Ferret: Vector plots of velocity on a C-grid

[William S Kessler <William.S.Kessler@xxxxxxxx>]

This is only partly a Ferret question, but I suspect many Ferret users
have thought about how to deal with this problem.

I have OGCM output on a C-grid. The temperature and velocity components
are on a staggered grid structure. A single gridbox looks like:

  v  
u T u 
  v  

where the u-values are the currents in/out the east-west sides of the
box, and the v-values out the north-south, and T is the temperature
value defined in the center of the box. All these are on the same
vertical level. 

I want to make a vector plot of the velocity components, but since the
u's and v's are not at the same location this is not straightforward. 

One possibility is to define a grid consisting of all the u and v points
together, and interpolate all the velocities to all the points that
exist on either grid.

Another is to average the u's in x and v's in y onto the temperature
gridpoint.

A third is to do no interpolation, but treat the upper v and right-side
u as the flow at the northeast corner of the box, and the lower v and
left-side u as the flow at the southwest corner.

The problem becomes particularly acute when I want to make vectors in a
region of complicated land masses, with narrow straits, but I want the
plot to accurately reflect the model's actual flow, and not to omit any
gridpoints.

All of the above ideas result in losing some values, because, for
example, there is no left-side u-value on gridpoints against a western
boundary, while the right-side u and upper and lower v's do exist in
this case.

Anybody have a good idea how to do this as correctly as possible?

Billy K