Monday, February 15, 2010

So, About That January UAH Anomaly

In a previous post I discussed that high anomaly reading from UAH's January measurements, pointing out that it's possible UAH's statistical adjustments to scan readings could have been thrown off by the unusual weather. In another post I discussed how the AMSU data used by UAH has known errors in nearly all of its scans. The error for channel 5, which is used to create the UAH anomaly readings, can go as high as -10 degrees K. In yet another post I discussed the raw scan line data that I've pulled from the AMSU on the Aqua satellite. It just so happens the data I pulled occurred during January.

So in this post, I want to combine those three posts and show why I'm still not convinced the January anomaly reading from UAH was correct. Basically, the data I pulled from January had an error of -14 degrees K in the channel 5 reading, which is 40% higher than the the maximum error according to the journals.

I can't help but wonder how well the statistical methods used to correct the known channel errors react to errors so far out of range of expected values. Actually, I can't help but wonder if errors of this size have ever been tested at all.

Which means I can't help but wonder if the January UAH anomaly has any validity at all.

It also makes me wonder how much additional data falls outside the expected error range. Because I didn't put any effort at all into finding data 40% beyond that range.

Some Useful Climate Code
A Note On UAH's High January Temperature
Taking A Look At Raw UAH Data


  1. So let me understand some of the basics regarding the Advanced Microwave Sounding Unit (AMSU) flying on NASA’s Aqua satellite:

    1) The Aqua satellite takes 99 mins to orbits the earth.

    Therefore it makes 14.55 orbits in 24 hours
    24 hrs x 60 mins = 1,440 minutes
    1,440 / 99 = 14.545454

    i.e. Aqua flies over the equator in daylight 14 times in 24 hours.

    2) The earth circumference at the equator is 40,075 km

    Therefore, the AMSU should scan 2,862.5 km of the equator on each orbit.
    40,075 km / 14 = 2,862.5 km

    3) This WUWT article says the instrument scans across the subtrack of the satellite,
    the radiometer’s antenna views thirty separate ‘footprints’, nominally 50 km in diameter

    Therefore, the AMSU only scans 1,500 km of the equator on each orbit.

    Using simple maths this means the AMSU only scans 52.4% of the equator.
    1,500 / 2,862.5 = 0.524

    The AMSU has a Swath of 1650 km according to
    This might increase the coverage to 57.64%

    Therefore, I would like to know how we get UAH global temperatures by sampling only 52.4% or 57.64% of parts the globe….


  2. The 15 scans directly below the satellite scan about 50 km each, for a total of about 750 km. However, the other 15 scans are at progressively sharper angles from the center of the satellite and scan progressively more area. These remaining 15 scans combine for another ~1500 km of cover, giving a total of about 2250 km of coverage.

    For more info, see:

  3. magicjava--
    UAH uses the Aqua satellite, not the channel 5 AMSU. Channel5 AMSU posts live-- so it's convenient for getting a general notion of what's going on, but it is error prone.

    I don't know what RSS uses.

  4. Hmm. I double checked what Dr. Spencer said in his WUWT post here:

    Here's the quote:
    we use a weighted difference between the various view angles of channel 5 to probe lower in the atmosphere, which a fairly sharp weighting function which is for our lower-tropospheric (LT) temperature estimate.

    If this isn't correct, or if I'm misunderstanding it, could you point me to a source that explains which channel(s) are used by UAH for their published temperatures anomalies?

  5. ok-- I may be mistaken. Spencer is definitely the right source!

  6. lucia -

    I'd guess we're both right. Channel 5 gets used for a live feed and has errors because they don't have time to perform al the processing to it live. But it also get used later for the "real" calculations. And just to be clear, the errors I noticed in this post aren't due to skipping some processing during live feeds, they're due to "out of range" errors in the data itself. UAH may properly handle these conditions, or it may not. I can't tell without seeing the source code.

    Malaga View -

    I realize the swath number I gave you differs from what NASA has on its web site. With all due modesty, I would say my numbers is right and NASA's is wrong. The source for my numbers explains exactly how they're calculated, whereas NASA doesn't.

    Also, grabbing numbers off a NASA web site can be a hit or miss proposition. See my series of posts on Redshift Z from August, 2009 for an example. It's those bad numbers from NASA's website that were the whole reason I started this blog in the first place.


    I am probably a lot more cynical... the lack of clarity and differing statements do not give me any confidence...

    I wonder what they do with the parts of the globe that are not scanned... do they ignore them? do they infill and estimate?

    I wonder what they do with the parts of the globe that are scanned 14 times in 24 hours. Do they average these scans? do they choose the first or the last?

    I wonder about the sequence of the scan strips. A storm system (for example) can move a long way in 24 hours and could appear in more than one scan strip in 24 hours... so is there double counting? or are some short durations anomalies simply missed?

    Overall, the techniques really need to be verified and proved correct... the other "scientific" methods have been manipulated and (for me) the satellite measurements are no different... the problems are always in the details!