Guest Post by Willis Eschenbach

A learned man was arguing with a rube named Nasruddin. The learned man asked “What holds up the Earth?” Nasruddin said “It sits on the back of a giant turtle.” The learned man knew he had Nasruddin then. The learned man asked “But what holds up the turtle”, expecting Nasruddin to be flustered by the question. Nasruddin simply smiled. “Sure, and as your worship must know being a learned man, it’s turtles all the way down …”

I’ve written before of the dangers of mistaking the results of the ERA-40 and other “re-analysis” computer models for observations or data. If we just compare models to models and not to data, then it’s “models all the way down,” not resting on real world data anywhere.

I was wondering where on the planet I could demonstrate the problems with ERA-40. I happened to download the list of stations used in the CRUTEM3 analysis, and the first one was Jan Mayen Island. “Perfect”, I thought. Middle of nowhere, tiny dot, no other stations for many gridcells in any direction.

Figure 1. Location of Jan Mayen Island, 70.9°N, 8.7°W. White area in the upper left is Greenland. Gridpoints for the ERA-40 analysis shown as red diamonds. Center gridpoint data used for comparisons.

How does the ERA-40 reanalysis data stack up against the Jan Mayen ground data?

Figure 2. Actual temperature data for Jan Mayen Island and ERA-40 nearest gridpoint reanalysis “data”. NCAR data from KNMI. Jan Mayen data from GISS.

It’s not pretty. The ERA-40 simulated data runs consistently warmer than the observations in both the summer and the winter. The 95% confidence intervals of the two means (averages) don’t overlap, meaning that they come from distinct populations. Often the ERA-40 data is two or more degrees warmer in the winter. But occasionally and unpredictably, ERA-40 is 3 to 5 degrees cooler in winter. Jan Mayen’s year-round average is below freezing. The average of the ERA-40 is above freezing. The annual cycle of the two, as shown in Figure 3 below, is also revealing.

Figure 3. Two annual cycles (Jan-Dec) of the ERA-40 synthetic data and Jan Mayen temperature. Photo Source

The ERA-40 synthetic data runs warmer than the observations in every single month of the year. On average, it is 1.3°C warmer . In addition, the distinctive winter signature of Jan Mayen (February averages warmer than either January or March) is not captured at all in the ERA-40 synthetic data.

So that’s why I say, don’t be fooled by people talking about “reanalysis data”. It is a reanalysis model, and from first indications not all that good a reanalysis model. If you want to understand the actual winter weather in Jan Mayen, you’d be well-advised to avoid the ERA-40, or February will bite you in the ice.

The use of “reanalysis data” has some advantages. Because the reanalysis data is gridded, it can be compared directly to model outputs. It is mathematically more challenging to compare the model outputs to point data.

But that should be a stimulus to develop better mathematical comparison methods. It shouldn’t be a reason to interpose a second model in between the first model and the data. All that can do is increase the uncertainty.

In addition, due to the fact that both models involved (various GCMs and the ERA-40) are related conceptually (being current generation climate models), we would expect the correlations to be artificially high. In other words, a model’s output is likely to have a better fit to another related model’s output than it does to observational data. Data is ugly and has sudden jumps and changes. Computer model output is smooth and continuous. Which will fit better?

My conclusion? The ERA-40 is unsuited for the purpose of validating model results. Compare model results to real data, not to the ERA-40. Comparing models to models is a non-starter.

Regards to everyone,

w.

[UPDATE] Several people have asked about the sea surface temperatures in the area. Here they are:

Figure 4. As in Figure 2, but including HadSST sea surface temperature (SST) data for the gridcell containing Jan Mayen. SST data from KNMI

Figure 5. As in Figure 3, but including HadSST sea surface temperature (SST) data for the gridcell containing Jan Mayen. SST data from KNMI

Note that SST is always higher than the Jan Mayen temperature. This is not true for the ERA-40 reconstruction model output.

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