Update 2 February 2015
Rereading this, I just noticed a stupid error: I had said Brisbane’s Acorn trend is less than its neighbours’ Acorn trend, but it is actually more- as with the other outliers.
In my previous posts I showed how the Acorn adjustments to the ‘raw’ minimum temperature data have the effect of enormously increasing the apparent trend across the whole network, and very differently in different regions. In this post I am looking more closely at the six locations where the adjustments cause a change in trend of greater than +2 degrees Celsius. These are: Brisbane Airport, Amberley RAAF, Dubbo, Rutherglen, Rabbit Flat, and Carnarvon.
And I am mystified.
The purpose of homogenisation adjustments is to remove discontinuities in data, which show up as differences between the ‘candidate’ site’s record and those of its neighbours, the ‘reference’ sites. The Acorn method of detecting discontinuities uses pairwise comparison with up to 40 neighbouring sites, and this includes sites many hundreds of kilometres distant. Adjustments are made with a Percentile Matching algorithm which compares with up to 10 neighbouring sites.
I use my own method to compare sites with neighbours. When comparing any sites, anomalies from a common base period (1961-1990) are used. Only sites with data (at least 15 years) in this period can be used. Sites also need long data records. While in closely settled areas there will be a selection of observation sites, very few meet these requirements. Therefore I compare the data of each of these six locations with those of their nearest surrounding Acorn sites’ ‘raw’ data, (adjusted by me only when necessary to create a long combined series), individually and with their mean.
Even with only five neighbours, for Carnarvon and Rabbit Flat these can be over 500km away.
I then repeat this using Acorn (adjusted) data for the neighbours.
The results are surprising.
Note the remoteness of Rabbit Flat and Carnarvon.
The neighbours are: Amberley, Cape Moreton Lighthouse, Bundaberg, Gayndah, Miles, and Yamba Pilot Station.
Note the adjusted trend (+1.95C per 100 years) is greater than the mean of the neighbours (+1.06) by nearly +0.9C.
As you would expect, the data are now very similar, and the trend for Brisbane is thus 0.23C per 100 years more than the trend for the mean of the neighbours’ Acorn data.
Neighbours are the same as Brisbane’s, including Brisbane, 50km away.
Note the trend is more than one degree steeper than the trend of the neighbouring Acorn sites’ raw data.
Amberley’s adjusted trend is +0.87C greater than that of the mean of its neighbours’ adjusted data.
Neighbours are: Gunnedah, Scone, Bathurst, Cobar, Wyalong
Fig. 3d: Dubbo Acorn vs neighbours’ mean (Acorn)
Now only +1.29C per 100 years greater than the neighbours.
Neighbours are: Deniliquin, Wagga Wagga, Sale, Kerang, Cabramurra
Note that Rutherglen’s cooling trend is only 0.3C different from that of its neighbours.
+0.51C per 100 years greater than the neighbours.
Rabbit Flat is a roadhouse in the Tanami Desert on the track between Alice Springs and Halls Creek. Climate Data Online shows the current Rabbit Flat site 015666 as being 71km away from the old closed site 015548, though the Acorn Station Catalogue says it’s only 200 metres. This in itself is peculiar.
The nearest non-Acorn site is Balgo Hills 211 km away.
Acorn neighbours are: Giles (567km), Halls Creek (328km), Victoria River Downs (433km), Tennant Creek (440km), and Alice Springs (568km).
+1.17C more warming than neighbours.
Rabbit Flat adjustments give it a trend +0.75C more than the neighbours’.
Carnarvon’s Acorn neighbours are Learmonth (298km), Wittenoom (560km) , Meekatharra (524km), Geraldton (447km), and Morawa (538km). The only non-Acorn site with continuous data for the early part of last century is Hamelin Pool 6025 (174km away).
Now note the effect of just one of the neighbours- Morawa.
Note the much closer comparison.
Note the trend is +1.58C per 100 years more.
The difference is +1.49C.
The Acorn trend at Carnarvon is also greater than the Acorn trends at each of the neighbours separately.
The Acorn adjustment algorithm creates homogenised data by comparing with up to 10 neighbouring sites. I have shown that the adjustments have made the Acorn trends greater than, not only the raw data trends for each site, not only the raw data trends of the closest neighbours in the Acorn dataset, but in every case but one, greater even than the trends of Acorn homogenised data from the same neighbouring locations. The adjustments created thus appear to be spurious and the algorithm faulty.