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.
The three surface trends (HadCrut, NOAA and NASA) seem fairly well correlated, although with varying “bumps” toward the end of the year. And RSS is reasonably close the surface trends, albeit with a slightly different annual pattern.
But the UAH monthly trend shows a smooth, broad annual fluctuation through the year. The February trend at 0.18 deg/decade is above all the other trends, while the May trough at only 0.07 deg/decade is only half that month’s trend in the other data sets.
The difference between UAH and RSS global LT average monthly anomalies for two recent periods since the baseline period (which ended in 1998) shows clearly the annual cycle that results in this extreme UAH trend divergence.
Here we can see that the UAH February peak has diverged more and more over recent years; in 2004-2008 it was an average of 0.o8 degrees higher than RSS. Even more striking is the May divergence, which reached an average of 0.11 degree in the same five-year period.
The trend divergence is even wider for the tropics.
As noted previously, the UAH tropical trend is lower in all months. And there does seem to be a modest seasonal variation in RSS that is not present in the surface data (NOAA).
But the UAH divergence is extreme to say the least, ranging from a high of 0.12 deg. per decade in February to -0.03 deg per decade in June. And, as in the global case, this fluctuation is far from random, but is clearly led by the recent annual cycle.
Recent developments have shed some light on the possible source of the UAH annual cycle.
As I noted in a comment at my previous post, UAH switched last year from NOAA-15 to the AQUA satellite and is apparently using its data exclusively. The change was noted by one of the UAH team leaders, Dr. Roy Spencer:
The biggest adjustment is the fact that we don’t even use NOAA-15 right now…we are using the AMSU data from NASA’s Aqua satellite in the final UAH product.
The extent of the overlap period between the two satellites is not clear from the documentation, but the above statement does seem imply that most of the period from late 2002 on (when the AQUA was deployed) is based on the AQUA AMSU alone. [Update April 9, 2009: The documentation does refer to an overlap period with both satellites being used up to January, 2008. However, there is no clear statement about the overlap period in the current data set, or from which date the current data set reflects AQUA data only].
The UAH team considers this satellite more stable, as it is not subject to “diurnal tempearture drifts”, as noted above.
“I believe it has to do with the differences in how diurnal variation is tracked and adjusted for. … For that reason, UAH has been using data from the AQUA satellite MSU, and RSS to my knowledge does not, and makes an adjustment to account for it. I believe our data [UAH] is probably closer to the true anomaly temperature, and if I’m right, we’ll see the two datasets converge again when the diurnal variations are minimized.”
In other words, RSS has to make a correction for diurnal drift, while UAH does not. But a comparison of the UAH pre-AQUA LT data set to 2007 with the current data set for the same period shows that the UAH seasonal divergence is stronger now than it was before the change!
I have obtained a copy of the UAH LT data set as of November, 2007, before the merging of AQUA data ( many thanks to cce for sending the data). This archived data set permits comparison of the recent UAH 5.2 data to the pre-AQUA version (also dubbed 5.2) over the 2000-2007 period.
For some reason, there are differences even before 2002, suggesting other processing changes have taken place between 2007 and present. In any event, early year peaks over and above the pre-AQUA data set can clearly be seen in every year from 2001 through 2006.
The result is an increase in trend divergence; the newer UAH data has a stronger peak in February, and a lower trough from May through October.
Since the pre-AQUA LT data was subject to adjustment for diurnal drift, while the current data is not, at least for recent years, the discrepancies can not be due to diurnal drift adjustment in UAH (or RSS for that matter).
Finally here’s a quick look at the T2 (or “mid-troposphere”) from UAH and RSS. (Essentially, these are based on the same raw data as the LT data sets, but have not been adjusted to remove cooling stratospheric influence, and therefore tend have significantly lower trends).
There is an annual cycle in the discrepancy between UAH and RSS in recent years, but it’s less severe than in the LT case (about 0.04 deg peak to trough, as opposed to almost 0.2 deg in the LT set).
The resulting fluctuations in monthly trends can be seen below.
Here both data sets seem to have a moderate annual (or semi-annual) cycle, but there is a much lower peak to trough discrepancy compared to UAH LT. The UAH-RSS discrepancy is still apparent but varies less through the year.
To summarize, there is a severe annual cycle in the UAH LT data set that results in a noticeable divergence in both the global and tropical monthly temperature trends over the 1979-2008 period. This annual cycle and resulting divergence can not be explained by diurnal drift adjustments in the UAH or RSS data sets. Rather, it appears that a moderate annual cycle (of hitherto unknown origin) in T2 channel data has been exaggerated by the UAH LT extraction process.