometimes affected by that change, even when the flow was supercritical. This appears to violate the results from simple shallow-water theory.

The first part of this study focused on the effects of the coastline geometry. An idealized terrain was used, varying the terrain characteristics as follows: Piecewise linear coasts with constant terrain height, varying the change in coastline orientation; Piecewise linear coast, varying the along-coast terrain height; Curved coastline change, varying the curvature; Piecewise linear coast with constant terrain height inserting a simplified cape and varying the height of the cape. As expected, it was found that the angle by which the coast turns away from the flow regulates the acceleration of the flow in the expansion fan; The larger angle, the faster flow. Along-coast terrain height variations had a significant impact, while the geometry (linear or curved) of the changing coastline appeared to have a smaller impact. The terrain-height effect was largest when the terrain slope coincided with the change in coastline orientation. Inserting a cape perpendicular to the coast resulted in a partial blocking of the upstream flow also with relatively low heights of the cape. On the lee-side of the cape, significant flow acceleration appeared, due to gravity-wave breaking. Even when supercritical, the flow gradually accelerated along the upstream coastline, except when the cape was inserted.

Several hypotheses for the cause of the gradual upstream acceleration of the flow have been tested and rejected. In the second part of this study we have focused on the transient behavior of this type of flows and its effects. As the initialization of the model was studied in more detail, it became clear that this might affect the results. This can also be interpreted in terms of the way the flows actually appear in reality. It was evident in the model that the terrain to a great extent determines the flow characteristics; the time for an initially subcritical flow to become supercritical, using the same background meteorological conditions, differs substantially with different terrain. This highlights the importance of using a high-resolution model to simulate the flow along coastlines with complex terrain. Not resolving the terrain properly will result in an incorrect flow structure. On the other hand, it also illustrates the difficulty of initializing high-resolution models.