The ACORN-SAT station at Butlers Gorge in central Tasmania
A new homogenised daily temperature set, the AustralianClimate Observations Reference Network – Surface Air Temperature (ACORN-SAT)data set, has recently been released by the Australian Bureau of Meteorology.
This data set contains daily data for 112 stations, with 60 of them extending
for the full period from 1910 to 2011, and the others opening progressively up
until the 1970s. (1910 is taken as the starting point, as it was only with the
formation of the Bureau of Meteorology in 1908 as a federal organisation that
instrument shelters became standardised).
The new data set applies differential adjustments to
different parts of the daily temperature frequency distribution, using an
algorithm which matches percentile points in the frequency distribution with
those at reference stations before and after an inhomogeneity. This takes into
account the fact that some inhomogeneities in temperature records have
different impacts in different weather conditions – for example, if a site
moves from a coastal location to one further inland, the difference in
overnight minimum temperature will normally be greater on clear, calm nights
than on cloudy, windy nights, and clear, calm nights are also likely to be the
coldest nights (at least in the mid-latitudes). If such effects are not
accounted for, adjustments which homogenise mean temperatures may not
homogenise the extremes.
The method is conceptually similar to that of Della-Marta
and Wanner (2006), while differential adjustment methods of this type have also
been developed by a number of other authors. The ACORN-SAT data set, however,
is believed to be the first implementation of such methods in a national,
year-round data set.
A detailed evaluation of the method was carried out before
finalising its implementation. One of the interesting findings from the
evaluation was that, while some earlier studies had suggested that reference
stations required a very high correlation (0.8 or above) for use in daily adjustment
methods, the ACORN-SAT evaluation found that correlations of 0.6 or above
produced satisfactory results. The reasons for this difference are yet to be
fully evaluated. Two potential explanations are that ACORN-SAT uses multiple
reference stations (normally 10) whilst the other methods were evaluated using
a single reference station, and that the ACORN-SAT evaluation took place using
real data, whilst the other evaluations used artificial data sets whose
properties may not necessarily match real-world inhomogeneities. This result is
critical for the use of such methods in Australia, where observing networks are
sparse in many areas by the standards of developed countries; had a minimum
correlation of 0.8 been necessary many locations would have had no available
reference stations.
Documentation of the data set at all points was considered
to be critical. The homogenised and raw daily data, all of the transfer
functions used in the adjustments, and other relevant documentation, are
available on the ACORN-SAT website. This information would allow the data set
to be reproduced from the raw data should anyone wish to do so.
The methods used in the development of the data set have
recently been published in a paper in the International Journal of Climatology.
A more extensive technical report has also been published by the Centre for
Australian Weather Climate and Research and is available on the ACORN-SATwebsite. Other material published on the website include a second technical report which compares outcomes from the new data set with other national and
international data sets, and a station catalogue containing metadata for the
112 locations.
(One of the author’s personal goals is to visit all 112
locations; the 91st, Cape Moreton in Queensland, was visited in
April. Occasionally his ambitions have exceeded his vehicle’s capabilities –
drowning his last car while attempting to get out of a remote site in the far
north of Western Australia).
The new data set will be used for operational climate
analysis by the Australian Bureau of Meteorology, including reporting of annual
national temperature anomalies. (Preliminary analyses indicate that the warming
trend in the new data set, about 0.9°C
over the 1910-2011 period, is similar to that in the data set previously in
use). It will also allow, for the first time, analyses of century-scale changes
in Australian temperature extremes. Such analyses are expected to be released
over the next few months.
References
Della-Marta, P.M. and
Wanner, H. 2006. A method of homogenizing the extremes and mean of
daily temperature measurements. J.
Clim., 19, 4179-4197.
Trewin, B.C. 2012. A daily homogenised temperature data set
for Australia. Int. J. Climatology,
published online 13 June 2012.
