date: Tue Nov 16 16:47:48 2004

from: Phil Jones <p.jonesatXYZxyz.ac.uk>

subject: Re: Fwd: Re: Mean Monthly Temperature

to: "Hans Teunissen" <HTeunissenatXYZxyz.int>, "Greg Hammer" <gregory.r.hammer@noaa.gov>, <Thomas.C.PetersonatXYZxyza.gov>, "H Kontongomde" <HKontongomde@wmo.int>, "Richard Thigpen" <RThigpenatXYZxyz.int>

Dear All,

Encouraging Malta to send its GSN data for Luqa (the airport) to Greg is fine. Perhaps

at the next AOPC meeting we should consider how we should be responding to

questions about how to calculate monthly averages. What is in the WMO Guide

in 1983 may be correct, but if you look in the WWR volumes for 1951-90 (all 4) you

will see that Malta has calculated its monthly mean pressures based on 4 synoptic

hours and the mean temperature based on the max and min.

So the advice to change will introduce a homogeneity into the Luqa record. The

example given is fine, but other equally plausible possibilities could be conjured up.

The whole question is are we after a consistent long record or the best estimate of the

true temperature. I would argue for the former. Whether the method gives a better estimate

of the truth is not the issue - long-term homogeneity should be paramount. I can easily

show that anomalies (from a common base period) derived from these two methods

(mean of 4 or 8 synoptic hours and the mean of min/max) will give virtually

indistinguishable

different trends. There will be absolute differences though when the change is made and

this will screw up the homogeneity.

This same issue came up with the Antarctic stations in the last teleconference call.

Can

we discuss what advice we should be giving? Can this Hans be added to our agenda for

April? We will likely have to discuss this with other commissions. As far as I believe,

most inhomogeneities in station temperature time series are due to two things:

- site moves

- changes in the way monthly mean temperatures are calculated

in a roughly equal percentage.

So, the advice being given for Luqa WILL introduce an inhomogeneity. Tom's software is

so good it will spot it but only after about 5 years !

Perhaps we need a Statement of Guidance on calculating daily and monthly averages.

Why am I saying this as you'll only say go ahead and write it, but at the moment we seem

to be giving the wrong advice. I hope GSN is not getting countries to think too deeply

into

their data. They need better advice if they do. Maybe by April we can see if Malta has

sent

anything !

Cheers

Phil

At 16:03 16/11/2004, Hans Teunissen wrote:

Dear All:

The original corrspondence that was the basis for the exchange I just sent you (some as

bc's), FYI.

Hans.

=================================================================

Dr. Hans W. Teunissen Tel: +41.22.730.8086

Global Climate Observing System (GCOS) Fax: +41.22.730.8052

c/o World Meteorological Organization E-mail: [1]HTeunissenatXYZxyz.int

7 bis, Ave. de la Paix

CP 2300, CH-1211 Geneva 2

Switzerland

=================================================================

Date: Tue, 16 Nov 2004 13:53:56 +0100

From: "Alan Thomas" <AThomasatXYZxyz.int>

To: "Hans Teunissen" <HTeunissenatXYZxyz.int>

Subject: Fwd: Re: Mean Monthly Temperature

Mime-Version: 1.0

Content-Type: text/plain; charset=US-ASCII

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Dr Alan R Thomas

Director, GCOS Secretariat

c/o WMO

Case postale 2300

7bis, Avenue de la Paix

CH-1211 Geneva 2

41-22-730-8275

41-22-730-8052 (fax)

>>> Alexander Karpov 16/11/2004 1:42:10 PM >>>

Dear Mr Porter,

Thank you very much for your message related to the procedure for MMT

calculations. Accordingly, the secretariat had approached an expert in

this field and the principle Author of "handbook on CLIMAT and CLIMAT

TEMP Reporting", Dr Oleg Alduchov (World Data Centre B, Russian

Federation) to provide comments on your query. Based on his input, we

would like to inform you on the following.

