Sunday, March 25, 2012

2857.txt

date: Fri, 16 Jul 2004 08:45:49 -0600
from: Tom Wigley <wigleyatXYZxyz.ucar.edu>
subject: thanks again
to: Sarah Raper <sraperatXYZxyz-bremerhaven.de>

Sarah,

Thanks again for all that useful material. I will absorb
it into the text.

Although it will not make much difference, I guess
I should use an exponent for V in the final version
of the model. I will stick to n=1 for the analytic
results -- but n=1.2-1.4 seems necessary to be
state of the art. As far as MAGICC goes it makes
no difference since I solve the equation numerically.

My 1880 start year comes from TAR p. 667 which
refers to 'the climate of 1865-1895'. What is confusing
is the T1990 term is relative to 'the late 19th century'
(p. 682, which I take as 1865-1895, equal near enough
to the 1880 value in MAGICC) --- but the g1990
is relative to the glacier steady state. There is a 0.15degC
correction for the warming from this time to 1880, so the
steady state time must be well before 1880. In MAGICC
with DT2x=2.6degC the 1765-1880 warming is only
0.1degC.

All this is accounted for in MAGICC strictly following
the TAR, but there is still an inconsistency problem that
I noted before (and it's worse than I said before). The
AOGCM gu(1990) range is 1.5 to 2.7cm (I use the
average in MAGICC). The corresponding gs(1990)
value is pretty much the same. These must be changes
from the glacier steady state point, which must be well
before 1880 and probably before 1765. Table 11.10
(1910-1990 data) suggests a change larger than this,
2.4cm over 1910-1990
.
The question is, 2.4cm from 1910 equals what from the
glacier steady state point? Even though 1910 was a
cool spot in the global temperature curve, the use of
the 0.15 correction implies that there must be positive
GSIC melt to 1880, and hence almost certainly positive
melt to 1910. So the 'observed' gs(1990) must be more
than 2.4cm.

I guess there is nothing we can do about this. At worse
we will be about 1cm too low with gs(1990) if we stick
to the TAR AOGCM mean, and this won't affect the
longterm behaviour which must tend to V0. What is
needed is a model that runs from the steady state point
rather than 1990. Probably easy to do, but it would make
everything more messy and less directly comparable with
the TAR.

Thwarted by Jonathan's ad hockery!

Tom.

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