Author: Dr. Michael Chase
Photo above: A recent picture of the weather station at Rutherglen, Australia, from the BoM webpage cited below. Other photos are shown at the end of the post.
Post Summary and Conclusions
This post documents some analysis of changes in minimum temperatures (Tmin) at Rutherglen, a rural weather station in South East Australia. It is found that:
- Early 20th century Tmin measurements are around 1.0C higher (annual average) than those that would have been measured if the recording system/location/environment of today had been in place then. There is some variation between months that make up this annual average.
- The annual average ACORN-SAT(2012) correction of 1.7C for early data is therefore substantially too high
- The daily ACORN-SAT(2012) corrections for 1920/21/22 (the only years examined) show a nonphysical discontinuity between the end of November and the start of December
Background
Rutherglen (BoM id 82039) is a rural weather station at a research farm, with no nearby man-made structures, at least from 1975, as revealed by photos and descriptions from the BoM webpage given below:
http://www.bom.gov.au/climate/change/acorn-sat/rutherglen/rutherglen-station.shtml
The RAW Tmin data from Rutherglen, and from many nearby stations, show a net cooling over the last 100 years, as revealed in the following figure:
Questions have been asked about why the raw temperature trend of net cooling has been adjusted in ACORN-SAT to a net warming trend, and the BoM have responded with the webpage cited above.
ACORN-SAT Corrections
The dates and sizes (annual average) of Tmin corrections applied by ACORN-SAT(2012) are given in the following extract from its adjustment summary document:
A later (September 2014) summary from the BoM about Rutherglen does not mention the 1928 Tmin correction:
Click to access station-adjustment-summary-Rutherglen.pdf
but it is unclear if that correction has been disowned (without saying so) or simply not mentioned. The original 2012 documentation is taken to be definitive, as it matches daily temperature data available in October 2017.
Data prior to the last-listed correction in 1928 is reduced, on average, by 1.7C, the sum of all corrections. The following figure shows the daily corrections for 1920/21/23 (the only years examined):
The corrections appear to change in jumps from month to month, in particular with a very large jump (marked A in the figure above) from November to December, surely an undesirable and erroneous artifact rather than a genuine weather phenomenon.
My Analysis
I have estimated the monthly average corrections that would be needed to be applied to raw Rutherglen Tmin data to remove non-climatic influences relative to those present in recent years. The methodology is being documented in a separate blog:
https://diymetanalysis.wordpress.com
The following figure shows the annual average correction needed for periods of data (the bold blue lines are the moving averages) deemed to be stable, tracking the regional average (in red) reasonably closely:
The required correction is the temperature difference between the bold blue and dashed red lines, which are respectively the 15-year moving average of raw Rutherglen Tmin data, and the 15-year moving average of the regional average temperature variations. The figure also shows the 12-month moving average of weather-corrected raw Tmin data at Rutherglen.
The key features of the data shown in the figure above are as follows:
- 1914 to 1926: The average correction needed for Tmin data in this early period of stability is around 1.0C, the ACORN-SAT(2012) correction of 1.7C is too much
- 1914: There was a step change in temperatures, probably associated with the station move in January 1914 (source: Torok thesis 1997), a move that fails to get a mention or a correction in ACORN-SAT(2012)
- 1928: There was a step change in temperatures around 1928, but they recovered around 1936. ACORN-SAT (2012) has the step down in 1928, but not the recovery in 1936, an example of errors caused in ACORN-SAT by transient perturbations.
- 1966: There was a large drop in temperatures
- 1974: There was another drop in temperatures, but note that this was the date of some heavy rainfall (see below), and the temperature drop looks a bit like the sharp edge of a sawtooth perturbation
- 1984: This marked the start of a long period of stable temperatures with a trend matching that of the regional average
- 1998 (29th January): This was the date of a switch to an AWS system, which does not appear to have had a significant impact on measured temperatures
- 2012: There was a drop in temperatures at that date, possibly associated with a period of heavy rainfall, more on that below
The regional moving average temperature history was derived by averaging periods of stable temperature (such as the ones shown above in bold for Rutherglen) across stations in the region.
Monthly Corrections
The following set of figures show eyeball-estimated corrections for each month, being the average temperature difference between the raw data (in black, red for its average) and the regional average (in blue/mauve):
The figures shown above confirm that the periods 1914-1966 and 1984 to 2012 were roughly stable in terms of non-climatic influence, justifying the use of these periods in obtaining the regional average temperature history. If a corrected (“homogenised”) version of Rutherglen Tmin data is required then early data (before 1966) must be reduced by around 1.0C, with some monthly variation in that figure.
Regional Average
The following figure shows more of the periods of data used to form the regional average temperature history:
The complete set of the data periods used in regional averaging at Rutherglen is shown here:
Finally, the following figure shows a summary of the regional average Tmin and rainfall history back to 1885, indicating the heavy rain that may explain some of the anomalous changes in temperature around 1974 and 2012:
Conclusions: See the start of this post.
Photos
The following photo of the Rutherglen station is from the ACORN-SAT station catalogue:
Photos of the Rutherglen site from the BoM website cited above (click to enlarge):
climanrecon 