Monday, June 11, 2012

5079.txt

cc: p.jones@uea.ac.uk,K.Briffa@uea.ac.uk
date: Wed, 06 Jun 2001 10:38:37 +0100
from: Tim Osborn <t.osbornatXYZxyz.ac.uk>
subject: Re: volcanoes
to: tom crowley <tomatXYZxyzan.tamu.edu>

Tom,

thanks for the recent message (and the previous one which I hadn't had time
to answer till now!). Here are my comments/thoughts:

At 10:18 31/05/01 -0500, tom crowley wrote:
>we have just completed a comparison a seasonal model simulation of climate
>change over the last 1000 years with the Briffa et al data. the seasonal
>ebm allows us to sample the model field over the area (20-90N land) and
>time (April-Sept) of the Briffa et al. 2001 JGR paper. Correlations of
>decadally smoothed model and observations is 0.76 for the whole record and
>0.82 over the post-1500 interval. The amplitudeand shapte of the very
>large decadal oscillationis in the 17th century are generally very well
>simulated.
>
>We are now comparing the model results with the Briffa et al 1998 volcano
>paper for individual years. Inspection of the annual model time series
>shows many of the same peaks you identify (we force our volcano record with
>a composite of ice core time series from Crete and Gisp2).
>
>BUT I am still a little unsure about how we should average our data, since
>your 1998 paper - if I understand your paper correctly you are not
>averaging over the same area as the 2001 paper - I read your 1998 paper as
>indicating all NH April-Sept.
>
>Is this correct? Please verify so we can analyze our model fields in an
>identical manner.

The NH time series in the 1998 volcano paper was not calibrated in a
particularly rigorous manner, though you are correct to say that we did
calibrate it against the full NH mean (i.e., land AND marine, everything
north of the equator) for April-September, subject of course to the
incomplete coverage of observed temperatures over certain regions during
the 1881-1960 period. The data used in the 1998 paper were the
standardised tree-ring densities (i.e., dependence of density on tree-age
was removed by fitting a Hugershoff function to each tree-core series),
which have less low-frequency variance than the age-banded approach used in
our 2001 JGR paper. We have also used the "Hugershoff standardised" data
set to reconstruct the April-Sept land north of 20N series (i.e., the same
series as we reconstructed in the 2001 JGR paper using the age-banded
version). This improved quasi-NH reconstruction is described in a paper
(Briffa et al.) that has been provisionally accepted for a special issue of
The Holocene, which will probably appear in 2002. If you're interested in
a series with the short-timescale variations but not the long-timescale
variations, then it is probably better to use the 2002 Holocene calibrated
reconstruction rather than the 1998 volcano series (they're obviously
similar, but not identical). I'm not sure exactly when we'll release that
series, however, since the paper won't appear for some time - I'll have to
check with Keith about that.

>Thanks for any help and of course I will send you a preprint of the paper
>when it is written up.

We would, indeed, be keen to see the preprint - and also to make a
contribution as co-authors if that is appropriate or necessary.

At 14:55 25/04/01 -0500, tom crowley wrote:
>we are in the preliminary stages of comparing our 2d ebm simulations
>(April-Sept, >30 N land) with the Briffa et al time series you sent me. We
>simulate a larger early 20th century temp rise than you reconstruct, but
>yours is also smaller than the Jones et al observations you illustrate in
>your JGR paper.
>
>do you have any idea why you are not reconstructing the full magnitude of
>the 20th century rise? it certainly can not be attributable to low
>sensitivity of your index, for it is very "excitable" in the earlier part
>of the record.

