cc: Tim Osborn <t.osbornatXYZxyz.ac.uk>, k.briffa@uea.ac.uk, jones@gkss.de, cubaschatXYZxyzz.de

date: Fri, 04 Feb 2000 12:42:51 +0100

from: Reiner Schnur <schnuratXYZxyzz.de>

subject: Re: INTEGRATE -- comments on draft of part B.

to: Simon Tett <sfbtettatXYZxyzo.gov.uk>

Hi all,

just a few comments on Simon's comments, sorry that I haven't got to

this earlier ..

> WP5

> Methodology/work description

>

> line 1/2 solar output -> total solar irradiance.

> line 3 forced -> force

> line 4 ". Both models" -> which [HadCM3 has 1200 years, what does

> ECHAM4 have??]

ECHAM4/HOPE (the one we're going to use) has a 1000-yr control run

completed. You should use this acronym since ECHAM4 is only the

atmospheric component, and there is also another coupled version,

ECHAM4/OPYC3, which uses a different ocean model.

We should stress that the other project providing the natural run from

gkss is already in progress, as opposed to submitted, so the reviewers

will not think the success of INTEGRATE depends on acceptance of another

project.

> add to end of methods.

>

> The Natural simulations will be used to estimate the

> contribution of natural forcing to climate variability while the

> All experiments will be used to compare the simulations with proxy

> data in order to evaluate the veracity of the simulations. [Think

> we may need to work a bit harder to justify the Natural only

> simulations..]

I see it more the other way round: the natural-only is easily justified

because we want to get better estimates of natural variability (I think

also this was what we started out with) to be used e.g. in detection

studies. And this is according to the introduction (B4), page 4) the

ultimate thrust of the project. The natural+anthro ("All") runs wouldn't

be needed for this. Also, in WP5, if we want to answer the question "How

does external forcing alter the climate and its variability" by

comparing unforced with forced simulation, we would only need the

naturally forced runs because there are already many experiments which

answer the question how anthropogenic forcings alter the climate (this

is not meant to mean that we couldn't use more :-).

The justification for performing the "All" runs since 1750 is harder ..

Of course, as Simon mentions, they should be verified against the proxy

data but this doesn't really give a justification for performing the

runs in the first place. Maybe we have to work a little harder to

describe the actual uses for the "All" simulations.

The proxy data after about 1750 will be contaminated by a possible

anthropogenic signal (although week in the beginning). If we only had

the natural runs then we could only compare the model with proxy data in

the 250 years 1500-1750. So the "all" run is the one corresponding to

the proxy data over the full time period. In comparing all three runs

(control, natural, all) we can better

investigate periods where the forced model runs differ from the unforced

control runs, and why (natural vs. anthropogenic).

There can be periods which could be explained by natural

forcings alone, but there might also be periods which can be best

explained by natural+anthropogenic, e.g. the warming around the 1940's.

The combination of all forcings has been considered e.g. in Tett et al.,

1999 to see if the "all" forcing can explain certain periods better than

anthro alone, but they didn't use a model simulation that used all

forcings at the same time but a linear combinations of the different

signals (you don't have experiments with all forcings together, Simon,

do you?). We could check if the G+s+Sol+Vol experiments is in better

agreement with the obs than any linear combination of the single

signals.

> Workpackage 6

>

> Objectives.

> #1 Replace "statistical" with "quantitative".

>

> #4 Add "from workpackage 5". [Um interesting idea to

> synthesize model and proxy data. Do we think we'd end up with

> NH coverage then using proxy data to tie down some kind of

> model based interpolations]

This was what I wrote some time ago ... One idea I have on this would be

to use Bayesian analysis to merge the proxy and model estimates of

natural variability (there is variant of the classical Bayes analysis

which only deals with priors and posteriors in terms of expectation,

variance and covariance, instead of full probability distributions .

this might be interesting to explore). Another possibility (but

computationally more demanding) would be do some kind of optimal

interpolation between model and proxy data (something done in NWP to

reduce analysis errors by incorporating teh error structure of

observations) .. this would essentially be some kind assimilation of

proxy data into the model data to reduce errors.

> Page 12 -- WP 4. Some comments. Why 2000 years? I would think given the

> nature of the model/proxy comparison we should be talking about 500

> years. [Reiner -- what is your take on this?] -- this would of course

> make life easier in getting the stuff out quickly enough. Why

> additional coverage for the last 1000 -- why not the last 500 ??.

Well, I could imagine that there is some added value in having long

proxy time series e.g. when we compare statistics like return periods

(for the models we have shorter time series, but two 500-year runs). we

have 2x500 years of model estimates of natural variability, but if we

only had 500 years of proxy data since 1500 we'd have only 250 years of

proxy data representative of natural variability (until 1750). If we

have more proxy data back in time we can compare better, I think, how

realistic the estimates since 1500 are or, at least, how they relate to

a longer time period.

We can also use a relationship we find between proxy data and naturally

forced climate models to reconstruct climate variables prior to 1500.

This is usually done by finding this relationship between proxy data and

recently observed instrumental climate variables, but since these

already contain the anthropogenic signal there might be some added value

in using the natural model runs for this (between 1500 and 1750).

Cheers, REiner

--

Dr. Reiner Schnur <schnuratXYZxyzz.de>, phone: +49 40 41173-379, fax: +49

40441751

Max-Planck-Institut fuer Meteorologie, Bundesstr. 55, 20146 Hamburg,

Germany

courier service only: DKRZ, 1. Stock, Beim Schlump 58, 20146 Hamburg,

Germany

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