Garbage In, Garbage Out- Horn Island

After last year’s major project of checking the compliance of 666 Australian weather stations with the guidelines set out by the Bureau of Meteorology, and finding that nearly half did not meet siting specifications, I decided to take a break from analysing BOM climate data.

It might now be time to re-enter the fray.

The pride of the BOM climate fleet, Version 2 of the Australian Climate Observation Recording Network- Surface Air Temperatures (ACORN-SAT, or Acorn 2) was launched with no fanfare at all in late 2018 and now is the basis for the Bureau’s climate claims and predictions.  I summarised many of its faults in May 2019.

Now I am going to look at some individual examples of Acorn 2 nonsense- firstly, Horn Island’s minimum temperatures.

Horn Island is our most northerly Acorn site.  It is the airport for Thursday Island and is closer to Port Moresby than any other Australian town except Weipa and of course TI.  Figure 1 shows the station’s only neighbours on Cape York Peninsula the BOM used for comparison. 

Figure 1:  Google Maps image of Cape York Peninsula showing Horn Island and its neighbours

Figure 2 shows the annual average minima for all of these weather stations.

Figure 2:  Annual minima, Horn Island and neighbours

Horn Island 27058 has a very limited data record commencing in March 1995.  To construct a longer record back as far as 1951, BOM merged Horn Island’s observations with those of Thursday Island Township 27021 and Thursday Island Meteorological Office 27022.  Figure 3 shows the result of the BOM’s merge.

Figure 3:  Horn Island and Thursday Island minima

The 27022 TI Met Office annual average Tmin has been reduced in Acorn by 0.5 or 0.6 degrees for all years before 1993.  (The Adjustment Summary claims there was an annual impact of -0.53 degrees.)  There were other adjustments at 01/01/1967 and 01/01/1958, but the 1993 adjustment has the largest effect.

Unfortunately, there are only 6 months of overlap between 27021 and 27022 (September 1992 to February 1993), and 11 months between 27021 and 27058 (March 1995 to January 1996), as figure 4 shows.

Figure 4:  Extent of overlaps in monthly minima

In September, October, and November 1992 TI Township 27021 recorded minima 1.1 to 1.4 degrees cooler than the Met Office site in “a relatively exposed location” on top of the hill 900 metres west.  However, Acorn tracks 27021 and approximately splits the difference from 27022.

Figure 5:  Extent of overlaps in monthly minima, including Acorn 2

To do this, the BOM uses two comparative stations, Lockhart River 28008 and Coen Airport 27006.  Coen is 385km from Horn Island. 

Figure 6 shows a plot of monthly anomalies from 1981 to 2010 means for these stations.

Figure 6:  Monthly minima anomalies from 1981-2010 means, all stations

Note that 27022 is close to 27021 from December 1992 to February 1993, but not before, while Coen is nothing like either.  I cannot see the justification for the adjustment.

I constructed a merge of annual data using 27021 for 1993, 1994, and 1995, joined to 27022 and 27058 with no adjustment to construct a “raw” record.  This is entirely artificial, but no more so than Acorn.  Figure 7 shows plots of both with annual trends.

Figure 7:  Horn Island minima, Acorn 2 and “raw”

Note Acorn has enormously increased the warming trend.  Figure 8 plots the differences between Acorn and my “raw” record.

Figure 8:  Horn Island minima adjustments

The earlier adjustments were also large and based on “statistical” breakpoints.

The object of adjusting temperatures using neighbours for comparison is to endeavour to produce a record that more truly reflects climate trends of the area.  The resulting record should be more like the neighbours than the original raw record.  We can test this by plotting the differences between Acorn and the raw record and the average of the neighbours.  If the comparison is good, while individual years’ differences may vary, the trend should be close to zero: the station should not be warming or cooling more than the neighbours.  Figure 9 shows the results for Horn Island minima for the period that the 1993 adjustment applied.

Figure 9:  Differences between Horn Island and two neighbours

You will note that the blue trend line, showing the trend of the difference between annual data merged with no adjustment and the average of Coen and Lockhart River, is almost flat, indicating the “raw” record already compares well with the neighbours.  In contrast the red trend line shows the adjusted data is warming faster than the neighbours, indicating a very poor reflection of the climate of the area.

Conclusion

The Horn Island record should never have been merged because of the lack of suitable overlap.

Once merged, it should never have been adjusted downwards so much.

 Lockhart River and Coen are far too distant to be suitable for comparison.

The result is nonsense.

Garbage in, garbage out.

Tags: Acorn, adjustments, bom