SPARC Temperature Trends Assessment group also Shigeo Yoden, Carl Mears, John Nash Nathan Gillett Jim Miller Philippe Keckhut Dave Thompson Keith Shine me Ulrike Langematz Dian Seidel John Austin

Why Dave isn’t here:

Issues regarding SSU data 1.Construction of data set (Nash up to 1998, NOAA to 2005) - why are climatologies different for x-channels? - why different seasonal cycles? (especially for 47x) 2. How are tidal effects treated? What are the associated uncertainties? What are the other important corrections used in constructing the SSU time series? 3. Evidence for SSU uncertainties, especially after NOAA-14 a)SSU15x comparisons with sondes and MSU4 b)comparison of channels with similar weighting functions (25 and 26x, 27 and 36x) c) unphysical nature of trends after ~1995

4. Where are ‘we’ with respect to original SSU data? (‘we’ = research community) 5.Comments on using SSU in future reanalyses? -> recommendations from SPARC (to WOAP)? WOAP = WCRP Observations and Assimilation Panel

Problems with using analyses / reanalyses for stratospheric trends change in models and satellite data (TOVS to ATOVS)

even larger problems in upper stratosphere (associated with satellite changes) SSU data analyses / reanalyses Net result: don’t use analyses / reanalyses for stratospheric trends

This discussion regards analysis of the SSU data set covering These are derived from the SSU data set that John Nash constructed (covering Jan May 1998), with updates to Dec 2005 made by appending the NOAA-14 data. The NOAA-14 record started in 1995, and the overlap period is used to normalize the entire NOAA-14 record to John’s previous time series for each of the SSU channels. The analysis here focuses on comparing time series and trends between the different SSU channels.

This shows when data were available from the individual operational satellites. Note the significant drift in NOAA-14 measurement time. equatorial crossing time

Weighting functions for the individual SSU channels, plus MSU4 (on bottom). Channels 25, 26, 27 are the original channels, while 15x, 26x, 36x, and 47x are synthetic channels derived by differencing measurements from different scan angles (Nash, 1988, QJRMS). Note the strong overlap between channels 15x and MSU4, 26x and 25, and 36x and 27 (we examine differences of these below)

The synthetic x-channels from SSU are calculated from the following formulas (from Nash, 1988): 15x = 0.99 (-18.7 * D(25) + 26) 26x = 1.13 (-17.8 * D(26) + 27) 36x = 0.96 ( 25.7 * D(26) + 25) 47x = 0.87 ( 25.0 * D(27) + 26) Here D(25) is the difference between the 35 o and 5 o scan angles for SSU channel 25 (and likewise for 26 and 27). Note that each of the x-channels depends on channel 26 data.

Generation of additional SSU channels by differencing nadir minus off-nadir scans: nadir minus off-nadir for Channel 26 + = Channel 25 ‘new’ Channel 36x

SSU15x and radiosonde trends for large differences in tropics

Time series of SSU15x and equivalent radiosondes in tropics SSU15x difference radiosondes

Time series of MSU4 and SSU15x note recent trend differences -> Overall comparisons suggest biases in SSU15x trends

Upper stratosphere: time series from SSU EP

Vertical profile of global trends: Shine et al. model summary satellites radiosondes

satellite orbit drifts note orbit drift for NOAA-14

Global trends for result: suspicion of trends for SSU26, 26x, 15x

For comparison:

This shows the latitudinal structure of trends derived from 25, 26, 27 and 47x. Note the small trends for 26.

The overall behavior for channel 26 seems curious to me. Given the substantial overlap with channels 25 and 27, it would require some unusual atmospheric structure (narrow in the vertical, with a peak near 40 km) to produce the different time behavior and trends seen in channel 26. A little more digging into the x-channels: given the similarity between 15x and MSU4, 26x and 25, and 36x and 27, I have looked at the differences between these data. Note that 25 is independent from 26x (derived from 26 and 27), and 27 is independent from 36x (derived from 25 and 26). The following plots examine these differences.

As an example, here is a time series of the difference between 15x and MSU4 at the equator (left). The difference time series has a large contribution from the QBO (which is real, and results from the slight differences in weighting functions together with the downward propagation of the QBO temperature signal). We have removed this QBO variation by a regression onto QBO reference time series (right panel), in order to focus on other variability. QBO removed note the changes during the last decade

We have generalized these calculations by performing an EOF analysis of the latitude-time field of differences for 15x - MSU4. This is intended to capture dominant patterns of variability in the difference fields. The first EOF has a spatial pattern that maximizes in the tropics (top), with a time variation shown at bottom. (This mode captures 79% of the overall variance in the difference fields). The key point is that the time series shows a discontinuous behavior, with a trend during the last decade. It would take an unusual atmospheric structure to generate this behavior in reality.

Here’s a similar calculation for 26x – 25. Again the time series (bottom) shows a trend at the end.

Likewise for 36x – 27. The time series (bottom) shows a trend at the end.

In summary, all of the difference fields between the x-channels and original channels are dominated by a time variation that is relatively flat for ~ , and then changes monotonically after This suggests to me that something systematic is happening. Recall that all of the x-channels are tied to channel 26. The curious behavior of channel 26 seen in the time series and trends (in comparison with 25 and 27), together with the systematic behavior seen in the x-channel difference fields, suggests to me that something may be amiss in the channel 26 data for the most recent decade. Note this is the period when the record is based on NOAA-14, and there is a significant orbit drift during this time. Overall, I am suspicious of the detailed results derived from channel 26, and all of the x-channels, for the NOAA-14 period.

Some questions: Is there anything suspicious about the NOAA-14 instrument (channel 26 in particular)? Could this somehow be a tidal effect? The spatial structure of the difference fields peaks at the equator, and covers ~ 30 N-S (from the EOF spatial structure plots), and there are stronger tides at the equator. But, why an influence on channel 26 and less on 25 and 27? Any ideas from others?