Until I recently examined the underlying technical literature on the construction of the UAH and RSS satellite data sets, I had little appreciation of the complicated adjustment and estimation procedure involved in the satellite temperature indices.
The UAH and RSS satellite temperature indices are constructed from many different satellites (TIROS-N, NOAA-6,7,8,9,10,11,12,14,…), each of which has distinctive instrumental and orbital properties, which need to be allowed for in the estimation of the “true” tropospheric temperature. The estimation of these parameters is done differently by Christy and his associates on the one hand and Mears and his associates on the other. Controversies over the merits of these adjustment procedures have been ongoing for over a decade.
From the perspective of CA readers, there is a substantial statistical component to the estimation of the various adjustment and bias correction parameters, as can be seen from examining Christy et al 2000 url and Mears et al 2003 url , both of which describe statistical procedures, though not always expressing things in ordinary statistical terms.
In looking at tropical tropospheric results recently, I happened to do a crosscut of UAH versus RSS, which seemed to me to provide an interesting perspective on this debate. I am not nearly familiar enough with the overall issues to venture an opinion on who is “right” and who is “wrong” in any of these disputes. I am merely presenting a graph that intrigued me.
Before doing so, I want to present a few graphics that illustrate the history of a few relevant parameters for the satellites considered in the two articles references above (which do not include the most recent satellites), but which illustrate the scale of variation which can occur.
First, here is a graphic from Christy et al 2000 showing their adjustment by satellite for orbital decay. Aside from particulars of orbital decay, this graphic also provides information on satellites used for construction of the index and major transitions. For example, in 1986, there was a switchover from N-8 to N-10, with N-6 being briefly called out of retirement because of the short lifespan of N-8. In 1992, there was another transition from N-10 to N-12 with overlap being provided by N-11.
Second, here is another graphic showing changes in local equatorial crossing time LECT), another effect adjusted for by both parties. This is presented here primarily to help readers get their eye into when the satellite switches took place.
Next is a crosscut comparing UAH and RSS presented by Tamino last year in a surprisingly sober assessment of the differences between UAH and RSS.
There are two differences which are apparent to the eye. First, there’s a “step change” at 1992, with RSS being higher than UAH after that but lower before that. Second, in the most recent time period (from about 2003 on) there’s an annual cycle, with RSS being relatively higher during northern hemisphere summer and UAH relatively higher in northern hemisphere winter.
1992 and Other Steps
The existence of a 1992 step appears to be common ground among the parties. It was specifically mentioned by John Christy in a recent email to me following my recent post on satellites (in which he kindly also provided a variety of references on the interesting adjustment issues.)
The 1992 step has been attributed by the various parties to differences in handling the NOAA-10 to NOAA-12 satellite switch.
Recently, I experimented with strucchange on RSS versus UAH in the tropics, stratifying land and ocean series separately, yielding a pretty interesting crosscut relative to the above debate. I think that there are a variety of interesting points in this graphic, which I’ll discuss below.
First, there is a remarkable lack of similarity between the Ocean and Land differentials. Over tropical oceans, RSS increases relative to UAH quite consistently, whereas the patterns over tropical land are quite erratic. Since 2004, UAH tropical land has a dramatic increase relative to RSS overlying the strong annual cycle in the difference series.
In the land series, strucchange picks out several breakpoints, each of which can be plausibly identified with a satellite changeover. The 1992 breakpoint at the transition from NOAA-10 to NOAA-12 is picked out. However, the 1992 breakpoint is not as unique as presumed in previous discussion nor even primus inter pares. There is a substantial breakpoint in 1986 at the transition to NOAA-10 (from a patchwork of satellites in the immediately prior period. There is a noticeable breakpoint in 1998 at the end of NOAA-12.
There is also a noticeable breakpoint around 2004-2005. Mears et al 2009 observe:
In order to continue the atmospheric temperature record past 2004, when measurements from the last MSU instrument degraded in quality, AMSU and MSU measurements must be intercalibrated and combined to extend the atmospheric temperature data records.
While the earlier history of the respective RSS and UAH adjustments has been much debated, it looks like another chapter needs to be written on the MSU/AMSU changeover, where, far from differences diminishing from prior analyses, the differences are increasing, with RSS/UAH difference trends being opposite over tropical land and over tropical ocean – a point not apparent in the combined difference series, where the trends offset.
Both Christy et al (J Clim 2009) and Mears et al ( Journal of Atmospheric and Oceanic Technology, 2009a,b) see url have recent technical discussions, but I didn’t notice any discussion of this point in a quick perusal.