Three Tree Point Form Drag Experiment


Form Drag in the Ocean


The purpose of the Three Tree Point Experiment is to measure the pressure drop over a topographic feature caused by the currents flowing over top of it.  We have developed special sensors to measure the force that a ridge can exert on the overlying flow, known as “form drag.”  Our objective is to relate this force to other variables we can easily measure, such as the tidal strength and the density structure in Puget Sound.  Three Tree Point represents an ideal natural geophysical laboratory for us to conduct these important experiments because the tidal currents are predictable and deviations from them can be associated with form drag. 

This research has been funded by the National Science Foundation

In the atmosphere, meteorologists acknowledge the importance of “form drag”, a phenomenon which represents a pressure drop as winds pass over mountains.  This produces a force in the upper atmosphere that slows down the wind.  These forces must be included for accurate weather prediction.

Similar phenomena must occur in the ocean, but they are presently poorly understood.  This is because few measurements have been made in the ocean to confirm their existence.  As a result, their influences are not incorporated into ocean models. Lack of inclusion of form drag in ocean circulation models may be an oversight. 

The ocean is layered, so warm lighter fluid rests over cold, dense waters.  When this so-called “stratified fluid” flows over an obstacle, layers are perturbed, and waves can form that are trapped behind the bump, such as when water flows over a rock in a river.  Alternatively, perturbations formed near an obstacle may radiate away as “internal waves”.  When either of these “break”, they generate turbulence that helps to mix the ocean.