cc: "Phil Jones" <p.jonesatXYZxyz.ac.uk>

date: Wed, 26 Mar 2008 12:32:16 +0000

from: Ian Harris <i.harrisatXYZxyz.ac.uk>

subject: Re: further CRUTS2.1 vs 3.0 comparisons

to: t.osbornatXYZxyz.ac.uk

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Hi Tim,

No criticism inferred! Sorry if I was snappy - as you can imagine I'm

heartily sick of this dataset! In a professional way of course.

My proposal is as follows:

� Derive TMN/TMX from gridded DTR and TMP anomalies.

Special cases:

� Where TMP is missing, TMN/TMX/DTR marked missing.

� Where DTR is missing, TMN/TMX = TMP, DTR = 0.

Please agree or adjust these (I've probably misunderstood somewhere),

then I'll code up a converter later today.

Cheers

Harry

On 25 Mar 2008, at 9:17, Tim Osborn wrote:

> Hi Harry,

>

> sorry for any implied criticism of you, Harry -- I appreciate the

> difficulties in working from incomplete documentation (having just

> been

> modifying ClimGen over the last week!). Just trying to get through

> all

> this fast, since some partners have collaborators arriving TODAY

> and need

> the observational data to calibrate their hydrological model with.

>

> The preference for gridding TMP and DTR and then deriving TMN and TMX

> arises in regions where only TMP exists or early on when only TMP

> exists.

> Suppose one such month has a TMP ANOMALY of -4 C. With no DTR, TMN

> or TMX

> data, gridding each of these would lead to them being relaxed to their

> climatological normals, i.e. anomalies of 0 C. Yet users will ask,

> how

> can both TMN and TMX have zero anomaly at the same time that TMP

> anomaly

> is -4 C?!!! The answer might be "well look at the station coverage

> data

> and you'll see that we don't in fact know what TMN and TMX are, so

> just

> don't use them at all in this instance". But the alternative would

> be to

> say that our best guess in the absence of real information is that

> TMN and

> TMX anomalies are also -4 C, which is what we'd get if we derived them

> from TMP and DTR=zero. Not likely to be completely correct, but

> the guess

> will have some skill over assuming their anomalies are zero.

>

> I'm not sure what you want to do for CRU TS 3.0, but for ClimGen I

> shall

> go ahead with TMP, DTR as provided, and then calculate TMN and TMX as

> described earlier.

>

> Cheers

>

> Tim

>

> On Sun, March 23, 2008 10:53 pm, Ian Harris wrote:

>> Hi,

>>

>> I did indeed follow a different process from 2.10. This was

>> remiss, but

>> can be explained.

>>

>> Firstly, I don't really think the published papers reflect the

>> actualit� of how the dataset was produced. Neither do the read me

>> files. They contain elements of truth but there are glaring

>> inaccuracies (including the gridding method, possibly the most

>> important single issue). For this reason I've not been as slavish to

>> them as I should have been.

>>

>> Secondly, the tmin and tmax databases are very good. I've removed

>> many

>> duplicates and 'badly augmented' stations and the two databases I

>> used

>> are completely aligned. I think that if stations are measuring a

>> parameter then that's the truth of the matter (conceding the

>> complications arising from measurement times).

>>

>> There are climatologies for DTR, TMN and TMX. This also pushed me

>> towards treating them as primary parameters.

>>

>> I derived a DTR database from the TMN and TMX databases, and gridded

>> all three. That's what you're playing with.

>>

>> If you want TMN and TMX to be derived from TMP and DTR, so be it

>> (though it seems superfluous since it's simply derived).

>>

>> The problem comes when one considers this process - when we are

>> talking

>> about TMN and TMX as theoretical constructs that may not have existed

>> at all.

>>

>> The TMN, TMX (and therefore DTR) databases each have 13654 stations.

>> Even if they require further cleaning, and I'm sure that they do,

>> that's still a lot more than TMP.

>>

>> Incidentally, sorry but I don't really follow this deduction:

>>

>>>> Since TMN and TMX are both correlated with TMP, the v2.1 method is

>>>> clearly the right way to go.

>>

>> When either are correlated with TMP, then the strength of that

>> correlation is of great interest, isn't it? But the approach of TMN =

>> TMP - DTR/2, TMX = TMP + DTR/2 just creates dummies with a

>> correlation

>> of 1.0. And although the correlation between TMP and DTR is very

>> important, DTR is not the only information contained in the TMN

>> and TMX

>> databases.

>>

>> Cheers

>>

>> Harry

>>

>> On 21 Mar 2008, at 18:17, P.JonesatXYZxyz.ac.uk wrote:

>>

>>>

>>> Tim,

>>> As far as I'm concerned, the T fields that get gridded

>>> are Tmean and DTR. It might be that the climatology

>>> is for Tmean, Tx and Tn. If this is the case then we

>>> need a DTR climatology.

>>>

>>> Then what you say below is how I would calculate as you say.

>>>

>>> Hopefully this is what Harry has done.

>>>

>>> It might be that Harry has done a Tx and Tn gridding as the

>>> two datasets aren't exactly the same - so you lose a bit of

>>> data getting DTR, but it is very, very small.

>>>

>>> Cheers

>>> Phil

>>>

>>>

>>> 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.

>>>> Here's the comparison when I make my own TMN from TMP and DTR for

>>>> v3.0 and compare with TMN from v2.1.

>>>>

>>>> Tim

>>>>

>>>> --------------------------

>>>> 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?

>>>>

>>>> 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,

>>>>

>>>> TimDr Timothy J Osborn, Academic Fellow

>>>> Climatic Research Unit

>>>> School of Environmental Sciences

>>>> University of East Anglia

>>>> Norwich NR4 7TJ, UK

>>>>

>>>> e-mail: t.osbornatXYZxyz.ac.uk

>>>> phone: +44 1603 592089

>>>> fax: +44 1603 507784

>>>> web: http://www.cru.uea.ac.uk/~timo/

>>>> sunclock: http://www.cru.uea.ac.uk/~timo/sunclock.htm

>>>>

>>>>

>>>

>>>

>>

>>

>

>

>

>

Ian "Harry" Harris

Climatic Research Unit

School of Environmental Sciences

University of East Anglia

Norwich NR4 7TJ

United Kingdom

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