A lot of interest was generated by my last post on the Acorn outliers, especially the dependence on very distant sites for homogenisation adjustments. In this post I will compare Carnarvon’s closer neighbours – excluding Wittenoom, Meekatharra, and Morowa- to show how a better understanding of Carnarvon’s minimum temperature record may be derived, and how reliance on more distant sites with different climate regimes can distort adjustments.
Carnarvon Airport 6011 has been recording temperatures since 1945. Before that, Carnarvon Post Office 6062 has data from 1885 to 1950 so there is useful overlap.
These records can be spliced by reducing the Post Office data for 1946-1948 by 0.4C (as the Post Office recorded increasingly warmer minima than the Airport in these years as shown by the monthly temperatures for 1949-1950) but making no changes to PO data before this, to produce a long composite record. I commence at 1910 to compare with the official BOM figures.
Note how Acorn reduces minima from 1974. Note the size of these adjustments.
So at 1910 the Acorn record shows minima 1.8C less than the raw data.
Now lets look at how Carnarvon’s neighbours compare. To do this we need to convert to anomalies from the 1961-1990 mean. Neighbours are listed at Climate Data Online, ranging from 19 km to 296 km away, but most are too short or have too much missing data to be useful.
The neighbours that I have used for comparison are: Hamelin Pool (174.4 km away), Winning (211.3 km), Emu Creek (248.5 km), and Learmonth (296.3 km). Learmonth is almost due north of Carnarvon, and like Carnarvon right on the coast, while the others are inland sites.
We can see that there is close agreement for most of the time. There are minor periods of disagreement in the 1970s and 1990s, but the major disagreement is 1926-1950. Which is a more accurate reflection of Carnarvon minima- Carnarvon PO or Hamelin Pool? To find which is the odd one out, we need to look at other sites with data for this period.
Winning, although confirming the approximate agreement 1910-1920, has very little data for 1926-1950, so we have to look further afield… all the way to Geraldton, which like Carnarvon is on the coast, but 447 km south.
Hamelin Pool is clearly the outlier, so we can accept Carnarvon raw temperatures as reasonably accurate from 1910 to 1970. There is a short period in the 1970s of disagreement, but little difference after that… and Acorn does not adjust after 1974 anyway.
How does this compare with Acorn?
Can you pick the outlier?
We can only presume that the Acorn homogenisation depends on data fed into the algorithm from much, much further away.
I can see no justification for any major adjustment to the raw record at Carnarvon.