climate change wa

Climate change or instrument change?


Automatic weather stations within the Bureau of Meteorology's Australian Climate Observation Reference Network (ACORN) use various brands of electronic platinum-resistance probes (mostly Rosemount) that may influence temperature trends.

In An updated long‐term homogenized daily temperature data set for Australia, which explores the update of ACORN from version 1 to version 2 in 2019, the bureau authors write about response time changes within these probes ...

temperature probe response times

Below are extensive tables showing the date of temperature probe installation or replacement at each station, the probe type, and variables including maximim/minimum temperatures, days of rainfall, very hot 40C+ days and solar exposure in the two years before and two years after.

The before and after comparisons will test the assertion that the ACORN average (including 81 unaffected AWS stations) impact of probe changes is in the order of +0.01°C for maximum temperature and between zero and −0.01°C for minimum temperature.

Comparison of average minimum and rainfall days before and after most recent probe installation or replacement pre-2017

aws probe influence on minimum temperatures

Average minima were 0.07C warmer in the two years after probe installation/replacement compared to the two years before, with an average 1.06 more rainy days.

Stations with more than the average change in rainfall days (an increase in rainy days) had an averaged 0.15C increase in minima and those with less than the average change in rainfall days (mostly a decrease in rainy days) had no change in averaged minima whatsoever.

Comparison of average maximum and rainfall days before and after most recent probe installation or replacement pre-2017

aws probe influence on maximum temperatures

Average maxima were 0.31C warmer in the two years after probe installation/replacement compared to the two years before, with an average 1.18 more rainy days.

Stations with more than the average change in rainfall days (an increase in rainy days) had an averaged -0.04C decrease in maxima and those with less than the average change in rainfall days (mostly a decrease in rainy days) had an average 0.62C increase in maxima.

Download Excel for two tables above

Comparison of average minimum and rainfall days before and after first ever probe installation

aws probe influence on minimum temperatures

Average minima were 0.06C warmer in the two years after probe installation compared to the two years before, with an average 3.05 more rainy days.

Stations with more than the average change in rainfall days (an increase in rainy days) had an averaged 0.15C increase in minima and those with less than the average change in rainfall days (mostly a decrease in rainy days) had an average -0.5C decrease in minima.

Comparison of average maximum and rainfall days before and after first ever probe installation

aws probe influence on maximum temperatures

Average maxima were 0.11C warmer in the two years after probe installation compared to the two years before, with an average 3.30 more rainy days.

Stations with more than the average change in rainfall days (an increase in rainy days) had an averaged -0.06C decrease in maxima and those with less than the average change in rainfall days (mostly a decrease in rainy days) had an average 0.32C increase in maxima.

Download Excel for two tables above

Comparison of average minimum and rainfall days before and after probe replacements pre-2017

aws probe influence on minimum temperatures

Average minima were 0.07C warmer in the two years after probe replacement compared to the two years before, with an average -0.71 fewer rainy days.

Stations with more than the average change in rainfall days (an increase in rainy days) had an averaged 0.17C increase in minima and those with less than the average change in rainfall days (mostly a decrease in rainy days) had an average -0.01 decrease in minima.

Comparison of average maximum and rainfall days before and after probe replacements pre-2017

aws probe influence on minimum temperatures

Average maxima were 0.41C warmer in the two years after probe replacement compared to the two years before, with an average -1.08 fewer rainy days.

Stations with more than the average change in rainfall days (an increase in rainy days) had an averaged -0.04C decrease in maxima and those with less than the average change in rainfall days (mostly a decrease in rainy days) had an average 0.74C increase in maxima.

Download Excel for two tables above

Another way to test if AWS probe installations and replacements influenced average maxima is to compare solar exposure across all stations in the two years before and two years after.

Solar exposure is arguably a better gauge of cloud cover than rainy days. As explained by the bureau:

Global solar exposure is the total amount of solar energy falling on a horizontal surface. The daily global solar exposure is the total solar energy for a day. Typical values for daily global solar exposure range from 1 to 35 MJ/m2 (megajoules per square metre). The values are usually highest in clear sun conditions during the summer, and lowest during winter or very cloudy days.

Comparison of average maximum and solar exposure before and after first ever probe installation

solar exposure influence on maximum temperatures

Average maxima were 0.20C warmer in the two years after probe installation compared to the two years before, with an average 0.04 increase in solar exposure.

