Saturday, December 10, 2011

1031762366.txt

From: Mike Hulme <m.hulme@uea.ac.uk>
To: "Iain Brown (UKCIP)" <iain.brown@ukcip.org.uk>
Subject: Re: temporal interpolation for UKCIP scenarios
Date: Wed Sep 11 12:39:26 2002
Cc: geoff.jenkins@metoffice.com,x.lu,j.turnpenny

Iain (and Geoff),
Definitive explanations are always dangerous! The reasoning behind this is as follows:
- the report only analysed and pictured seasonal and annual data (DJF,MAM, etc.) [in fact,
nearly all published maps of climate model outputs show changes in seasonal - 3-month -
averages]. This applying a uniform filter over 90 or 360 days.
- the requested datasets are at monthly time-steps. The default option for this is in
effect applying a uniform 30-day filter. [one might also conceive of weekly or daily
time-step files - e.g. changes in Week 13 for the 2050s for precip. for Medium-High or
changes for Julian day number 256 for the 2080s for Tmin for Low].
- these are all arbitrary choices of course, dictated by convention. But the important
point it seems to me is again a signal to noise issue - the shorter the time-averaging
period, the weaker the S/N ratio [i.e., we have more confidence that averaged over a year,
Tmin in the UK will increase by, say, 2.7degC for certain scenario, than that for the same
scenarios Tmin on 13 June will increase - on average - by 2.6degC and on 14 June only by
2.3degC - is this difference between 2.6 on 13 June and 2.3 on 14 June really meaningful?
No - it is most likely due to noise - natural variability].
- this reasoning suggests that as the time-averaging period decreases, one should pay less
attention to small differences between adjacent time-averaged periods, e.g. if June precip.
goes down by 10%, is the fact that July precip. goes down by 20% and August by 5% really
meaningful?
-
At 10:13 11/09/02 +0100, Iain Brown (UKCIP) wrote:

Mike,
For the UKCIP Scenarios datasets - both 98 and 02 - temporal interpolation
was applied to the raw model data in the form of a 1-2-1 filter. This had
the effect of smoothing out monthly values so that there are not as abrupt
transitions between adjacent months.
Can you provide us with the definitive explanation for the interpolation?
Some users (eg. in the recent London study) have noted that there are
differences between the maps they have derived from the data and the maps in
the UKCIP02 report.
best wishes,
Iain
-----------------------------------
Dr. Iain Brown
UK Climate Impacts Programme
12 St. Michael's St.
Oxford
OX1 2DU

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