Friday, April 13, 2012

3400.txt

cc: "Birgitte von Christierson" <b.v.christiersonatXYZxyzallingford.co.uk>
date: Tue Jul 28 12:05:07 2009
from: Phil Jones <p.jonesatXYZxyz.ac.uk>
subject: Re: Sampling UKCP09?!
to: "Steven Wade" <s.wadeatXYZxyzallingford.co.uk>

Steven,
We'll discuss this with Geoff next week.
A few things
In your 3) the way MOHC produced the probabilistic projections GCMs and RCMs
(with their perturbed physics) were given greater weight if they simulated the
20th century better. This is a part validation, so better models and parameter sets
get preferred.
In 4) not clear what 'more homogeneity than the 11 RCMs' means?
Your key bullet lower down is the third one. You do need to sample the full range -
somehow.
This is difficult to do in the joint probability case.
In 5) totally agree that you can get very different results depending on how you sample
UKCP09. This should be clear from reading the reports, but it is difficult to get across
when people use UKCP09 as a tool - without complete understanding.
The one thing you don't mention relates back to the work we did for the EA
project in 2005/6. This is that it is still very likely that historic droughts will
be more extreme than future ones. This is because the GCMs and RCMs and
the WG don't do low-frequency variability very well. The WG will have reasonable
extremes, but is unlikely they will bunch together into drier and wetter decades
and bi-decades. This is discussed in the large report under blocking, and also
in the WG report. The WG may not do 2-3 year droughts that well - but neither
will the RCMs.
Cheers
Phil
At 10:19 27/07/2009, Steven Wade wrote:

Phil,

I wondered whether you might have a view on how to make UKCP09 a little more practical
for water companies?

I attach a note on our attempt to capture the range of UKCP09 by using Optimal Latin
Hypercube Sampling that makes sure we can sets of factors that cover a reasonable range
of UKCP09 and maintain characteristics of the Met Office emulator outputs (combined
probabilities in P and T for each season and between seasons)

We are suggesting that using 10-20 samples is a good first step and use of say 100
Weather Generator runs would come later for sites where there are significant risks.

You must of tackled some of the same issues trying to make your WG outputs consistent
with UKCP09 (?) so I would welcome your opinion on this.

Regards,

Steven

Steven Wade - Group Manager
Water Management
HR Wallingford Ltd
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