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

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Date: Tue Sep 15 2009 - 18:48:04 EDT

On Sep 15, 2009 2:24pm, Bill Powers <> wrote:
> Rich:

> Just a short reply. The problem is how are the codes obtaining long term
> temporal results? All the codes operate on the presumption of
> infinitesimal time steps. At each step presumably energy and the like is
> conserved (not true of all codes). In looking at longer temporal scales
> are we taking larger time steps or simply taking the same time steps, but
> looking further down the road.

The time steps are same and run for more steps. The real information that
is compared with the models has more "noise" at the smaller time scales.
Most people refer to said noise as weather. Since most of this noise is
cyclical over a long enough time it averages out to nearly zero making your
later predictions in the same run more accurate than your earlier ones. The
short-term noise is treated as if it was truly random with multiple
ensembles being run with different initial conditions. What doesn't go away
(the anthropogenic climate change) changes smoothly. What then comes out is
a range of values bounded by the amount of short-term noise. The size of
the bounds gives a feel for the accuracy of the modelling.

In order to accurately model the short-term noise as a signal rather than
noise -- even for such things as average temperature -- you need smaller
grid sizes which is why the NASA folks are excited about their new
computer. As the grid sizes have been decreasing over the years even
climate models are starting to show ENSO and other such phenomena which are
normally associated with weather rather than climate.

> It does not seem that it is the longer time scale that is the salient
> feature, but the kinds of variables that are being examined on longer
> time scales. Examining average temperature will likely be fairly reliable
> at all time scales.

That's true with one important caveat. Volcanic eruptions. These can lower
global temps for a couple of years and go away. Over the long term it is as
if it never happened. Yet another reason for the counter-intuitive result
of climate predictions being more accurate at longer time scales than
shorter ones.


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Received on Tue Sep 15 18:48:46 2009

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