[MITgcm-support] Wind stress without interpolation

[Martin Losch]

Hi Jody,

from looking at eesupp/src/get_periodic_interval.F I understand, that the forcing record is assumed to be representative for the interval externForcingPeriod, and the first one is assumed to start a t=0, so for t=0 (or n*externForcingPeriod), the weights should be 0.5. If you don't want that, I guess, you'll have to change get_periodic_interval.F,

Martin

On Feb 4, 2013, at 6:09 PM, Jody Klymak <jklymak at uvic.ca> wrote:

> Martin et. al.
> 
> I found a "feature" of the external forcing, which I guess I should track down more clearly: t=0 does not seem to correspond to the first time step of the forcing, but rather its interpolated from the last step to the first step, which appears to get lined up with t=externForcingPeriod.  I guess I expected the first entry to correspond to t=0.  
> 
> Either that or I did something wrong...
> 
> With 63o I have:
> 
> externForcingPeriod=3600.,
> externForcingCycle=604800.,
> 
> and 168 timesteps in my forcing files.  
> 
> Thanks,   Jody
> 
> On Feb 4, 2013, at  0:41 AM, Martin Losch <Martin.Losch at awi.de> wrote:
> 
>> Hi Massimo,
>> 
>> I don't think that there are run time flags for this. The only way I see how you can do it is by modifying model/src/external_fields_load.F
>> Please have a look at the file. In this file the forcing fields are read and interpolated, e.g., for wind stress:
>>           fu(i,j,bi,bj)    = bWght*taux0(i,j,bi,bj)
>>    &                       + aWght*taux1(i,j,bi,bj)
>> For "fu" to be constant over an "externForcingPeriod", you need to set either aWght or bWght to zero, or remove one of the terms in this sum.
>> 
>> Martin
>> 
>> 
>> On Feb 3, 2013, at 4:21 AM, massimo novello <massimonovello at yahoo.com.br> wrote:
>> 
>>> Please, is it possible to use a periodic wind force without interpolate the values? I need that the wind change suddently according to the files (wind_x.bin and wind_y.bin).
>>> 
>>> I started the simulation using the exp4 sample with some changes on its configuration (bathymetry, coast line, wind stress, open boundaries,...). 
>>> 
>>> Many thanks,
>>> Massimo Novello
>>> UERJ - Rio de Janeiro
>>> 
>>> From MITgcm manual:
>>> "There is also the possibility of prescribing time-dependent periodic forcing. To do this, concatenate the successive time records into a single file (for each stress component) ordered in a (x,y,t) fashion and set the following variables: periodicExternalForcing to ’.TRUE.’, externForcingPeriod to the period (in s) of which the forcing varies (typically 1 month), and externForcingCycle to the repeat time (in s) of the forcing (typically 1 year – note: externForcingCycle must be a multiple of externForcingPeriod). With these variables set up, the model will INTERPOLATE the forcing linearly at each iteration."
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>> 
>> 
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> --
> Jody Klymak    
> http://web.uvic.ca/~jklymak/
> 
> 
> 
> 
> 
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