# [MITgcm-support] exf interp

Martin Losch Martin.Losch at awi.de
Mon Oct 19 03:28:03 EDT 2009

```Hi Matt,

I don't know if this helps, but the wind stress is something like
Cd*sqrt(u^2+v^2)*(u,v). Then the wind stress curl is A*dv/dx - B*du/y
+ C*du/dx - D*dv/y with A-D some simple function of Cd,u,v (basically
u^2/sqrt(u^2+v^2).

Now if you your wind field is piece-wise linear (because of the linear
interpolation between the 1-deg grid points), then your du/dx,dv/dx,du/
dy,dv/dy are piecewise constant. In other words they have
discontinuities at cell interfaces, which explains your pattern. This
pattern can only be eliminated with a higher order interpolation
scheme (as Chris points out at least c1 continuous, so that the
deriviative is continuous, ie, at least piecewise linear). We should
consider making bicubic default (only) for wind interpolation.

Martin

On Oct 17, 2009, at 11:53 PM, Dimitris Menemenlis wrote:

> The issue was mainly one of negative rain
> (and perhaps also negative downward radiations and humidity).
> These will probably not be a problem in your limited domain.
>
> When you run your test on Monday, could you also save the wind
> velocity fields:
>
> 179 |EXFuwind|  1 |SM      U1      |m/s             |zonal 10-m wind
> speed, >0 increases uVel
> 180 |EXFvwind|  1 |SM      U1      |m/s             |meridional 10-m
> wind speed, >0 increases uVel
> 181 |EXFwspee|  1 |SM      U1      |m/s             |10-m wind speed
> modulus ( >= 0 )
>
> to see if there are any discontinuities are in the curl of the wind
> velocity.
> If Chris' hypothesis is correct you will not see a rectangular
> pattern in
> the curl of the wind.  if you do, then we will need to use Chris'
> interpolation scheme.
>
> D.
>
> On Oct 17, 2009, at 1:38 PM, Matthew Mazloff wrote:
>
>> Hi Chris,
>>
>> Yeah, its relatively easy to test.  I'll give it a try on monday and
>> let you know
>>
>> -Matt
>>
>> Dimitris, what issues arose when using bicubic before for buoyancy
>> terms -- anything specific I should look out for?
>>
>> -Matt
>>
>>
>> On Oct 17, 2009, at 1:14 PM, Chris Hill wrote:
>>
>>> Matt,
>>>
>>> I think the bicubic is worth trying. As I understand Benny's code it
>>> is meant to be c1 continuous
>>> which should help.
>>> My hope is that the bilinear temps are affecting the wind stress
>>> (because atemp/SST feature in static stability calcs in bulk formula
>>> and in turbulent mixing calcs and so affect momentum flux).
>>> Is it quick(ish) to run a test?
>>>
>>> If this doesn't work I have a couple of other things that would be
>>> kind of interesting to try. They involve doing stuff offline
>>> initially.
>>>
>>> Chris
>>>
>>> On Sat, Oct 17, 2009 at 1:36 PM, Matthew Mazloff <mmazloff at mit.edu>
>>> wrote:
>>>> Hi Chris,
>>>>
>>>>> Are the surface data sets 1/2 degree res?
>>>>
>>>> 1 degree res -- starting at the half degree:
>>>> uwind_lon0         = 229.5D0,
>>>> uwind_lon_inc      = 1.D0,
>>>> uwind_lat0         = 26.5D0,
>>>> uwind_lat_inc      = 15*1.D0,
>>>>
>>>>> My guess is it is the piecewise constant in the aTemp/aQ/
>>>>
>>>> I'm confused, I'm plotting windstress curl, and the wind speed uses
>>>> bicubic
>>>> Do you think this is a feedback from buoyancy forcing be linearly
>>>> interploated?  I can check the wind speed to ensure the signal is
>>>> not coming
>>>> from the (ocean) relative speed in the stress calculation.
>>>>
>>>>> If the spline is done right
>>>>> the overshoots should be small in a limited area domain, so you
>>>>> could try
>>>>> that.
>>>>
>>>> You mean trying bicubic for buoyancy components?
>>>>
>>>>
>>>> Thanks for the help!
>>>> -Matt
>>>>
>>>>
>>>>
>>>>>
>>>>> Chris
>>>>>
>>>>> On Sat, Oct 17, 2009 at 11:38 AM, Matthew Mazloff <mmazloff at mit.edu
>>>>> >
>>>>> wrote:
>>>>>>
>>>>>> Hi Dimitris,
>>>>>>
>>>>>> See the lines at X.5 degrees.  Its not very noticeable in
>>>>>> snapshots, but
>>>>>> really stands out in the mean as the signal is accumulated
>>>>>>
>>>>>> -Matt
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> On Oct 17, 2009, at 7:42 AM, Dimitris Menemenlis wrote:
>>>>>>
>>>>>>> Matt, could you send an example figure of problem?
>>>>>>>
>>>>>>> On Oct 17, 2009, at 7:29 AM, Matthew Mazloff wrote:
>>>>>>>
>>>>>>>> Hi Dimitris,
>>>>>>>>
>>>>>>>> Ah yes, I see this now.  Very nice.  Unfortunately the gridding
>>>>>>>> is
>>>>>>>> still noticeable in my plots of wind speed gradient.  I am able
>>>>>>>> to
>>>>>>>> pick out the exact locals where the forcing is prescribed --
>>>>>>>> meaning
>>>>>>>> the interpolation is not smooth.  This is not good for plotting
>>>>>>>> wind-
>>>>>>>> stress curl. Have you noticed this on your high-res set-ups?
>>>>>>>> Do you
>>>>>>>> think this is a problem?  Should we (can we) try a smoother
>>>>>>>> interp
>>>>>>>> method?
>>>>>>>>
>>>>>>>> -Matt
>>>>>>>>
>>>>>>>> On Oct 16, 2009, at 5:03 PM, Dimitris Menemenlis wrote:
>>>>>>>>
>>>>>>>>> Matt, I hardcoded bilinear interpolation for tracer fields
>>>>>>>>> because
>>>>>>>>> overshoots can be problematic and bicubic for wind velocity
>>>>>>>>> (see
>>>>>>>>> exf_set_uv.F) for stress fields because the second derivative
>>>>>>>>> matters.  I would recommend to leave as is.  Other
>>>>>>>>> combinations can
>>>>>>>>> (or did) cause trouble.  Dimitris
>>>>>>>>>
>>>>>>>>> On Oct 16, 2009, at 3:21 PM, Matthew Mazloff wrote:
>>>>>>>>>
>>>>>>>>>> Hello,
>>>>>>>>>>
>>>>>>>>>> It appears the model is equipped to do both bilinear and
>>>>>>>>>> bicubic
>>>>>>>>>> interpolation for exf_interp.F.  Interp_method, however, is
>>>>>>>>>> hardcoded
>>>>>>>>>> to bilinear interpolation.  This does matter for my 1/16
>>>>>>>>>> degree set-
>>>>>>>>>> up.  Can anyone confirm that the bicubic interpolation is ok
>>>>>>>>>> -- and I
>>>>>>>>>> can go ahead and use this option.
>>>>>>>>>>
>>>>>>>>>> -Thanks,
>>>>>>>>>> Matt
>>>>>>>
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