[asa] Re: Open Source Climate Modeling was Re: [asa] Crop Yields FaceNon-LinearEffects Due to Climate Change

Dave:

I think we've talked about this before.
What he means, I think, is that the codes produce less variance (e.g.,
less sensitivity to initial conditions) on "long" time scales than for
shorter ones. That is, the code is more stable.

This is slightly, at least, contrary to my experience. My experience would
be with shocked radiation hydrodynamics. And we would generally expect better
results for shorter time scales, (e.g., courant constraints).

I wonder if longer time scales are being time averaged in some sense.

Larger mesh size often produces "better" results because of spatial averaging,
although by missing much detail. But generally we don't think of longer time
scales as temporal averaging.

What do you think?

I would think that the efficient ability to change the number of
bits used to represent reals would be chip dependent. I know the old
Crays did this. I think you are right about SGI. But things change so
fast. Who knows what their capabilities are now.

bill

On Tue, 15 Sep 2009 08:36:58 -0400, Dave Wallace <wmdavid.wallace@gmail.com> wrote:
> Rich Blinne wrote:
>>
>>
>>
>>
>> The study used CCSM3 (Common Climate System Model 3). This is an open
>> source model so that differential studies can do true apples to apples
>> comparisons. As with all weather and climate circulation models CCSM3
>> solves Navier-Stokes fluid flow equations which as you probably know
>> is notoriously sensitive to proper initial conditions. When used in
>> climate modeling the approach used by thermodynamics is used in order
>> to get useful results far into the future, averaging. The model is run
>> multiple times, with slightly different initial conditions, and then
>> an average is computed. The spread of the runs gives a sense of how
>> accurate the results are.
> Another good way is to run the models with floating point precision
> doubled for example 128bit float rather than 64 bit float. I realise
> that not many processors support 128 bit float but some do, Power PC/AIX
> does and I can't remember but possibly Sun, Alpha or SGI as well.
>
>> Climate models are counter-intuitive in that they are less accurate on
>> shorter time scales than on longer ones. We want them accurate at
>> *all* time scales, short, medium, and long.
>>
> How do we know they are more accurate on longer scales? How long is
> longer?
>
> Dave W
>
>
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Received on Tue Sep 15 11:01:00 2009