Comments on: Perrott puts his foot in his mouth

By: Richard C (NZ)

Richard C (NZ) — Sat, 12 Feb 2011 04:26:10 +0000

“As these random adjustments are supposed to produce random results, they should cancel out over time, and make no difference to the trend. BUT NIWA’s adjustments mainly flow in one direction and accumulate.”

They’re not random though, they’re systematic and the accumulation is a result of that. NIWA’s formal term is “cumulative step adjustments” e.g. Masterton, see Table 1. The baseline is East Taratahi Aws and the cumulative steps are Waingawa Subtation (-0.08), Essex Street (-0.34), Workshop Road (-0.55). NIWA went to a lot of trouble to derive those steps using remote comparison stations, but if you extract the data for the last accumulation from CliFlo, 1912-1919, average it and subtract it from the baseline, you get -0.15 C.

So the further back in time, the greater the accumulation (the steps are a time sequence spanning 1991-1920).

The local isolated difference at the last step doesn’t coincide with the remote accumulated difference – and it’s a lot less.

By: Australis

Australis — Sat, 12 Feb 2011 02:23:02 +0000

NIWA also derives its anomalies by taking ADJUSTED data and relating it to a 1971-2000 baseline derived from different stations at different locations using different measurement equipment.

The only difference is that NIWA adjusts the data from older stations and AWFWY does not. As these random adjustments are supposed to produce random results, they should cancel out over time, and make no difference to the trend. BUT NIWA’s adjustments mainly flow in one direction and accumulate.

In any event, you are quite right in saying that a change in a benchmark cannot affect the trend. Subtracting a constant from all values leaves the trend unaffected.

By: Barry Brill

Barry Brill — Sat, 12 Feb 2011 02:15:52 +0000

The Review Report uses Riverhead Forest and Waiuku Forest (both much closer than Te Aroha) but then admits (p36, footnote 29) that they are subject to a ‘Forest Heat Island’ effect. That will explain why they were useless in a comparison seeking UHI effects.

By: Barry Brill

Barry Brill — Sat, 12 Feb 2011 02:09:11 +0000

“Neighbouring stations” means “stations subject to identical local weather conditions”.
See Rhoades & Salinger (1995)

By: Bob D

Bob D — Fri, 11 Feb 2011 20:49:47 +0000

Yes, I noticed they do that a lot, same as Salinger did. The need for the adjustment may be there, but the magnitude of the adjustment is usually way beyond common sense.

By: Richard C (NZ)

Richard C (NZ) — Fri, 11 Feb 2011 11:50:26 +0000

The last cumulative adjustment for Masterton is -0.55 C at Waingawa Workshop Road 2473.

But the difference in the average of all East Taratahi Aws 2612 and the average of all Workshop Road 2473 1909-2010 from CliFlo data is only 0.17 C (12.55652 – 12.38551).

Cumulative adjustment -0.55 C

Actual CliFlo difference -0.17 C

Is my reasoning flawed? It not, NIWA has introduced a cumulative error of -0.38 C.

By: Richard C (NZ)

Richard C (NZ) — Fri, 11 Feb 2011 11:06:13 +0000

Should have gone back to re-read the BOM review long ago. It has all the contributing stations for each location tabulated making it easy to retrieve the contributing stations from CliFlo using station codes. The review also states “Masterton Sites 1 to 6 are all less than 10 km from the reference East Taratahi site (Site 7)” so a 10 km radius search centred on East Taratahi would pull them all in.

The review also catalogues the adjustment process so now I’m much wiser but I will have to study in detail the CUMULATIVE STEP ADJUSTMENTS as plotted in Figure 9 Masterton section to really understand what is going on because that is where the progressively increasing downward adjustment towards the early years takes place that pulls down the early years and creates a steeper trend.

The CUMULATIVE part is crucial because there will have to be a very sound reason to apply the adjustments cumulatively, otherwise if there isn’t, the steep rising trend disappears.

By: Richard C (NZ)

Richard C (NZ) — Fri, 11 Feb 2011 10:36:32 +0000

Figure 5 Masterton section that should be (shows NZ climate relative to Masterton).

By: Richard C (NZ)

Richard C (NZ) — Fri, 11 Feb 2011 10:34:17 +0000

The BOM review Auckland section indicates that the use of Te Aroha as a comparison site for Albert Park was specifically “to diagnose the source of non-climatic warming” i.e. urban heating.

By: Richard C (NZ)

Richard C (NZ) — Fri, 11 Feb 2011 10:22:38 +0000

“That LITTLE issue I think should be re-framed not as a missing data issue but as a spatial balance issue”

Climate balance would be a better term than spatial balance e.g Lincolns climate is more compatible with Masterton than Hokitika (see the BOM review Masterton section).