Re: FYI: Question from Nanne Weber and reply by Haywood.

[Mark Chandler]

We are, in fact, very focused on understanding how the Pliocene climate system actually functioned - it is the closest analogue to a mature-stage globlal warming that we're going to get. _Mark

On Mon, Feb 9, 2009 at 4:37 PM, Alan Haywood <ear...@leeds.ac.uk> wrote:

Question emailed by Nanne Weber regarding why PRISM3 boundary
conditions are provided as absolutes rather than anomalies from
present-day.

Weber: I did have one question from my earlier browsing of the
webpage: you give all BCs as absolute values for the mid Pliocene,
rather than as differences between mid Plio and now. Is this on
purpose? in PMIP the BCs were used as differences, so I took Peltiers
LGM minus now orography and added that to my models' present-day
orography. It depends on your emphasis obviously (do you want to look
at simulated differences or simulated past climate), but was just
wondering about your rationale.

Haywood: Your question is a very good one.  I understand the rationale
for working with anomalies for the LGM but obviously the further back
in time you go the more problematic this becomes. The Eocene would be
impossible because of the altered land/sea mask.  In that sense the
Pliocene is in between, the continents had reached very nearly their
modern position but there are important regional differences (i.e.
west Antarctica, Hudson Bay etc) that need to be taken into account
which is why we peruse the method of prescribing the absolutes.

--
Mark A. Chandler
Columbia University - NASA/GISS
2880 Broadway, New York, NY 10025
NY office (212) 678-5644; cell (608) 445-0166

[DJ Lunt, Geographical Sciences]

Hi,
The question of absolute vs. anomaly orography is a tricky one.

We know that there are large biases between models in terms of absolute
modern climatology. Because of this, it is very rare in the IPCC that
absolute results are compared - it is always the anomaly relative to a
control. I expect that we will want to do the same, and present Pliocene
results in terms of anomalies.

We want these anomaly results from different groups to be completely
comparable - i.e. to reflect differences in the models rather than
differences in methodology of creating boundary conditions. Therefore, it
seems to me that we have two solutions -
(1) Groups use the absolute Pliocene orography from PRISM but also create a
new control using the PRISM modern.
(2) Groups use anomaly Pliocene orography, and their own local standard
modern orography.

(2) Leaves the problem of mis-match of land-sea mask between pliocene and
modern, but this can be overcome by using absolute Pliocene orography in
regions where there is no modern orography (if there are any such regions).

For the AGCM runs, carrying out a new modern control is probably no problem
for most groups, but for the AOGCM runs, i expect most groups will not be
able to do this. And we want the AGCM and AOGCM runs to be as consistent
with each other as possible. So i would prefer solution (2) (ie anomaly
orography).

cheers,
Dan

cheers,
Dan

>
> Question emailed by Nanne Weber regarding why PRISM3 boundary
> conditions are provided as absolutes rather than anomalies from
> present-day.
>
> Weber: I did have one question from my earlier browsing of the
> webpage: you give all BCs as absolute values for the mid Pliocene,
> rather than as differences between mid Plio and now. Is this on
> purpose? in PMIP the BCs were used as differences, so I took Peltiers
> LGM minus now orography and added that to my models' present-day
> orography. It depends on your emphasis obviously (do you want to look
> at simulated differences or simulated past climate), but was just
> wondering about your rationale.
>
> Haywood: Your question is a very good one. I understand the rationale
> for working with anomalies for the LGM but obviously the further back
> in time you go the more problematic this becomes. The Eocene would be
> impossible because of the altered land/sea mask. In that sense the
> Pliocene is in between, the continents had reached very nearly their
> modern position but there are important regional differences (i.e.
> west Antarctica, Hudson Bay etc) that need to be taken into account
> which is why we peruse the method of prescribing the absolutes.
>
> >

---------------------------------------------------------------
Dan Lunt Tel: +44 (0) 117 33 17483
School of Geographical Sciences
University of Bristol d.j....@bristol.ac.uk
University Road http://www.bridge.bris.ac.uk
Bristol BS8 1SS

GEOSCIENTIFIC MODEL DEVELOPMENT - A NEW EGU JOURNAL FOR
DESCRIPTIONS OF MODELS OF THE EARTH SYSTEM
http://www.geoscientific-model-development.net
---------------------------------------------------------------

[Nan Rosenbloom]

We plan to use topographic anomalies for our AOGCM experiments.

best regards -

nanr