[MITgcm-support] Friction velocity ustar in S/R exf_wind.F

Mon Apr 23 13:06:50 EDT 2018

> On 23 Apr 2018, at 09:58, Stanislav Martyanov <martyanov.sd at gmail.com> wrote:
> I went to git repository, but the bug-fix procedure seems a little bit
> too complicated…

If you don’t feel like making a full Pull Request, then simply open an issue, with as much specifics as you can, including line numbers and file names if possible.  Then someone with a development environment can fix it for you.

Cheers,   Jody

> Stanislav M.
> 
> 
> 
> 
>> Hi Stanislav,
> 
>> I think you got me convinced that there is a problem. The code has been around for a long time (the routine was first introduced in 2006), but the code was there even earlier in the ecco branch. I can’t figure out who added the code originally, in order ask if we are overlooking >something, but maybe someone on this list feels responsible and can check …
>> In a quick check, it seems that this does not affect any of the forward tests that we run regularily. That means that this code is probably not used very often, probably a reason why this did go unnoticed for such a long time.
> 
>> If you want to, you could contribute the bug-fix yourself to the still relatively new git repository, see here how to do that: <https://mitgcm.readthedocs.io/en/latest/contributing/contributing.html>
> 
>> Martin
> 
>> See further comments below:
> 
>> On 22. Apr 2018, at 15:04, Stanislav Martyanov <martyanov.sd at gmail.com> wrote:
>> 
>> Hi Martin!
>> 
>> Thank your for your reply. Yes, I understand that some optimization
>> was made for the computational reasons. But I still have some doubts
>> about the used formula for ustar in s/r exf_wind. I'll try to explain
>> what I mean in detail.
>> 
>> So, let's see the code in exf_wind.f with my brief comments:
>> 
>> Line 146-153: We calculate usSq = (ustress*ustress) +
>> (vstress*vstress). Both ustress and vstress have units [N/m^2] because
>> they are the wind stress vector's components along X and Y axes. After
>> that we obtain variable usSq which has unit [ (N/m^2)^2 ]. It is OK.
>> 
>> Line 156: wStress = SQRT(usSq). So wStress is now in [N/m^2]. That is
>> OK too, because wStress is just a module of wind vector.
>> 
> This is already wrong, because usSq is stress squared. It should be
> SQRT(SQRT(usSq))/atmrho). The rest are follow up errors.
>> Line 157: c            ustar = SQRT(usSq/atmrho). This is a correct
>> statement, but it is commented.
>> 
>> Line 176: recip_sqrtRhoA = 1. _d 0 / SQRT(atmrho). That is OK, we are
>> optimizing the code for the further computations.
>> 
>> Line 179: ustar = wStress*recip_sqrtRhoA.    Now that is a problem, as
>> I think. Because after that, ustar has units (N/m^2) / (kg/m^3)^0.5.
>> But it has to have units as friction velocity - [m/s]. The problem is
>> that we have not taken another one SQRT.
>> 
> I agree that this should fix it:
>> If we rewrite, adding additional SQRT, as:
>> ustar = SQRT(wStress)*recip_sqrtRhoA,
>> 
>> then the units of ustar will be [m/s] as it should be. Friction
>> velocity is basically determined as SQRT(stress/density).
>> 
>> Best regards,
>> Stanislav
> 
> 2018-04-23 16:30 GMT+03:00 Martin Losch <Martin.Losch at awi.de>:
>> Hi Stanislav,
>> 
>> I think you got me convinced that there is a problem. The code has been around for a long time (the routine was first introduced in 2006), but the code was there even earlier in the ecco branch. I can’t figure out who added the code originally, in order ask if we are overlooking something, but maybe someone on this list feels responsible and can check …
>> In a quick check, it seems that this does not affect any of the forward tests that we run regularily. That means that this code is probably not used very often, probably a reason why this did go unnoticed for such a long time.
>> 
>> If you want to, you could contribute the bug-fix yourself to the still relatively new git repository, see here how to do that: <https://mitgcm.readthedocs.io/en/latest/contributing/contributing.html>
>> 
>> Martin
>> 
>> See further comments below:
>> 
>>> On 22. Apr 2018, at 15:04, Stanislav Martyanov <martyanov.sd at gmail.com> wrote:
>>> 
>>> Hi Martin!
>>> 
>>> Thank your for your reply. Yes, I understand that some optimization
>>> was made for the computational reasons. But I still have some doubts
>>> about the used formula for ustar in s/r exf_wind. I'll try to explain
>>> what I mean in detail.
>>> 
>>> So, let's see the code in exf_wind.f with my brief comments:
>>> 
>>> Line 146-153: We calculate usSq = (ustress*ustress) +
>>> (vstress*vstress). Both ustress and vstress have units [N/m^2] because
>>> they are the wind stress vector's components along X and Y axes. After
>>> that we obtain variable usSq which has unit [ (N/m^2)^2 ]. It is OK.
>>> 
>>> Line 156: wStress = SQRT(usSq). So wStress is now in [N/m^2]. That is
>>> OK too, because wStress is just a module of wind vector.
>>> 
>> This is already wrong, because usSq is stress squared. It should be SQRT(SQRT(usSq))/atmrho). The rest are follow up errors.
>>> Line 157: c            ustar = SQRT(usSq/atmrho). This is a correct
>>> statement, but it is commented.
>>> 
>>> Line 176: recip_sqrtRhoA = 1. _d 0 / SQRT(atmrho). That is OK, we are
>>> optimizing the code for the further computations.
>>> 
>>> Line 179: ustar = wStress*recip_sqrtRhoA.    Now that is a problem, as
>>> I think. Because after that, ustar has units (N/m^2) / (kg/m^3)^0.5.
>>> But it has to have units as friction velocity - [m/s]. The problem is
>>> that we have not taken another one SQRT.
>>> 
>> I agree that this should fix it:
>>> If we rewrite, adding additional SQRT, as:
>>> ustar = SQRT(wStress)*recip_sqrtRhoA,
>>> 
>>> then the units of ustar will be [m/s] as it should be. Friction
>>> velocity is basically determined as SQRT(stress/density).
>>> 
>>> Best regards,
>>> Stanislav
>>> 
>>> 
>>> 
>>> 
>>> 
>>>> Hi Stanislav,
>>>> 
>>>> I am no expert on this, but did you see lines 156/157:
>>>>            wStress(i,j,bi,bj) = SQRT(usSq)
>>>> c            ustar = SQRT(usSq/atmrho)
>>>> 
>>>> so ustar should be sqrt(usSq/atmrho). for algorithmic reasons wstress=sqrt(usSq) is computed first and then divided by sqrt(atmrho) later in lin 179. To this makes sense, although it’s not very easy to read.
>>>> 
>>>> Martin
>>> 
>>>> On 4. Mar 2018, at 13:58, Stanislav Martyanov <martyanov.sd at gmail.com> wrote:
>>>> 
>>>> Hello!
>>>> 
>>>> I think it should be
>>>> 
>>>> ustar = SQRT (wStress(i,j,bi,bj) / atmrho)
>>>> 
>>>> rather than
>>>> 
>>>> ustar = wStress(i,j,bi,bj)*recip_sqrtRhoA
>>>> 
>>>> in exf package in S/R exf_winf.F (line 179)?
>>>> 
>>>> In the current version wStress is not affected by the function SQRT,
>>>> but should be, in my opinion.
>>>> 
>>>> Best regards,
>>>> Stanislav
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--
Jody Klymak    
http://web.uvic.ca/~jklymak/