Re: Ferret: Vector plots of velocity on a C-grid

["Gustafson, William I" <william.gustafson@xxxxxxx>]

> I don't have any extra techniques to share, but I can relay my experience with the WRF
> atmospheric model, which sounds like it uses a similar grid setup. I interpolate the U and V
> velocities to the T points using simple averages. In my case, this mimics what most of the
> physics routines inside WRF do so I have not worried too much about loosing information in the
> process. The one problem I did run into was that when I did the averages to the T grid, I
> smoothed out some numerical noise produced by the model at around the 2*dx frequency. So, I
> was under the impression that my fields were smoother than they actually were if I only plotted
> the interpolated winds. 
> 
> Here is a sample of my code to plot the vectors from WRF. I setup the variables "ugrd" and
> "vgrd" in two steps in order to get the labelling to look nicer in the final plots.
> 
> use wrfout_d01_2000-08-28_12:00:00.nc
> ! Winds:
> let uu1 = 0.5*(u+u[i=@shf:1])
> define var/title="unrotated de-staggered zonal wind"/units="m/s" ugrd = uu1[g=xlong]
> let vv1 = 0.5*(v+v[j=@shf:1])
> define var/title="unrotated de-staggered meridional wind"/units="m/s" vgrd = vv1[g=xlong]
> vector/k=1/l=1 ugrd,vgrd
> 
> Since we are on the subject of winds, if anybody has some nice code to rotate winds to earth
> relative for the lambert conformal grid used in WRF, that would be a great thing to share!
> 
> -Bill
> --------------------------------------------------------------
> William I. Gustafson Jr.
> Atmospheric Science and Global Change Division
> Pacific Northwest National Laboratory
> 3200 Q Ave., MSIN K9-30
> Richland, WA 99352
> (509)372-6110
> 
> 
> ----------
> From: 	owner-ferret_users@pmel.noaa.gov on behalf of William S Kessler
> Sent: 	Monday, June 13, 2005 8:21 AM
> To: 	ferret_users@noaa.gov
> Cc: 	billy.kessler@noaa.gov
> Subject: 	Ferret: Vector plots of velocity on a C-grid
> 
> 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
> 
> 
> 
>