the Air Vent

Because the world needs another opinion

John Christy Comments on 1992 Correction

Posted by Jeff Id on January 15, 2009

After my last post on the discontinuity in RSS vs UAH HERE, I emailed John Christy regarding my findings and asked if he knew of a reasonable explanation for the discontinuity. You know I was really impressed that he took the time to find a couple of articles on the subject which clarified the step. Dr. Christy has published a paper on this in 2007, the full text is not available yet as far as I know without payment.

Tropospheric temperature change since 1979 from tropical radiosonde and satellite measurements
Published in

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, 2007

——

Satellite and VIZ–Radiosonde Intercomparisons for Diagnosis of
Nonclimatic Influences
JOHN R. CHRISTY
AND
WILLIAM B. NORRIS
Earth System Science Center, University of Alabama in Huntsville, Huntsville, Alabama
(Manuscript received 14 September 2005, in final form 24 March 2006)

The first article is too new to find for free but it addresses exactly the issue in my previous post. Where my last article used only my eyes to identify the discontinuity, their 2007 paper uses statistical anlaysis of the differences between satellite data and radiosonde (weather balloon) data rather than ground measurements for determination of step points. The weather balloon data would be preferable for calibration of the satellites because it is a direct measurement of the LT profile rather than a good approximation. It does have the drawback of being a sparse measurement with less coverage than ground measurement but as in satellite data the corrections required for weather balloon data are substantially reduced they are less subject to human interaction and therefore corrections are more easily quantifiable.

If you have ever measured anything for science, you realize how easy it is to unintentionally influence data. This is a huge nearly undiscussed point in temp measurement. What sort of people monitor the sensors in their yards? Are they anti-agw or green folks who want a certain result? Even the most honest person can unintentionally influence data, you know just from the numbers you get all types so it is an average but it isn’t going to be perfectly balanced. Nobody is that lucky and there is a consensus after all.

Anyway back to the interesting stuff.

Unfortunately, satellite data can have spurious local shifts because of the insertion or deletion of observa- tions from a new or decommissioned spacecraft. For example, Christy and Norris [2006] report instances when local shifts were detected between the two satellite data sets at the point in time when a new satellite was incorporated, suggesting that differing merging methods were responsible.

This quote explains my reasoning for looking so closely at the discontinuity. The graph from my previous post has some very interesting implications to me. It is a bit complicated to take in, having four separate (color matched) trend lines. The red and black trend lines are a comparison of pre 1991 data and the blue and green are post 1994. The pre 1992 and post 92 slopes are almost exact matches yet the total trend between RSS and UAH is different. The cut point in the graph was also chosen by eye to insure that I removed the discontinuity completely with no science other than common sense examination of trends on either side of an ugly shift in RSS-UAH trend difference.

early-sat-and-late-sat-trend1

From Christy 2007.

Examination of Difference Series [36] The largest and most significant breakpoint in the UAH-RSS difference series occurs between October 1991 and March 1992 (Figure 7). The NOAA-12 spacecraft began adding data into the data stream around October 1991. In addition, adjustments applied to NOAA-11, which have their greatest effect in the latter half of the satellite’s operation, i.e., after October 1991, may also be a factor. The schematic in Figure 8 shows how a several month shift could occur in one of the data sets. This appears to have occurred in the RSS data set relative to UAH’s and is
reflected in the RSS-UAH difference series in Figure 7, although the diagram is in the opposite direction.

Figure 7 from Christy et. al 2007.

christy-trend-anomaly

The statement at the end of the above paragraph indicating a constant flat trend step followed by another flat trend has been solidly confirmed by continued data collection since 2005 when this paper was written as demonstrated in my graph above and below.

rss-uah-data

The shift is quite visible in this last graph. The explanation needed to be a non-instantaneous yet still fast change in measurement for the above. After correction for this discontinuity the two series match within a very high degree of certainty. Now that Dr. Christy has explained the rational and likely timing for the discrepancy, I need to tighten up (minimize) my correction window to keep as much original trend as possible using ground data. I had come up with beginning and ending numbers similar to his Oct-March explanation just by my own trial and error. This means two things to me, one the Doc. is right and two, my methods weren’t off the mark.

It will be interesting to see the result to the long term trends after I tighten up the correction window and keep as much original data as possible.


One Response to “John Christy Comments on 1992 Correction”

  1. Layman Lurker said

    The alignment of slopes by correcting for the step would stregnthen the argument (along with the short term variability confirming the amplification factor) that it is the surface data which is biased. The RSS and UAH data cooperate on data accuracy but there are still independant methods for data adjustment. Alignment of the slopes using independant data adjustment does not eliminate the possibility of a common bias but the the range of possible explanations becomes very narrow.

    It would be interesting to explore how (or if) supposed rural surface data station elimination could produce a bias which results in incorrect data adjustment to fill the gaps.

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