1. Unfortunately, it should be noted that changes in the estimation of

daily mean values introduced in your Service in January 1995 were not in

compliance with the WMO GUIDE TO CLIMATOLOGICAL PRACTICES (WMO-No. 100),

1983, second edition. The Guide is also accessible through

[2]http://www.wmo.ch/web/wcp/wcdmp/home.html. You may also wish to note

an appropriate excerpt from Chapter 8 of this Guide - CLIMAT REPORTS,

which reads as follows:

"For the mean monthly pressure, be it station pressure or standard

level pressure, it is advisable to use a true mean or a corrected value

to correspond to a mean based on 24 direct observations a day. The mean

of four equally spaced observations may suffice as a close approximation

to the true mean."

..........................................

For monthly mean temperature the same remarks apply as for computation

of monthly mean pressure."

It is understood that the same rule has to be applied for the

computation of all other monthly mean values. It should be also

mentioned that the use of 8 equally spaced observations gives slightly

better estimation of "true daily mean".

2. As regards the current practice of your service - half of a sum of

minimal and maximal temperatures - for daily mean values, it might give

a good estimation of monthly mean value only for the periods without any

significant changes of weather and "standard/normal" sinus-like

changes of observed parameters.

3. To illustrate the above, one could assume that during 21 hours we

had a very stable temperature 25 C, followed by a significant fall of

the temperature down to 18 C during last 3 hours. According to current

practice you mentioned, the calculation of the daily mean value would

give 21.5 C. However, the WMO-recommended procedure gives the value of

23.25 C. Naturally, in this case the last value would better represent

the daily mean temperature. More generally, for many stations it is

proved that anomalies for minimal and for maximal temperatures have

different statistical properties. Therefore, the daily mean values based

of the half sum of minimal and maximal temperatures often provide a

systematical bias with respect to the "real daily mean temperature".

Actually, the same is correct for many other observed meteorological

parameters.

4. In addition to the above, please be informed that the third edition

of the WMO GUIDE TO CLIMATOLOGICAL PRACTICES is now in preparation and

will comprise some new developments and changes. However, it would be

advisable to follow the guidelines laid down in the current version

until the third edition will be published.

I trust that the above information would help you to meet stated

requirements in compiling climatologcal reports. Should you have any

other specific questions, do not hesitate to contact the secretariat and

directly Dr O.Alduchov (His address is aoaatXYZxyzeo.ru).

Yours sincerely,

A. Karpov

>>> <saviour.porteratXYZxyztairport.com> 12.11.2004 10:44:09 >>>

Dear Mr. Karpov,

1. We have just received WMO/TD No.1188 and have noted that when

computing

the Mean Monthly temperature, the values for 00Z 06Z 12Z and 18Z should

be

taken into consideration. These should be added up, and then divided by

4

- this becomes the mean temperature for the day and so on until the

end of

the month, - the mean temperature for the month is then computed from

these daily values.

2. However my predecessors, since at lease January 1995, have computed

the

mean daily temperature differently - namely they found the maximum

temperature for the day, added to it the minimum temperature for the

night

and divided this by 2, and they called this the mean temperature for

the

day. The values for the Standard Deviation is also computed from these

values. We have inherited from them this procedure - however it gives

quite

a different value to the procedure described in my previous paragraph

(para.1) . One of these systems gives large errors.

3. I would be most grateful to you if you could verify which system

should

be used - and if this is the system outlined in para 1 would you

please

tell us what should be done to rectify matters.

Yours sincerely

Saviour Porter

Permanent Representative of Malta with WMO

Chief Meteorological Officer

Malta International Airport plc

Tel: (+356) 2369 6527

Fax: (+356) 2124 6694

MIA Weather Website : [3]http://www.maltairport.com/weather

Prof. Phil Jones

Climatic Research Unit Telephone +44 (0) 1603 592090

School of Environmental Sciences Fax +44 (0) 1603 507784

University of East Anglia

Norwich Email p.jonesatXYZxyz.ac.uk

NR4 7TJ

UK

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