Stations with more than the average change in solar exposure (an increase in solar exposure) had an averaged 0.46C increase in maxima and those with less than the average change in solar exposure (mostly a decrease in solar exposure) had an average -0.07C decrease in maxima.

Download Excel for table above

Comparison of average maximum and solar exposure before and after probe replacements pre-1972

solar exposure influence on aws replacement probes

Average maxima were 0.46C warmer in the two years after probe replacement compared to the two years before, with an average -0.01 decrease in solar exposure.

Stations with more than the average change in solar exposure (an increase in solar exposure) had an averaged 0.77C increase in maxima and those with less than the average change in solar exposure (mostly a decrease in solar exposure) had an average 0.15C increase in maxima.

Download Excel for table above

Another way to test if AWS probe installations and replacements influenced average maxima is to compare the number of very hot 40C+ days and rainy days across all stations in the two years before and two years after.

Comparison of very hot 40C+ days and rainy days before and after first ever probe installation

aws probe influence on very hot days

The average number of very hot days increased by 2.84 in the two years after probe installation compared to the two years before, with an average -2 decrease in rainy days.

Stations with more than the average change in rainy days (an increase in rainy days) had an averaged -4.88C decrease in the number of very hot days and those with less than the average change in rainy days (mostly a decrease in rainy days) had an average 10.95 increase in very hot days.

Download Excel for table above

Comparison of very hot 40C+ days and rainy days before and after probe replacements pre-1972

aws replacement probe influence on very hot days

The average number of very hot days increased by 19.79 in the two years after probe replacement compared to the two years before, with an average 13.38 increase in rainy days.

Stations with more than the average change in rainy days (an increase in rainy days) had an averaged 8.18C increase in the number of very hot days and those with less than the average change in rainy days (mostly a decrease in rainy days) had an average 28.77 increase in very hot days.

Download Excel for table above

Testing average maxima and very hot days against rainy days and solar exposure in the two years before and after AWS probe installation suggests a slight increase in minima and a significant increase in maxima that is even more pronounced in automatic weather stations that have had their temperature probes replaced.

There are numerous other factors and variables at each individual station that may influence a weather station's cooling or warming in the two years after an AWS probe is either first installed or in later years replaced.

However, the three separate tests suggest that there was considerably more probe influence than +0.001C maximum or -0.001C minimum.

Possible reasons

New and replacement probe installations are often tied to the simultaneous replacement of large Stevenson screens (230 litres) with smaller Stevenson screens (60 litres). Studies suggest instruments in smaller screens produce warmer temperatures than in large screen.

Impact of two different sized Stevenson screens on air temperature measurements, published in 2015, compared readings in the two different sized screens at the first-order meteorological station of Calamocha in Spain and found that the annual average maximum was 0.54C warmer and the annual average minimum -0.11C cooler in the smaller screen, with the bias particularly evident in the warm season.

Calamocha's average annual maximum temperature is 20.0C and average annual minimum is 9.0C. The BoM states that Australia's average annual maximum in 1961-90 was 28.6C and average annual minimum was 15.1C.

Report 4 for the Independent Peer Review of the ACORN-SAT dataset in 2011 found that the ACORN accepted tolerance limit for observations of +/- 0.5C does not meet the World Meteorological Organisation's 95% air temperature uncertainty specification of +/- 0.1C with a reasonable uncertainty of +/- 0.2C, and provides a table of small Stevenson screen start years showing that among 94 weather stations where installation has been since 1960, the averaged start year is 1994.

The average annual ACORN maximum anomaly in the 26 years of 1968-1993 was +0.05C and in the 26 years of 1995-2020 it was +0.78C - a maximum increase of 0.73C.

The average annual ACORN minimum anomaly in 1968-1993 was +0.10C and in 1995-2020 it was +0.58C - a minimum increase of 0.48C.

RAW absolute temperatures suggest Australia's 104 non-urban ACORN stations had an average maximum of 25.17C in 1969-1993 and 25.84C in 1995-2019 - an increase of 0.67C.

RAW absolute temperatures suggest Australia's 104 non-urban ACORN stations had an average minimum of 13.28C in 1969-1993 and 13.55C in 1995-2019 - an increase of 0.27C.

AWS temperature probe replacements and the installation of smaller Stevenson screens may help to answer the question : Is Australia warming more rapidly than the rest of the world?




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