Dave.C writes:
Probably a silly question but do all of the major models (ECM,
GFS,UKMO etc)
obtain independent raw data to produce their model output or is there
some/lots of overlap and it's the programming that causes the
differences?

.... there are so many variables to atmospheric modelling it is difficult
to know where to begin!

Firstly the analysis (the initial conditions) may be assessed in
different ways as between different centres - much as if you had two
forecasters faced with the same set of observations would produce a
slightly different analysis - location of front etc. The centres will
have *available* all the same raw data, but what they do with it may be
different. They may not assimilate *all* the data, or may be selective
about which dataset are assimilated (this applies particularly with
satellite-derived datasets). They may apply different weighting to some
datasets that another centre would not.

The method of analysis is now highly complex and as has been noted
elsewhere, data are not just used at the primary DT's (00, 06, 12 &
18Z), but used asynoptically - but the methods of doing this are
different from centre-to-centre, hence different analyses, and differing
forecasts.

Then of course there are different grid lengths (or wave numbers for
spectral models), and different numbers of vertical levels - and these
will be spaced differently by different centres.

Some models are 'tuned' to produce results at higher definition for
specific purposes - e.g. the tropical models, or GCM's used for climate
simulation vs. the mesoscale models used for short-range work.

The time-steps (or mathematical procedures used) will be different one
from another. The way some processes are parameterised will be
different - some may have explicit representation of surface type, some
may use climatology etc.

Some centres will skew the grid they use to make sure that the primary
area of interest is over them, so 'notional' poles will result which may
affect, even if only slightly, the forecast over the domain further
away from the primary centre of interest.

I'm sure I've missed some out! The only way to tackle this question
properly would be to visit each web site of the protagonists and read
through the literature.

But, to answer your original question about data, all data is made
*available* (via the World Weather Watch or Global Observing System of
WMO) - it's up to the individual centre how they use it ;-)

Martin.


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