Tuesday, May 22, 2012

4582.txt

date: Fri, 20 Feb 2009 09:51:40 +0000
from: Adrian Simmons <Adrian.SimmonsatXYZxyzwf.int>
subject: Re: Temperature trends and SST analyses
to: Phil Jones <p.jonesatXYZxyz.ac.uk>

Thanks, Phil. I think it is worth saying something about absolute
values. I'm just looking at them for precip, for which the
observation-based data do not come as anomalies.

Adrian


Phil Jones wrote:
>
> Adrian,
> The absolute values for ERA-Interim are interesting. Over the
> 1989-2007 period
> the range is 0.55 from the coldest in 1992 (13.74) to 2005 (14.29).
> These compare
> with HadCRUT3v from 1992 (+0.07) to 1998 (+0.53), so a difference of
> 0.46. The value
> is slightly smaller if 2005 is used instead of 1998.
> Using the 14K value for HadCRUT3v for 1961-90 indicates that the
> absolute
> values differ by about 0.3K (with HadCRUT3v warmer), a value that is
> consistent with
> your cold bias relative to SYNOPS of about 0.5K. I agree that the SST
> differences are
> an issue, but I think we are quite close in absolute terms.
> This might be worth reporting in a small section. A map of the
> absolute differences
> may show some larger cancelling errors across the world.
>
> The attached figure and bit of text comes from this paper - which is
> too large to email
> in full.
>
> Jones, P.D., New, M., Parker, D.E., Martin, S. and Rigor, I.G., 1999:
> Surface air temperature and its variations over the last 150 years.
> Reviews of Geophysics 37, 173-199.
>
> The range of absolute global averages is from 12 to 16K, almost the
> same as the second
> of the two plots you sent yesterday.
>
> [Aside] The absolute temperature over the ocean in this paper is one
> for air temperature.
> Even though we use SST anomalies as a surrogate for air T over the
> oceans, the
> absolute was derived through the addition of an air minus SST climatology.
>
> We could possibly use ERA-Interim's average absolute fields to reduce
> my presumed
> uncertainty in the 14K number, by perhaps doing better over the data
> sparse regions
> such as the sea-ice areas and parts of the Antarctic.
>
> It isn't just the media that are interested in the absolute figure, a
> lot of users would
> like better absolute figures for driving impacts models where absolute
> thresholds can
> be important.
>
> Cheers
> Phil
>
>
>
> At 17:12 19/02/2009, Adrian Simmons wrote:
>> Phil
>>
>> All I did for the ten warmest years was to make sure that the set from
>> the Hadley Centre/BBC website had the same average as the ERA-Interim
>> set. ERA-Interim then shows a larger range. I did think this was
>> predominantly because ERA-Interim shows stronger warming in the average
>> over land (due to high-latitude coverage problems in CRUTEM3v), but with
>> the SST differences discovered this morning perhaps things are more
>> complicated. Maybe the SST difference is enough to allow 1995 into the
>> list for ERA-Interim. Not one for the journalists, I guess, though I do
>> wonder how/whether one should convey uncertainty in this sort of thing.
>>
>> Absolute values for ERA-Interim, for the record, are:
>>
>> 1989 13.78483200
>> 1990 13.99974918
>> 1991 13.94549942
>> 1992 13.73666668
>> 1993 13.76133442
>> 1994 13.83608341
>> 1995 14.01591682
>> 1996 13.87950039
>> 1997 14.00908375
>> 1998 14.20900059
>> 1999 13.91508484
>> 2000 13.94616604
>> 2001 14.12133312
>> 2002 14.16433334
>> 2003 14.16483307
>> 2004 14.08841705
>> 2005 14.28650093
>> 2006 14.21850014
>> 2007 14.16433430
>>
>> See also the first panel on the attached plot. But one should treat
>> these numbers with caution. Not only because of all the values after the
>> decimal point, (needed to distinguish 2002 from 2007), but because over
>> land we have a bias of about 0.5K cold compared with the SYNOPS.
>>
>> Adrian
>>
>>
>> Phil Jones wrote:
>> >
>> > Adrian,
>> > I think you're definitely imagining the ship tracks! These are
>> > decadal averages. They potentially do show the drifter/ship offsets
>> > I was talking about, though - about 0.1 to 0.2 deg C. John
>> > Kennedy may have a difference for ships minus drifters.
>> >
>> > In the warmest year pdf, I assume you're adding 14 to the
>> ERA-Interim
>> > anomalies. If not these are amazingly close!
>> >
>> > It would be useful to see what ERA-Interim does get for the global
>> > average
