from: Tim Osborn <t.osbornatXYZxyz.ac.uk>
subject: Re: further CRUTS2.1 vs 3.0 comparisons
to: Phil Jones <email@example.com>,Ian Harris <i.harrisatXYZxyz.ac.uk>
Here's the comparison when I make my own TMN from TMP and DTR for
v3.0 and compare with TMN from v2.1.
Phil & Harry,
Next problem! :
I've now made comparisons for TMN, TMX and DTR for v3.0 vs. v2.1.
Attached is the result for TMN. You'll see we now have 4 plots per
month, for Jan-Apr-Jul-Oct. Plot 1 is the temporal
correlation. Plot 2 is the SD of v2.1. Plot 3 is the ratio of v2.1
SD to v3.0 SD. Plot 4 is the SD of v3.0.
Clearly the correlation is rather weak in many areas and the ratio of
standard deviations exceeds 1.5 across most of S America, Africa and
India, plus Greenland and N. Russia.
I think that this may be due to the way you've made the data. I've
checked Mitchell and Jones (2003) and it says very clearly that TMN
(and TMX) are derived variables, taken entirely from the grids of TMP
and DTR (presumably TMN = TMP - 0.5*DTR and TMX = TMP + 0.5*DTR???).
Now if you haven't followed this approach for making v3.0 but have
instead "independently" made gridded fields of TMP, DTR, TMN and TMP,
relaxing to climatology where there are no nearby observations, so
that they are all primary variables and none are derived, then if you
have TMP data but not DTR, TMN or TMX values, the latter 3 will be
relaxed to climatology. But the v2.1 methodology would relaxed DTR
to climatology, but TMN would be TMPactual - 0.5*DTRclimatology, and
hence would still have variations that paralleled the observed
variations in TMPactual.
Since TMN and TMX are both correlated with TMP, the v2.1 method is
clearly the right way to go. The only time when something different
to both approaches might be useful is if you have lots of
stations/months with only TMN or TMX but not both. But Harry says
that you generally have both or none. In which case v2.1 will be the
best we can do.
Harry -- now that I've confirmed what Mitchell & Jones did to make
v2.1, can you confirmed that you have made v3.0 in the way I described above?
Phil -- if Harry says "yes", then will I get exactly the "correct"
result if I ignore the TMN and TMX files that Harry's made and
instead make my own from TMP and DTR, using:
TMN=TMP-0.5DTR and TMX=TMP+0.5DTR?
I tested this and now get much better correlations with v2.1 for TMN,
and presumably (not checked yet) for TMX. The standard deviations
are now much more similar too. I'll attach the plot with the next
email (too big for this one!).
Given the time and effort I've put in to CRU TS 3.0, I shall expect
to be a co-author when the paper describing CRU TS 3.0 is written!
Cheers and happy Easter,
Attachment Converted: "c:\eudora\attach\cruts_tmp-0.5dtr_cmp.pdf"
Dr Timothy J Osborn, Academic Fellow
Climatic Research Unit
School of Environmental Sciences
University of East Anglia
Norwich NR4 7TJ, UK