As promised, I contacted Dr. John Christy regarding the seasonal signal that the anonymous blogger “deepclimate” says he/she has identified in the UAH data, seen below. He/she says: “I am a Canadian citizen residing in Canada. For private and professional reasons, I prefer to remain anonymous to the general public, at least for now.”
I’ve never understood the need for some people to remain anonymous while at the same time attempt to do science. Imagine the furor if scientists like Christy or Spencer created an anonymous blog and then were later discovered. I’m sure it would be immediately up there on sourcewatch with “tsk tsk” attached.
Science really should be done out in the open. Here’s Dr. Christy’s in the open response.
Dr. Christy has made a response in the readme file at the UAH website here:
Update 18 Jul 2009 ************************************
Corrected trend values (1700 CST)
It was brought to my attention by Anthony Watts that there has been some discussion about the noticeable annual cycle in the LT and MT trends when done by months. In other words, the trend for Februaries is on the order of 0.12 C/decade warmer than the trend for Mays. Other data sets don’t have such a large range in trends when calculated by months, RSS for example has a range of 0.05 C/decade. (Note, this issue doesn’t affect the overall trend.)
The feature arises when the AMSU data are adjusted and merged into the MSU data stream beginning with NOAA-15 in Aug 1998, then carries forward with NOAA-16 and AQUA (both of which are AMSUs too). The process involves at one point
the removal of a mean annual cycle in the anomaly differences from one satellite to another. It turns out that all satellites have a residual annual cycle due to each instrument’s peculiarities. In the end, all annual cycles are matched to NOAA-6 and NOAA-7.
Detecting the impact of this peculiarity is difficult. For example, it is not seen when
gridded data are directly compared against radiosondes (see Christy
and Norris 2006 and 2009.) However, an annual cycle in the difference time series is clear in RSS data when compared with balloons (see Fig. 2 of both papers.)
I’ve tested a number of alternate processing methods (basically versions of
not removing the annual cycle in the difference time series from the first
AMSU onward) and the range from the highest to lowest is reduced
to just under 0.09 C/decade. This in effect establishes a new annual
cycle for the AMSUs based on the first AMSU.
I think the magnitude of the annual cycle in the monthly trends is a
legitimate problem to address. The range in the current v5.2 LT looks too large
(about 0.12 C/decade). However, one should expect differences from month to month, especially when ENSOs and a volcano have different impacts by months so
so the range shouldn’t be zero. I’ll keep looking into this and if a
reasonable result is produced, I’ll rename the dataset v5.3.
The important point in all of this is that the overall global trend of the entire
time series ranges insignificantly from +0.123 to +0.125 C/decade even
under the different merging methods used to date. This is because the removal of the annual cycle of differences from satellite to satellite does not add any bias
to the time series, so the overall trend doesn’t change.