>> > surface absolute T for your 1989-99 base period. The work that
>> > estimated the 14
>> > number (it was 14.02 for the globe) made lots of assumptions over the
>> > sea ice
>> > areas and the Antarctic. I always thought this was accurate to about
>> > +/- 0.5 deg C.
>> > It was impossible to explain this accuracy of the absolute vs the much
>> > smaller error bars
>> > on the anomalies to any journalists.
>> >
>> > Cheers
>> > Phil
>> >
>> >
>> > Cheers
>> > Phil
>> >
>> >
>> > At 16:09 19/02/2009, Adrian Simmons wrote:
>> >> Hi everyone
>> >>
>> >> Here are the differences in map form. Shown in the first attachment
>> are
>> >> ten-year means relative to 1989-1999 (except the last month of 2008 is
>> >> missing, as ERA-Interim is still working on it). Please discount what
>> >> happens right near the coast and sea-ice regions, as I may not have
>> >> things quite right there.
>> >>
>> >> What is clear is that relative to 1989-1999, HadCRUT3v (or HadSST2) is
>> >> almost universally a little warmer than the SSTs we use in
>> ERA-Interim.
>> >> I think I can see ship tracks also, but maybe I'm over-interpreting.
>> >>
>> >> See also the second attachment. We should know by Saturday whether
>> 2008
>> >> makes it to the ERA-Interim list - I doubt it. ERA-Interim is
>> currently
>> >> analysing 18 December.
>> >>
>> >> Adrian
>> >>
>> >>
>> >> Phil Jones wrote:
>> >> >
>> >> > Adrian,
>> >> > I'm going down to the Hadley Centre next week on an unrelated
>> >> matter,
>> >> > and have arranged to talk to Kate and Peter about the HadCRUH
>> >> analyses,
>> >> > so we can add this one in as well. It would be worth showing these
>> >> > plots to
>> >> > John Kennedy and Nick Rayner to get their thoughts.
>> >> >
>> >> > HadCRUT3v uses HadSST2 anomalies over the ocean. HadSST2
>> should be
>> >> > similar to HadISST1 where there are data, but you require a
>> complete
>> >> > field so
>> >> > the latter is infilled - and there is also the sea ice component.
>> >> >
>> >> > The problem with the various SST forcing fields used by ERA and
>> >> NCEP
>> >> > is that
>> >> > it is made up of different components and these may have slightly
>> >> > different
>> >> > absolute fields. There is probably a document explaining how the
>> SST
>> >> > fields
>> >> > for Reanalyses was produced - maybe Nick will know about this.
>> >> Overall the
>> >> > differences will be small for most of the world's oceans, but are
>> >> > likely to be
>> >> > larger in data sparse regions and near the sea ice edges. So, if
>> >> you redid
>> >> > your plot for NH oceans, I'd expect it to be closer than a
>> similar one
>> >> > for the SH.
>> >> >
>> >> > As for the recent 8 years or so, there is an issue of the
>> dramatic
>> >> > increase in
>> >> > drifter based SST values, which may be nearer the true value, but
>> >> which
>> >> > seem
>> >> > about 0.1 to 0.2 deg C cooler than the ships. The HC are working
>> on a
>> >> > revised
>> >> > SST dataset (HadSST3 presumably) which I'd like to think would
>> >> solve these
>> >> > problems. The fact that precip looks better in more recent years
>> >> > suggests that
>> >> > the drifter-based SST may be nearer the truth.
>> >> >
>> >> > The important message here is that ERA-Interim must be
>> getting very
>> >> > good to
>> >> > be able to respond to barely detectable differences in SST
>> across the
>> >> > world's
>> >> > oceans.
>> >> >
>> >> > Cheers
>> >> > Phil
>> >> >
>> >> >
>> >> > At 09:35 19/02/2009, Adrian Simmons wrote:
>> >> >> Dear co-authors (also Saki, Per and Tim Stockdale)
>> >> >>
>> >> >> More interesting results. This time Kate can relax, as I'm back
>> on to
>> >> >> temperature trends.
>> >> >>
>> >> >> I wanted to produce a plot contrasting the recent temperature
>> changes
>> >> >> over land and sea. So I looked at the full HadCRUT3v dataset,
>> which is
>> >> >> based on Hadley Centre SST over the oceans, and compared the
>> >> ocean-wide
>> >> >> averages with the corresponding averages of ERA background 2m
>> >> >> temperatures. I chose the background fields not the analyses, as
>> the
>> >> >> later took in ship air temperatures and cannot be relied upon, as
>> >> >> discussed in Simmons et al.(2004), based on a comment from Simon
>> Tett.
>> >> >>
>> >> >> The upper panel in the attachment shows the result -
