Nov 27, 2016

Which Satellite Data?

by
Tamino
,
Open Mind
The satellite data set with the best correlation to RATPAC data is the one warming fastest, RSS TTT. Image: RSS, Tamino
The satellite data set with the best correlation to RATPAC data is the one warming fastest, RSS TTT. Image: RSS, Tamino

There are five data sets of global average temperature in the troposphere (the part of the atmosphere where our weather occurs) based on satellite data, from the two main providers, RSS (Remote Sensing Systems) and UAH (University of Alabama at Huntsville). UAH provides two of them: TLT (temperature in the lower troposphere) and TMT (temperature in the mid-troposphere); while RSS provides three: TLT, TMT, and TTT (temperature in the total troposphere). They’re all different, and each has gone through a number of revisions since the satellites began collecting data in 1979.

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Naturally one wonders, why are they so different?

The UAHTLT data are warming faster than the UAHTMT data simply because the lower troposphere seems to be warming faster than the mid-troposphere. But that raises the question, why is this reversed for the data from RSS? That’s because the RSS data for TMT are their latest version 4, but they haven’t yet published a version 4 for TLT, that’s still on the older version, v3.

In testimony before congress, retired admiral David Titley (formerly chief oceanographer of the U.S. Navy) mentioned three problems with satellite data sets: orbit drift, mismatch between different satellites, and stratospheric contamination. Addressing these problems is the reason RSS has revised their data products recently. Their newest version 4 addresses the problem of orbit drift and improves the match between different satellites. Doing so results in “substantially increased global-scale warming relative to the previous version”...

The impact is strong enough that their new TMT shows faster warming than their old TLT.

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The new RSS product, TTT, is designed to reduce the effect of stratospheric contamination. The stratosphere is actually cooling (because of CO2 increase), and if it contaminates a troposphere estimate it will introduce false cooling, leading to estimates of warming which are too small. As RSS states, “In the simpler TMT product, about 10% of the weight is from the lower stratosphere. Because the lower stratosphere is cooling at most locations, this causes the decadal trends in TMT to be less than the trends in the mid and upper troposphere..” ...

In short, the revisions to RSS products have directly addressed some of the known problems with satellite data, strongly suggesting that the new RSS data sets — especially TTT version 4 — are a more accurate measure of how tropospheric temperature is changing. If you use the UAH data, you still have to wonder about orbit drift and mismatch between different satellites, and if you use UAH or RSS data for TLT or TMT, stratospheric contamination is still a serious problem.

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Bottom line: the reasons for the recent revisions to RSS data and direct comparison to balloon data both lead me to believe that the RSS data products are distinctly superior to the UAH data. In particular, the RSS TTT product, because it has a much better correction for stratospheric contamination, seems best of all, and matches the balloon data best. It’s also the satellite data set showing the fastest warming.

We don’t live in the upper atmosphere, we live on Earth’s surface, where temperature is measured by thermometers. Some people believe (and climate deniers want you to believe) that the satellite data sets are better than surface temperature data sets based on thermometer measurements. ‘Tain’t so. In fact, Carl Mears, lead scientist processing the satellite data for RSS, says outright that the surface temperature records are more accurate. You can hear him say so here (21 seconds in, at 0:21).