Australia's B.O.M & "Homogenisation" - they must think we're stupid

Australian scientist and blogger Jennifer Marohasy has been leading a small but vocal group campaigning for the Bureau of Meteorology to explain and defend "homogenisation" of temperature data for Australian stations. Their new, improved, value-added and of course, scientifically and mathematically sound database has, in most cases, turned a long-term static or cooling trend into a warming trend. They call it ACORN-SAT. You and I might call it GHCN/GISS outback style - if the present isn't warming to suit their meme, then by god, they'll cool the past, and "prove" local and global warming.

In most cases, the process leads to lowering past temperature data in a series of steps, supposedly corresponding to breakpoints in the series due to station moves, claimed station moves, statistical "discontinuities", or undocumented and therefore unjustified adjustments. Now this was something I could really get my teeth, and my Excel 2000 skills into. Yes, it's the 2000 version - if it ain't broke, don't spend money on a newer version.

I started several draft posts, downloaded shedloads of B.O.M. data, unadjusted GHCN/GISS data, PDF documents, bookmarked scientific literature, created new spreadsheets, chart graphics, googled for images of towns, cities, weather stations. The B.O.M. got very defensive when challenged a couple of years ago. Jennifer and her small group were tilting at a very big and huffily self-righteous windmill. The B.O.M. actually produced some documentation. What right had these upstarts to question their methodology and intent?

Then, a couple of days ago, I had an epiphany; I'd clearly missed something, something very important. The fact was, the B.O.M. has never, ever made the adjustments they'd claimed were necessary! If there's a documented station move, for example from a town out to a rural site, a small temperature change would be expected. In this case, likely to be downward because of the removal of the "urban heat-island" effect. They've identified and quantified such steps in the record at many sites. They believe an appropriate adjustment should be documented and applied, in this case an upward adjustment. The ACORN-SAT database appears not to contain any such adjustments, despite what the B.O.M. has documented, and everyone, until now, has believed.

I'd been looking at the "Station Adjustment Summary" for Amberley, Queensland. There was just one significant adjustment, forward from 1980:

2.  1 January 1980—breakpoint detected by statistical methods.
•  Night-time temperatures started to appear much cooler relative to surrounding                   stations.
•  No accessible documentation for Amberley in 1980, but a breakpoint of this size              would normally be associated with a site move.
•  Min T adjusted by -1.28 °C; no detectable impact on Max T so no adjustments made.

Adjusted by -1.28°C - wouldn't they apply an increase if temperatures "started to appear much cooler relative to surrounding stations"? You'd expect so, but that -1.28°C adjustment was correct - applied to data prior to 1980, thus leaving the "much cooler" data from 1980 onwards unchanged! The shape and trend of the resulting "homogenised" plot becomes identical to what would have resulted from a positive adjustment, but shifted down by the amount of the reversed adjustment. This is the plot of the change to average minimum temperatures for Amberley; for all my graphics, click to see a larger version, or right-click and "save as" to save them:

From the Station Adjustment Summary for Amberley.

The first thing to note is that the "difference" is positive because the author has subtracted adjusted from raw data. If you're comparing A with B, the correct way is to subtract B from A, not vice-versa. Compare 10 with 8, and the difference is 2, not -2. There's no change to raw data after 1980; if post-1980 data was "much cooler relative to surrounding stations", then it remains so. I've plotted ACORN-SAT adjusted against raw for Amberley:

Raw data series (blue) versus ACORN-SAT (red); difference (black)

The raw and adjusted series track together from 1998 rather than 1980 because a second, undocumented adjustment was made after 1996; a 2-step change. From 1980 to 2016, raw minimum trends flat. That's right - no change since 1980. The B.O.M. has what might be termed a "positive attitude" to temperature series; they like to see upward trends - flat or negative has to be "corrected".

Average annual minimum or maximum series are no good metric for measuring or tracking anything, yet they're what the Bureau uses exclusively, and to my knowledge, no other large private nor state-funded meteorological organisation does this. I'll back my up assertions in a future post. In the meantime, here's a plot of monthly average minima for 1978-1983, which spans the Bureau's "breakpoint":

Monthly average minimum temperatures

There's no "step change" evident in 1980. The dip in winter 1982 was caused by a record cold June & July. Minima generally "drifted" down after 1981 to around 1998 - which point coincides with the undocumented adjustment.

Jennifer's campaign focussed on the record for Rutherglen; the story's the same there, though more complicated:

Raw data series (blue) versus ACORN-SAT (red); difference (black)

Raw average minimum and ACORN-SAT adjusted track together (minor differences) from 1974. There's no net upward change due to the claimed adjustments in the Adjustment Summary. I've checked the other Adjustment Summaries, and a few other sites which should show adjustments. In every case, the changes resulted in raising or lowering previous temperatures, not those from the adjustment point forward. The B.O.M. isn't in  the business of correcting anomalies to create ACORN-SAT; they're in the business of changing the past to suit an agenda.