We have thought about this in some detail, and also tested a few things
out. The correlation between the tree-density series and the observed
temperature series is stronger at the longer timescales (e.g. for decadally
smoothed data) than for shorter timescales (sub-decadal). Since the
regression slope (and hence the variance of the reconstruction) is always
reduced in situations where the correlation is poor, it seems that the
poorer performance at the shorter timescales has degraded the overall
variance of our reconstruction and hence resulted in a smaller magnitude of
early 20th century warming in the reconstruction. We have attempted to
create an alternative reconstruction, where we calibrate separately on the
high and low frequencies, and this results in a noticeably greater early
20th century warming. However, the degrees of freedom for the decadal
calibration are rather few, due to the smoothing, and therefore it is very
easy to overfit the decadal component of the reconstruction. We have a
paper describing this study coming out soon in Dendrochronologia - let me
know if you'd like a preprint/reprint.

>also can you send me the unsmoothed Jones et al NH land Ap-Sept time series
>you used so I can be sure I am using the same observations you are?

The file is attached (Apr-Sep land north of 20N), as anomalies in degrees C
from the 1961-1990 reference period mean. We only use 1881-1960 for our
final calibration.

Best regards

Tim





138
1860 -0.14
1861 -0.16
1862 -0.18
1863 0.03
1864 -0.28
1865 0.28
1866 -0.05
1867 -0.42
1868 0.38
1869 0.09
1870 0.19
1871 -0.22
1872 0.31
1873 -0.25
1874 -0.05
1875 -0.31
1876 -0.11
1877 -0.27
1878 0.26
1879 -0.28
1880 -0.10
1881 -0.14
1882 -0.38
1883 -0.39
1884 -0.71
1885 -0.59
1886 -0.25
1887 -0.22
1888 -0.45
1889 -0.21
1890 -0.27
1891 -0.42
1892 -0.31
1893 -0.35
1894 -0.21
1895 -0.22
1896 -0.20
1897 0.02
1898 -0.20
1899 -0.22
1900 -0.09
1901 0.02
1902 -0.63
1903 -0.60
1904 -0.57
1905 -0.31
1906 -0.06
1907 -0.80
1908 -0.42
1909 -0.48
1910 -0.35
1911 -0.28
1912 -0.70
1913 -0.54
1914 -0.31
1915 -0.27
1916 -0.40
1917 -0.46
1918 -0.49
1919 -0.23
1920 -0.14
1921 0.09
1922 -0.03
1923 -0.30
1924 -0.19
1925 -0.02
1926 -0.28
1927 -0.07
1928 -0.28
1929 -0.23
1930 0.05
1931 0.04
1932 0.15
1933 -0.06
1934 0.05
1935 -0.12
1936 0.17
1937 0.27
1938 0.38
1939 0.19
1940 0.08
1941 -0.05
1942 0.00
1943 0.23
1944 0.19
1945 0.01
1946 0.14
1947 0.12
1948 0.21
1949 0.02
1950 -0.09
1951 0.14
1952 0.20
1953 0.33
1954 -0.01
1955 0.06
1956 -0.33
1957 -0.07
1958 -0.07
1959 0.11
1960 -0.02
1961 0.15
1962 0.05
1963 0.02
1964 -0.23
1965 -0.35
1966 -0.06
1967 -0.01
1968 -0.18
1969 -0.20
1970 -0.02
1971 -0.14
1972 -0.30
1973 0.05
1974 -0.10
1975 0.19
1976 -0.23
1977 0.23
1978 -0.13
1979 -0.08
1980 0.07
1981 0.32
1982 -0.00
1983 0.18
1984 0.03
1985 0.01
1986 0.07
1987 0.11
1988 0.50
1989 0.37
1990 0.50
1991 0.49
1992 -0.19
1993 -0.00
1994 0.60
1995 0.54
1996 0.11
1997 0.42

Dr Timothy J Osborn | phone: +44 1603 592089
Senior Research Associate | fax: +44 1603 507784
Climatic Research Unit | e-mail: t.osbornatXYZxyz.ac.uk
School of Environmental Sciences | web-site:
University of East Anglia __________| http://www.cru.uea.ac.uk/~timo/
Norwich NR4 7TJ | sunclock:
UK | http://www.cru.uea.ac.uk/~timo/sunclock.htm

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