cc: "Phil Jones"
date: Mon, 6 Jul 2009 17:02:48 -0600 (MDT)
from: "Kevin Trenberth"
subject: Re: Your visit to ECMWF and a question on the PREC/L dataset
to: adrian.simmons@ecmwf.int
I agree with Phil on that: poor or no documentation and none that is
referencable does not help.
Kevin
> Kevin
>
> You'll see from later mails that it's a problem for PREC/L also, though
> its apparently better analysis method (compared at least to that which
> used to be used in the GPCC Monitoring Product) helps a bit. Also, Phil
> Jones grumbles to me from time to time how difficult it is to get the
> GPCC people to write up fully what they have done. He tells me he will
> see you at the IPCC scoping meeting in a couple of weeks time, so
> perhaps you'll be able to find a little time to discuss these matters
> then.
>
> I think the real issue is to get the countries to send comprehensive
> gauge records promptly to the data centres, not five or ten years after
> the event. This is important in the climate-service context.
>
> Best regards
>
> Adrian
>
>
> Kevin Trenberth wrote:
>> Hi all
>> My comment and concern is that when stations come and go it corrupts the
>> record and this is especially an issue for GPCC. How that is dealt with
>> whether by using anomalies or whatever is a big issue. In GPCC this was
>> poorly done earlier (not taken into account) as I understand it
>> Kevin
>>
>>> Aiguo
>>>
>>> I've just read the actual gauge numbers given with the PRECL data. For
>>> North America there is a sharp drop in gauge numbers at the end of 1996
>>> (not 1997). But for Australia there is a dramatic drop at the end of
>>> 1992 - from about 1400 gauges to about 90 gauges, and the number falls
>>> after that so there are only 40-50 gauges being used in the latest
>>> years. So it is not surprising that PREC/L appears to be the poorest of
>>> all the datasets for Australia. I am inclined though to add it into the
>>> paper, since it has been quoted in AR4 etc.. The Full Data GPCC v4
>>> product (and the forthcoming version of GPCC based on it) looks a much
>>> better bet to me.
>>>
>>> PREC/L looks better in terms of coverage going back in time, with
>>> ~13000
>>> gauges used globally for 1973, but the GPCC Full Data Product v4 has
>>> ~28000 for this year. Globally, PREC/L is down to ~2000 gauges in 2007
>>> compared with ~8000 in GPCC v4.
>>>
>>> Best regards
>>>
>>> Adrian
>>>
>>>
>>> Aiguo Dai wrote:
>>>> Dear Adrian,
>>>>
>>>> Thanks for hosting our visit to ECMWF and we enjoyed talking to you
>>>> and
>>>> other colleagues there.
>>>>
>>>> The CPC analyses rely heavily on the NCDC GHCN2 station data for
>>>> precipitation, and the GHCN2 has a smaller
>>>> number of gauges since 1997. I normally use GPCP (after adjustment for
>>>> mean difference) for years after
>>>> 1997.
>>>>
>>>> Aiguo
>>>>
>>>>
>>>> Adrian Simmons wrote:
>>>>> Dear Aiguo
>>>>>
>>>>> It was good to talk with you and June when you visited ECMWF on
>>>>> Friday. Thank you in particular for being willing to discuss matters
>>>>> you raised in your review of our paper on near-surface humidity over
>>>>> land. I'll amend it to make it clear that the GPCC full data product
>>>>> version 4 is based on gridding anomalies, and add concluding
>>>>> discussion of the evidence that leads us to conclude that the decline
>>>>> in relative humidity in recent years is not dimply due to our picking
>>>>> up shift in observational bias.
>>>>>
>>>>> After talking with you, I eventually found the PREC/L dataset on the
>>>>> CPC website. I've produced versions of Table 5 and Fig 12 with it
>>>>> included. Results are attached. To be honest, it does not look like
>>>>> much of an improvement over CMAP, and it really seems to be the "odd
>>>>> man out" when it comes to Australia. This is particularly the case
>>>>> since about 1997, as can be seen in Fig. 12. Are you aware of any
>>>>> particular problem with PREC/L after then, particularly with regard
>>>>> to
>>>>> Australia? I ask particularly because in your review you mentioned
>>>>> that the gauge coverage for PREC/L may not be as good as for GPCC
>>>>> after 1997.
>>>>>
>>>>> Best regards
>>>>>
>>>>> Adrian
>>>>
>>> --
>>> --------------------------------------------------
>>> Adrian Simmons
>>> European Centre for Medium-Range Weather Forecasts
>>> Shinfield Park, Reading, RG2 9AX, UK
>>> Phone: +44 118 949 9700
>>> Fax: +44 118 986 9450
>>> --------------------------------------------------
>>>
>>
>>
>> ___________________
>> Kevin Trenberth
>> Climate Analysis Section, NCAR
>> PO Box 3000
>> Boulder CO 80307
>> ph 303 497 1318
>> http://www.cgd.ucar.edu/cas/trenbert.html
>
> --
> --------------------------------------------------
> Adrian Simmons
> European Centre for Medium-Range Weather Forecasts
> Shinfield Park, Reading, RG2 9AX, UK
> Phone: +44 118 949 9700
> Fax: +44 118 986 9450
> --------------------------------------------------
>
___________________
Kevin Trenberth
Climate Analysis Section, NCAR
PO Box 3000
Boulder CO 80307
ph 303 497 1318
http://www.cgd.ucar.edu/cas/trenbert.html