[MITgcm-support] Floating point overflows in gad_os7mp_adv_r

[Martin Losch]

Christopher,
since no-one competent is answering ...

as far as I can see the "+Eps" is only supposed to avoid a division  
by zero. I assume that this "technique" is carried over to some  
extent from, say gad_dst3fl_adv_, where there  is no "abs".
So my assumption is that you can even replace all instances of abs(Y 
+eps)/abs(X+eps) by (abs(Y)+eps)/(abs(X)+eps) and the algorithm  
should work, but it's up to you to prove it (to yourself). And if it  
works we should consider changing it in the code for everyone, so I  
am curious about your experience. The fact that this scheme is not  
really really documented (e.g. you need a overlap of 4 for it to  
work), tells you that this scheme is not fully established.

Incidentally, I use this scheme for very high resolution grid and  
also for coarse resolution grid, and so far there haven't been any  
problems at all. Why, in your opinion, do you get the overflows and I  
(and others) don't?

Martin

On 21 May 2008, at 00:02, Christopher L. Wolfe wrote:

>
> Well, replacing all the instances of division by abs(X + Eps) with  
> division by abs(X) + Eps in gad_os7mp_adv_? caused the overflows to  
> stop. However, since I don't know that much about the algorithm, I  
> don't know if this change will have a detrimental effect on the  
> algorithm. Anyone knowledgeable care to comment?
>
> Thanks,
> Christopher
>
> -----------------------------------------------------------
> Dr. Christopher L. Wolfe                   858-534-4560
> Physical Oceanography Research Division    OAR 357
> Scripps Institution of Oceanography, UCSD  clwolfe at ucsd.edu
> -----------------------------------------------------------
>
>
>
> On May 19, 2008, at 6:45 PM, Christopher L. Wolfe wrote:
>
>>
>> Hi all,
>>
>> I'm getting floating point overflows in gad_os7mp_adv_r. I haven't  
>> pinned it down to the exact line that's causing it yet, but I've  
>> found several places where there's division by arbitrarily small  
>> quantities and I was wondering if someone familiar with the OS7MP  
>> algorithm could comment on it (or point me to a detailed  
>> exposition of the algorithm). Anyway, on lines 154, 157, and 182  
>> there's division by abs(DelIp + Eps), where DelIp = ( Qip - Qi ) *  
>> MskI. DelIp does not appear to be guaranteed non-negative, so abs 
>> (DelIp + Eps) can be made arbitrarily small. There's similar  
>> division by abs(X + Eps) (where X is some possibly negative  
>> quantity on lines 181 and 183.
>>
>> My question is this: Would it be a good idea to replace the  
>> instances of abs(X + Eps) with abs(X) + Eps, or would that totally  
>> screw up the algorithm?
>>
>> Thanks,
>> Christopher
>>
>> -----------------------------------------------------------
>> Dr. Christopher L. Wolfe                   858-534-4560
>> Physical Oceanography Research Division    OAR 357
>> Scripps Institution of Oceanography, UCSD  clwolfe at ucsd.edu
>> -----------------------------------------------------------
>>
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