This is the second episode in the Cobar ACORN-SAT series examining BoM adjustments to the CDO temperature data – here I start to look at adjustments to minimum temperatures. The 1st episode looked at maximum temperatures. A list of ACORN adjustments to Cobar data is here and you can see the first min adjustment listed is 1st Jan 1972 meaning the adjustment factor applies to all data earlier than that. You will see it is labelled as “Statistical” meaning there is no evidence for it in station diaries or admin records but it derives from computer driven comparisons sifting data differences from multiple stations as far away as Parkes and Hillston – see map. In this case of the 4th adjustment the following stations data was used.
Making the chart of Cobar annual minimum temperatures compared to ACORN-SAT my eye was caught by the adjustment starting in 2006 and affecting all earlier years which I have marked with a blue 6. That is unlisted in the ACORN-SAT documentation and is substantial at about -0.4 degrees C. The slight mismatch between Cobar Met Office and ACORN from 2007-2013 is due to rounding differences because I have made my ACORN annuals by averaging a year of daily data which I leave as produced by Excel with multiple decimal places.
The next adjustment to look for is at 1971 where I have the blue 4, which is the 4th adjustment in the ACORN list and is listed at -0.49 degrees C. The increased departure of ACORN cooler than Met Office to about -0.9 is obvious on the chart.
Examining this adjustment in greater detail I have made a chart comparing Cobar MO and ACORN version with nearest neighbours Bourke, Wilcannia and Nyngan. The average difference between the 1971 & 1972 readings for these 3 stations is +0.2 at Cobar MO, +0.4 at Bourke PO, +0.4 at Nyngan, and -0.4 at Wilcannia, an average for the 3 Cobar neighbours of +0.13, not very different from the +0.2 that we know happened at Cobar Met Office. But instead of leaving the higher quality Cobar Met Office readings well alone – what does the BoM decide to do with their adjustment #4? They take off 0.49° making the 1971-1972 difference now 0.7 – greater by 0.3 than any of the neighbours. Presumably the BoM justify this by their computer driven comparisons with sites as distant as Parkes.
If the reasons for an adjustment can not be seen in nearest neighbours then it must be an exercise in fantasy to search for a reason in a cherry picked array of more distant stations which are all of poorer quality than Cobar Met Office.
It is interesting to check the differences in annual minimums between Cobar Met Office and Cobar Airport which are only about 7 or 8 km apart. You might expect them to be very similar and in lockstep – not so from the chart.
Note the BoM never refer to Cobar Airport data in ACORN-SAT – but we are free to check it out.
First there is no evidence here of a step or jump around 2006 – 2007.
While there are such wildly varying and apparently random differences between these two very adjacent sites – what on earth can the BoM learn by comparing Cobar with Parkes – or indeed any other station in their adjustments list.
These are the sort of unsafe foundations that pro-IPCC climate science is based on.