[MITgcm-support] Velocity cubed sphere + pickup seaice

Wed Jun 4 14:13:29 EDT 2008

Thank you all for your advice! I changed the CS grid files, and I think that my results will be much more reliable...

I still have a (maybe very trivial) question about obtaining the correct zonal and meridional velocities from the cubed sphere coordinates. Martin, you wrote that I first have to interpolate the velocities and after that I have to rotate the velocities according to the values of angleCosC and angleSinC. I found the definitions of angleCosC and angleSinC:

angleCosC :: cosine of grid orientation angle relative to Geographic direction
                  at cell center: alpha=(Eastward_dir,grid_uVel_dir)=(North_d,vVel_d)
angleSinC :: sinus  of grid orientation angle relative to Geographic direction
                  at cell center: alpha=(Eastward_dir,grid_uVel_dir)=(North_d,vVel_d)

Actually, I do not understand this explanation; is there an image available, explaining the angles? 
I think that the velocities are something like:

U_final = U_cs_grid * angleCosC  -  V_cs_grid * angleSinC
V_final = V_cs_grid * angleCosC  +  U_cs_grid * angleSinC

Is this correct?

Elja

-----Oorspronkelijk bericht-----
Van: mitgcm-support-bounces at mitgcm.org namens chris hill
Verzonden: wo 6/4/2008 6:09
Aan: mitgcm-support at mitgcm.org
Onderwerp: Re: [MITgcm-support] Velocity cubed sphere + pickup seaice

Hi Elja (and others doing non-Earth modeling on cube),

  The CS grid files are generated using an assumed radius of 6370.10^3 m 
(which is somewhere between the Earth's polar and equatorial radius).
  At the moment there is not an automatic way to change this, i.e. you 
must do it by hand per Martin's recipe.

Chris
Martin Losch wrote:
> Elja,
> I guess I misunderstood. For cs-runs (and generally for 
> usingCurvilinearGrid=.true.) the grid parameter have to be provided to 
> the model, in your case by the grid_cs32.face[n],bin files.
> 
> In fact you need to compute the correct quantities for your Jupiter moon 
> application yourself, sorry. The easiest would be to read the grid 
> files, scale the fields with (rMoon/rEarth)^n (n=1 for dx*, n=2 for ra*, 
> etc) and write the to a file again. (you could also do this in 
> ini_curvilinear_grid, but that's probably more dangerous.
> 
> Martin
> 
> On 4 Jun 2008, at 16:45, Elja Huibregtse wrote:
> 
>> Hi Martin,
>>
>> The angles are indeed independent of the radius. But I'm still 
>> confused about how to change the parameters dxc, dyc, dxg etc.
>>
>> The default of the cube-sphere integration (with sea ice on the cs32 
>> grid) is:  usingCurvilinearGrid=.TRUE.
>> Trying usingSphericalPolarGrid=.TRUE. to make sure that dxg etc are 
>> adapted to the correct configuration, doesn't work; I get a 
>> segmentation fault.
>>
>> I use the cubed sphere configuration (cs32), which means that the 
>> grid_cs32.face[n].bin files, with n=1..6, are read and used. Are these 
>> files (containing all cubed-sphere grid lengths, areas and grid-point 
>> positions) 'fixed' or based on the current configuration (radius)? In 
>> the model, I often use recip_dxc etc, which are (as far as I 
>> understand) dependent on these grid_cs32.face[n].bin files. Because 
>> recip_dxc etc, haven't been adapted, I use the wrong values.
>>
>> I guess that the grid_cs32.face[n].bin files change if 
>> usingSphericalPolarGrid=.TRUE., but this doesn't work.
>> Thus, do I have to adapt the grid_cs32.face[n].bin files myself? Or is 
>> there another way to automatically change the parameters?
>>
>> Thanks for your help,
>>
>> Elja
>>
>>
>>
>> -----Oorspronkelijk bericht-----
>> Van: mitgcm-support-bounces at mitgcm.org namens Martin Losch
>> Verzonden: wo 6/4/2008 12:17
>> Aan: mitgcm-support at mitgcm.org
>> Onderwerp: Re: [MITgcm-support] Velocity cubed sphere + pickup seaice
>>
>> Elja,
>> you don't need to change set_defaults.F, you can set most of the
>> relevant parameters (such as rSphere, rotationPeriod or omega, etc in
>> "data"; have a look at ini_parms.F to see which parameters are
>> available in namelists.
>>
>> rac=dxg*dyg in meters^2 (etc.), dxg etc are computed based on the
>> value of rSphere (if "usingSphericalPolarGrid=.true."). the radius is
>> only hard-coded in cubeCalcAngle.m, but does that matter? The angles
>> are absolutely independent of the radius, right?
>>
>> Martin
>>
>> On 4 Jun 2008, at 11:06, Elja Huibregtse wrote:
>>
>> > Thanks, Martin and Dimitris, for your useful replies.
>> >
>> > My study object is a moon of Jupiter, thus the radius and other
>> > parameters differ from those used for the Earth. I changed all
>> > these parameters in set_defaults.f. Trying to calculate the values
>> > of angleCosC and angleSinC, I found out that the radius stored in
>> > RAC (calculated from: r_earth=sqrt(sum(sum(RAC))./(4.*pi)) in
>> > cubeCalcAngle.m, line 23), is still the radius of the Earth.
>> >
>> > Now, I think that all the 'coordinate' files, like RAC, RAS, RAZ
>> > etc. still contain the configuration of the Earth, which will, of
>> > course, influence my results. Is it possible to adapt those files
>> > to the correct configuration?
>> >
>> > Thanks!
>> > Elja
>> >
>> >
>> > -----Oorspronkelijk bericht-----
>> > Van: mitgcm-support-bounces at mitgcm.org namens Martin Losch
>> > Verzonden: wo 6/4/2008 8:54
>> > Aan: mitgcm-support at mitgcm.org
>> > Onderwerp: Re: [MITgcm-support] Velocity cubed sphere + pickup seaice
>> >
>> > Hi Elja,
>> >
>> > in addition, the orientation of u and v-velocity is along grid
>> > coordinates, which will give you funny results, if you try to plot
>> > them on some sort of map-projection. After interpolating, you need to
>> > rotate the velocities according to the values of angleCosC and
>> > angleSinC. Unfortunately, these fields are often not set, because the
>> > model does not need them for hydrostatic integrations (so that they
>> > are 1 and 0). There is a matlab script to compute the angles in
>> > <utils/matlab/cs_grid/cubeCalcAngle.m>
>> >
>> > I am also attaching a matlab function that does the interpolation and
>> > takes care of the edges (for me, so I am not sure that this will work
>> > for you or anyone else, too).
>> >
>> > Martin
>> >
>> > _______________________________________________
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>>
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