The NOAA-N Polar Orbiting Weather Satellite
There have been some questions posed recently about how the satellite global temperature record is created by UAH and RSS from the MSU (Microwave Sounder Unit) on weather satellites. For example, Andrew recently posted this comment:
“I keep hearing rumors that the satellite data was “adjusted” to match the surface data better. Is that true? If so, that would be disturbing.”
It is always best to ask the source, so I put the question to Dr. John Christy, who is the lead scientist at UAH that produces this dataset:
“I’ve had some queries on my blog recently that are suggesting that the UAH and RSS satellite data is somehow “tuned” to the surface data, or that the surface data is used to provide some offset function. Given that the MSU looks at microwave emissions from oxygen, essentially a first principles measurement, I don’t see any reason that surface data would be used in any way to adjust the MSU data.
But I figured I’d ask the source, if you’d care to elaborate. If not, no worries.”
To which Dr. Christy graciously responded within a couple of hours:
“No other data are used in the construction. That is why we can do comparison studies without any interdependence.”
So from Dr. Christy’s response it is clear that there is no data sharing or comparative adjustment of any kind between that satellite global temperature record and the land-ocean global temperature record such as is produced by HadCRUT and GISS.
The Advanced Microwave Sounder Unit on NOAA-N satellites
For further reading there is an overview on satellite temperature measurements on and details on the microwave sounder unit both Wikipedia.
UPDATE: An additional question has been brought up related to why the data seems to be missing from the poles. Dr. Christy also responded:
As the spacecraft rolls over the pole it does so at an inclined orbit so
that the highest nadir latitude is about 82 deg with the scanner looking
out a bit closer to the pole. Since we apply the scan line data mostly to
the nadir area directly below the satellite, the actual data only go to
about 83 deg. In the gridded data I interpolate over the pole, but I
wouldn’t trust the data too much beyond 85 deg.
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sooner_wx,
Would the disagreement in decadal trends be from those differing methods?
JM,
Randall and Herman (2008) conclude the differences between the trends (shown in the LT channel over land) of UAH and RSS is primarily due to diurnal corrections.
REPLY: By chance would you have a link to that paper? Thanks, Anthony
Anthony,
Here is the info:
Randall, R. M., and B. M. Herman (2008), Using limited time period trends as a means to determine attribution of discrepancies in microwave sounding unit–derived tropospheric temperature time series, J. Geophys. Res., 113, D05105, doi:10.1029/2007JD008864.
http://www.agu.org/pubs/crossref/2008/2007JD008864.shtml
REPLY: Thanks so much!
I know this is now a rather old thread, since there have been so many new ones recently, but these dated posts about the satellite trends being different from the surface trends seem relevant:
http://science.nasa.gov/newhome/headlines/essd06oct97_1.htm
http://science.nasa.gov/headlines/y2000/ast21jul_1m.htm
Patrick:
Those links are from 1997 and 2000. Over the years since then, several errors were discovered in the methods used by Christy et al for processing the satellite data. The MSU temperature trends now match the surface records quite closely (IIRC, RSS’s trend is slightly higher than GISS, while UAH’s is slightly lower).
In the previous thread, I was defending the proposition that the satellite microwave radiometer data were not “tuned” to match the surface temperature trend. I’m glad that Anthony took my advice and posed this question to Dr Christy. Sometimes it seems like the same arguments keep getting recycled over and over again. Here’s hoping that this particular allegation has now been conclusively put to rest.
J.
I thought that the RSS used a climate model for the diurnal correction, so the question is what are the various inputs used for the climate model and how are the radiative properties of the climate model and how sensitive are they to atmospheric CO2 quantities?
Andrew,
The process of obtaining the temperature from the satellite radiance data is an ill posed problem, i.e. one is inverting an integral and this inversion is especially difficult in the presence of clouds. It is my understanding (based on experience) that the temperture retrieval is an iteration process that uses surface temperature readings in the iteration process. Please ask Dr. Christy again if this is the case or not. If he claims it is not the case, I will dig out the mathematics to prove the point.
Jerry
Andrew,
I just read the info on wikipeidia and it states very clearly that the conversion from satellite radiances to temperature is not a simple process and in fact different results are obtained by different methods. I think Dr. Christy needs to be asked about this in a bit more detail.
Jerry