Saturday, May 19, 2012


date: Wed, 1 Oct 1997 15:29:06 -0600 (MDT)
from: Tom Wigley <>
subject: Forwarded mail....
to: Mike Hulme <>

Dear Mike,

Can you send me data on grid box elevations for the USA? I'd like these
for both the HadCM2 GCM and for the real world. I realize that there are
different ways to define a grid-box elevation, so I'll leave that choice
up to you. My main concern is to determine elevation differences, so I
can account for this effect in validating HadCM temperatures. This is
probably an issue you have though about, so I'd appreciate your insights
and opinions. For example, does one normally (or can one) account for any
global-mean bias; are there any seasonally-specific effects, etc.


P.S. See also attached item sent to John Mitchell. Do you have any
thoughts on this?

---------- Forwarded message ----------
Date: Mon, 29 Sep 1997 11:44:56 -0600 (MDT)
From: Tom Wigley <>
To: "J.M. Gregory" <>,
John Mitchell <>
Cc: Robert Wilby <>

Dear John and Jonathan,

Have you ever done any work on explaining why precipitation changes in GCM
runs (or do you know of any such work)? The key physical factors should
be: changes in the circulation (e.g., vorticity, divergence), changes in
the moisture held by the atmosphere (e.g., as measured by specific
humidity), or changes in cloudiness. Rob Wilby and I have looked at the
circulation and moisture factors statistically in HadCM2 and find that
they are nowhere near enough to explain the precipitation changes. From
other models, I know that cloud changes in perturbation experiments have
low SNR, so I doubt that they are the answer. So what is it? Could there
be some effect through the way the precipitation on/off is parameterized
in the model -- i.e., something that may not be realistically linked to
what happens in the real world?

As a related issue, we recently looked at precipitation changes in the
9-box region around Hawaii. In the control run, there is little change
between roughly 1990 and 2040, but then an increase of around 30% (similar
in all seasons) between 2040 and 2090. This seems a very big change,
given no forcing. I wonder whether this could be an artifact of the
strong zone of coherence HadCM2 has in the tropics -- if this were a wet
zone, and it showed a low-frequency northward (or poleward) change, Hawaii
could be mainly outside the zone to around 2040 and then "move" to
mainly within it? Any ideas?

How is HadCM3 going? Is there any point in trying to understand HadCM2 if
it is soon to be replaced by HadCM3?


*Tom M.L. Wigley *
*Senior Scientist *
*National Center for Atmospheric Research *
*P.O. Box 3000 *
*Boulder, CO 80307-3000 *
*USA *
*Phone: 303-497-2690 *
*Fax: 303-497-2699 *
*E-mail: *

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