cc: m.manton@uea.ac.uk, david.parker@metoffice.gov.uk
date: Tue Nov 16 16:47:48 2004
from: Phil Jones
subject: Re: Fwd: Re: Mean Monthly Temperature
to: "Hans Teunissen" , "Greg Hammer" , , "H Kontongomde" , "Richard Thigpen"
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]HTeunissen@wmo.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"
To: "Hans Teunissen"
Subject: Fwd: Re: Mean Monthly Temperature
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Content-Disposition: inline
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 aoa@meteo.ru).
Yours sincerely,
A. Karpov
>>> 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.jones@uea.ac.uk
NR4 7TJ
UK
----------------------------------------------------------------------------