Merge Errors in ACORN-SAT

Author: Dr. Michael Chase

Summary

I believe that it was James Hansen and NASA/NOAA co-workers who first pointed out in print the problem of historical non-climatic warming in temperature reconstructions, as depicted in the following diagram:

hansen_uhi

Source of the figure above: https://pubs.giss.nasa.gov/docs/2001/2001_Hansen_ha02300a.pdf

This post shows examples of the problem manifesting itself in station mergers in ACORN-SAT, and quantifies the sizes of the resulting errors.

Methodology

I construct regional average temperature histories, separately for each month, of monthly average daily maximum (Tmax) and minimum (Tmin) temperatures. Full details of the methodology are documented in this blog:

https://diymetanalysis.wordpress.com

Normalising station data to the level of the most recent regional average reveals the time-history of non-climatic influence at the stations being merged, indicating the size of corrections that would be needed in the construction of composite station records.

Example 01: Wagga Wagga (NSW) Tmin

The following figure shows 12-month (and longer) moving averages of Tmin for Wagga Wagga Kooringal, and AMO, with consistent normalisation for both stations, i.e. allowing actual measured temperature differences to be seen:

WAGGA_Tmin_composite

 

The figure above also indicates typical sizes of temperature corrections that would be needed in the construction of a composite record. The composite corrections needed for early data (1910-25) are much lower than those applied in ACORN-SAT, which are shown in the following extract from its documentation:

DIY_P2_AS_WAGGA

Working backwards in time from the present, ACORN-SAT applies the following corrections to Tmin data:

  • 1968 (-0.46C): There was indeed a step change in temperature of around this size at that date
  • 1964 (-0.09C): The data are consistent with this small correction
  • 1948 (-1.62C): It appears that this enormous correction is the consequence of the anomalous warming at Wagga Kooringal between around 1930 and 1950, a period that I mark as being too anomalous to include in the regional averaging process.
  • 1928 (+0.43C): This correction achieves some damage limitation, but the net correction (the sum of all of them) applied to data before 1928 is -0.46 – 0.09 -1.62 + 0.43 = -1.74 C, which results in considerable over-correction

 

 

Regional Average Stations

The list of stations used in constructing the regional averages in the analysis above is as follows, indicating which ones were omitted:

id BoM-id From To
01 82039 1912 2017;… % RUTHERGLEN RESEARCH
02 74128 1867 2003;… % DENILIQUIN WILKINSON
03 74258 1997 2017;… % DENILIQUIN AWS
04 82001 1908 1986;… % BEECHWORTH COMPOSITE
05 82002 1903 2006;… % BENALLA
06 82170 2006 2017;… % BENALLA AIRPORT
07 72151 1871 1950;… % WAGGA WAGGA KOORINGAL
08 72150 1942 2017;… % WAGGA WAGGA AMO
09 80015 1881 2017;… % ECHUCA AERODROME
10 80023 1903 2017;… % KERANG
11 82053 1901 1987;… % WANGARATTA
12 80002 1907 1986;… % BOORT
13 77042 1899 1996;… % SWAN HILL PO
14 77094 1996 2017;… % SWAN HILL AERO
15 74034 1907 2014;… % COROWA AIRPORT
16 74110 1914 1975;… % URANA PO
17 72023 1922 2017;… % HUME RESERVOIR
18 74009 1907 1975;… % BERRIGAN PO
19 80043 1908 1977;… % NUMURKAH PO **** OMITTED ****
20 82016 1909 1976;… % EUROA
21 73127 1913 1975;… % WAGGA WAGGA AG
22 80091 1965 2017;… % KYABRAM
23 81084 1965 1985;… % LEMNOS **** OMITTED ****
24 80049 1940 1975;… % ROCHESTER **** OMITTED ****
25 72000 1907 1994;… % ADELONG PO **** OMITTED ****
26 74039 1947 1977;… % DENILIQUIN FALKINER
27 74069 1949 1969;… % MATHOURA STATE FOREST
28 75080 1914 1927;… % WANGANELLA
29 74106 1970 2017;… % TOCUMWAL AIRPORT
30 81049 1965 2017;… % TATURA
31 82138 1987 2017;… % WANGARATTA AERO
32 72097 1970 1986;… % ALBURY
33 82100 1968 1986;… % BONEGILLA
34 82056 1954 1968;… % WODONGA
35 82038 1903 1921;… % RUTHERGLEN PO **** OMITTED ****
36 82085 1903 1937;… % RUTHERGLEN VITI **** OMITTED ****
37 82034 1927 1969;… % MYRTLEFORD PO **** OMITTED ****
38 81057 1965 1975;… % YARRAWONGA PO **** OMITTED ****
39 81124 1993 2017;… % YARRAWONGA
40 75038 1940 1956]; % KOONDROOK **** OMITTED ****

More examples to follow later.

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