How Do These Things Happen
Posted by Jeff Condon on December 17, 2008
I took some flack on how I missed such an obvious problem in the data. Well, things are rarely as clear as they first seem. Let’s look at the facts, below is a plot of the uncorrected bootstrap data.
See the offset at 1987, no how about the same graph zoomed in.
There it is, that little scribble after 1987.5. That’s the entire difference between my first and second posts. But it’s worse than just this, here is some text from the actual pages.
In computing the total ice-covered area and ice extent, pixels must have an ice concentration of 15 percent or greater to be included; thus, total ice-covered area is defined as the area of each pixel with at least 15 percent ice concentration multiplied by the ice fraction in the pixel (0.15-1.00). Total ice extent is computed by summing the number of pixels with at least 15 percent ice concentration multiplied by the area per pixel. Sea ice concentrations are assumed to be 100 percent around a circular sector centered over the Northern Hemisphere pole (known as the pole hole) which is never measured due to orbit inclination. The Southern Hemisphere also has a pole hole; however, it does not affect this sea ice data set; since only land is under this hole. For SMMR, the hole is 611 km in radius and is located poleward of 84.5 degrees north. For SSM/I, the hole is 310 km in radius and is located poleward of 87 degrees north.
As I read this paragraph the data at the north pole is simply filled in for both “area and extent” and would have little or no offset due to the sensor area change. This is in regards to the data I used.
It gets worse.
Important Note: The most recent revision (Version 2, released September 2007) has tie-points adjusted to be consistent with the AMSR-E Bootstrap algorithm. This involved a complete reprocessing of the entire SMMR-SSM/I time series. Because Version 2 is inter-calibrated with AMSR-E, it is NOT consistent or compatible with the original version of this data set (Version 1). Users should acquire the entire Version 2 data set in order to update their time series.
This is in regards to how the olde SMMR data is aligned to the SMM/I data. The AMSR-E is the new style data. The paragraph clearly states that the correction made the data consistent but you must have V2 data. Which I did. There’s also some info on how the data was aligned here.
Think were done yet. No way.
The selection of the SSM/I F8 tie points (table 1, p. 40) was based on an analysis of SSM/I TBs, PR-GR distributions, histograms of sea ice concentrations, and on comparisons with near simultaneous measurements from the Nimbus-7 SMMR during July and August 1987. The two sets of SSM/I tie points (one for the Northern Hemisphere and one for the Southern Hemisphere) represent a global set designed for mapping global ice concentrations. While this global set of tie points provides a uniform measure of sea ice concentration on the large scale, improved accuracy is obtained with the use of regionally selected tie points (Steffen and Schweiger 1991).
A very interesting paragraph. The data’s alignment is being discussed including histograms of sea ice concentrations. The tie point for the two datasets was calculated using near real time data from the two systems.
Not yet, there’s more. Remember my anomaly data, well I happened to check it on the site at this link.
This looks a lot like my original anomaly plot (below) on a slightly different scale.
If this anomaly plot is correct then my entire original post is correct but wait there’s more.
Here’s the IUIC cryosphere graph which looks impressively similar to my original data. Click for the large version.
Below was part of tamino’s reply from a completely different site (link here )that points to a dataset which I didn’t use. Still if you look into the source of the data (SMMR and SMM/I) they are the same and I did find the step in the second graph above and admitted as far and widely as I could my screw up.
Here’s the text from the link.
Important Note: The “extent” column includes the area near the pole not
imaged by the sensor. It is assumed to be entirely ice covered with at
least 15% concentration. However, the “area” column excludes the area not
imaged by the sensor. This area is 1.19 million square kilometers for SMMR
(from the beginning of the series through June 1987) and 0.31 million
square kilometers for SSM/I (from July 1987 to present). Therefore, there
is a discontinuity in the “area” data values in this file at the June/July
As you see this paragraph is quite different from the info I was reading and it represents a different dataset. Another point is that the little jig in the second graph above occurs at the July/August boundary not at the Jun/July boundary as stated above.
Well, I think there is still some small question as to whether adding the area back in is appropriate. Were the tie points discussed above the result of a rescaling operation that offset the data to a point where a trend analysis can be done? I doubt it now but why would they introduce the data as a continuous time series with a simple correction left out.
I emailed the National Snow and Ice Data Center for clarification.
It should be interesting.