>> temperatures are
>> >> >> adjusted to have the same average for the period 1989-1998 as
>> before.
>> >> >> The most obvious feature is a shift between HadCRUT3v and ERA
>> >> around the
>> >> >> turn of the century. The lower panel shows the corresponding
>> >> comparison
>> >> >> between HadCRUT3v and the SST used in ERA (the same for
>> ERA-Interim as
>> >> >> ERA-40). Clearly there is a shift in recent years between the
>> SST used
>> >> >> in ERA and that used in HadCRUT3v.
>> >> >>
>> >> >> Now, ERA used the following SSTs:
>> >> >>
>> >> >> (i) HadISST1 monthly up until November 1981
>> >> >>
>> >> >> (ii) Then Reynolds 2D-Var weekly up until June 2001
>> >> >>
>> >> >> (iii) Then NCEP operations until September 2008
>> >> >>
>> >> >> (iv) Then OSTIA
>> >> >>
>> >> >> The lower panel clearly indicates a shift of between 0.1 and 0.2C
>> >> in SST
>> >> >> in 2001. So apparently there is a shift in going from the
>> 2D-Var to
>> >> >> NCEP operations, assuming that HadCRUT3v uses a homogeneous SST
>> >> product.
>> >> >>
>> >> >> Then there is a question of whether it is the 2D-Var or the NCEP
>> >> >> operations that is closer to HadCRUT3v. Remember the anomaly curves
>> >> are
>> >> >> normalised to 1989-1999 (just because that's what's convenient for
>> >> other
>> >> >> plots in the paper), but it could be that the absolute values are
>> >> closer
>> >> >> for the end of the period.
>> >> >>
>> >> >> Finally, I wonder whether this is related to the shift in
>> >> precipitation
>> >> >> over land in ERA around the turn of the century relative to
>> GPCC. It's
>> >> >> in the right sense (ERA shifting to relatively cooler SSTs, less
>> >> >> evaporation, and thus less precipitation over land). As Dick and I
>> >> >> discussed yesterday after looking at one of Per's plots for the
>> >> >> comparison of ERA with GPCP and CMAP, when looking at the absolute
>> >> >> values the precip is in better agreement in recent years than in
>> the
>> >> >> 1980s and 1990s, so perhaps the more recent SSTs are better also.
>> >> >>
>> >> >> I'd appreciate some input on this from the SST experts, at ECMWF
>> >> and the
>> >> >> Hadley Centre. Clearly we need to resolve this before the next
>> >> >> reanalysis, and I hope ERACLIM will get funded to enable us to
>> do this
>> >> >> thoroughly.
>> >> >>
>> >> >> Adrian
>> >> >>
>> >> >> PS Yesterday's interesting result was that the temperature
>> trends over
>> >> >> land from the ERA-Interim background fields are significantly
>> >> closer to
>> >> >> CRUTEM3v than are the trends from the ERA-40 background. The
>> analyses
>> >> >> themselves don't differ much, as there is plenty of 2m T data
>> >> >> assimilated by the OI analysis in grid boxes for which there are
>> >> >> CRUTEM3v values, and the OI analysis for 2m T is the same in ERA-40
>> >> and
>> >> >> ERA-Interim. But the result does suggest that more confidence
>> can be
>> >> >> placed in the ERA-Interim analysis than the ERA-40 analysis in data
>> >> >> sparse regions, where the background comes more into play.
>> >> >>
>> >> >> --
>> >> >> --------------------------------------------------
>> >> >> 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
>> >> >> --------------------------------------------------
>> >> >>
>> >> >
>> >> > 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
>> >> >
>> >>
>> ----------------------------------------------------------------------------
>>
>> >>
>> >> >
>> >>
>> >> --
>> >> --------------------------------------------------
>> >> 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
>> >> --------------------------------------------------
>> >>
>> >>
>> >
>> > 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
>> >
>> ----------------------------------------------------------------------------
>>
>> >
>>
>> --
>> --------------------------------------------------
>> 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
>> --------------------------------------------------
>>
>
> 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
> ----------------------------------------------------------------------------
>

--
--------------------------------------------------
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
--------------------------------------------------

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