Readers here (and elsewhere) have noted the release of a new version of the UAH satellite-derived tropospheric temperature record by John Christy and Roy Spencer. As noted by Spencer on March 5, version 5.3 seeks to remove, or at least mitigate, a spurious annual cycle that was apparently exacerbated in the changeover from the older MSU sensors to the newer AMSUs back in 1998.
The new data set is now available, so I’ll show some graphs that highlight the differences between the two versions. But first I’ll give a brief background on the matter, including my role.
[Update, March 10: It appears that John Christy was first notified of the annual cycle issue in October 2008, although it is unclear whether he understood the implications at that time. ]
In a recent post, I examined seasonal divergence in troposphere temperature (LT) trends produced by teams at RSS (Remote Sensing Systems) and UAH (University of Alabama at Huntsville). The strong annual cycle in the recent UAH data set has led to a wide divergence of temperature trends depending on time of year.
Since then, the blogosphere has been atwitter over the marked divergence between UAH and RSS for the month of February, which showed the UAH estimated anomaly (0.35) more than a full tenth of a degree higher than RSS (0.23).
Update (April 9, 2008): The March LT global temperature anomalies are out and they show both RSS (0.17) and UAH (0.21) down from February. The divergence has narrowed from 0.12 deg C in February to 0.04 deg C in March, which is exactly the divergence seen for that month in 2004-2008 (see below). So far in 2009, the UAH annual cycle is alive and well.
However, a detailed look at the divergence month-by-month shows that this latest discrepancy is not so surprising. I’ll also take a look at the effect on UAH of the recent switch to the newer AQUA satellite, which has actually resulted in an enhancement of the UAH annual cycle. AQUA has a self-correcting propulsion system, so the UAH annual cycle, and the cyclic component of UAH-RSS divergence, can not be the result of differing methods of correction for diurnal drift resulting from orbital decay.
I start by showing the global temperature trends for each month for the 1979-2008 period for both LT and surface temperature sets.