NOTE: ****Check the appropriate MSU data directory for the latest version of this file****

Update 3 Jan 2008*****************************

We now have data from AQUA added to the time series beginning with day 221 of 2002. AQUA is a spacecraft with on-board propulsion and thus has stable station-keeping. Thus, AQUA's AMSU will not be subject to diurnal temperature drifts. Upon comparison with NOAA-15's AMSU, we find only minor differences for their 5+ year overlap, with NOAA-15 being slightly warmer near the end of the time series for LT and MT. The error values for NOAA-15 are much smaller than what we indicated below. At this time we are merging AQUA into the time series while keeping NOAA-15, with its slight, spurious warming, in the mix through the present. There will be some slight month-to-month differences versus the version from last month, but these are very small so we will keep the version numbers as they are since nothing has changed except the merging of AQUA into the time series. We note that 1-11 Dec 2007 are missing in AQUA and are hoping these will be recovered. We do have all daily values from NOAA-15 for December.

Update 12 July 2007 *****************************

We are still relying on NOAA-15 for the current MT and LT products. We have not instituted a diurnal correction on these, so they are likely running a little on the warm side as NOAA-15 is "backing" into a warmer period of the diurnal cycle. Regarding the long term trend, this will have a very small impact. When the new MSU diurnal corrections as well as the AMSU diurnal corrections are applied, we suspect the resulting trend will be almost identical to the current trend, though there will be some changes in the interannual variations.

Update 15 Dec 2006 ******************************

Due to a dumb mistake, the values for MT were in error when loaded up for the period ending Nov 2006. Rather than eliminating NOAA-16 data (the bad satellite) I had eliminated NOAA-15 (the good satellite) after Sept 2005. So, the values for MT have all been rerun and replaced. There are slight changes throughout the time series since the mean annual cycle was affected. I've also replaced all of LT to make sure they were consistent.

Update 5 Dec 2006 *******************************

Data products are still 5.2 and 5.1. For LT 5.2 and MT 5.1 we have eliminated the data from NOAA-16 after September 2005 when NOAA-16 began to diverge in a manner that suggested NOAA-16 was having problems. Thus, the data since Oct 2005 is based on NOAA-15.

Update 10 Nov 2006 *******************************

Notice that data products are back to version 5.2 for LT and 5.1 for MT and LS. We had hoped to solve the inconsistencies between NOAA-15 and NOAA-16 by this time, but we are still working on the problem. The temperature data for LT and MT are diverging, and we had originally thought that the main error lay with NOAA-15. However, after looking closely, there is evidence that both satellites have calibration drifts. We will assume, therefore, that the best guess is simply the average of the two. This is what is represented in LT 5.2, MT 5.1 and LS 5.1. These datasets have had error statistics already published, so we shall stick with these datasets for a few more months until we get to the bottom of the calibration drifts in the AMSUs. However, the error statistics only cover ther period 1978 - 2004. The last two years cover the period where the two AMSUs are drifting apart, so caution is urged on the most recent data.

Update 12 Oct 2006 ******************************

This is a note just to emphasize that the current products will be changing and conclusions should not be made on versions appended with "p". The older version 5.1 has been analyzed in the literature with published levels of confidence. There are no confidence levels for any of the "p" versions, which means confidence should be assumed to be poor until published magnitudes are available.

Update 6 Oct 2006 ******************************

Another month and the same story, we are still finalizing the production of version 6.0. The values which will be placed on the website contain the earlier v5.2 data through Dec 2004 with the preliminary values from v6.0 from Jan 2005 to the present (Sep 2006). The largest changes occur in the last two years as NOAA-15 was drifting into warmer temperatures and its target temperature effect became evident. Again, this is a preliminary dataset designed only to give an idea of what is going on at the moment. We will update the daily files when the full v6.0 is ready. The monthly gridded files and the uahncdc files also reflect the use of the newer data in 2005 and 2006 only.

Update 6 Apr 2006 ****************************

Roy is working on a diurnal adjustment for the AMSU instruments as they have now drifted over an hour from their initial crossing time. NOAA-15 has backed up from 7:30 to 5:48 and NOAA-16 has drifted forward from 1:54 to 3:10. Be on the lookout for a new version that will have these additional adjustments. Based on early results we expect LT and MT to have warmer temperatures in the past year or so by a few hundredths of a degree.

Update 20 Aug 2004 *****************************

NOAA-15 and NOAA-16, the first two spacecraft carrying the AMSU instrument, are due for diurnal corrections. The global mean intersatellite drift error is 0.016 C/year for MT for the period of their overlap (Feb 2001 - present). In general, the newer spacecraft drift less than the earlier ones, so corrections should not be substantial and the net effect may be a wash. NOAA-15 is drifting earlier, which introduces a spurious cooling trend in LT, while NOAA-16 is drifting later in the diurnal clock which introduces a spurious warming in LT. The opposite occurs for both MT and LS. Sometime in the next few months we shall apply the new diurnal corrections, likely changing the file names to 5.2.

We are also experimenting with a more direct method of calculating the brightness temperatures which avoids the NESDIS nonlinear adjustments altogether. Preliminary results suggest trends could be very slightly warmer, but less than 0.02 C/decade different.

Update 7 Mar 2003 *****************************

We have made some changes to the data processing that were quite minor. Even so, we decided to change the version number to 5.1 from 5.0. These changes will not affect scientific results for those of you in the process of publishing work from version 5.0. For all three products we have strengthened the requirement a bit for acceptable data to entire into the routine that calculates the intersatellite biases. This resulted in a very slightly more negative trend in LT by 0.004 c/decade and for MT by about 0.003 C/decade. In addition as noted in the 10 Jan 03 entry, we have updated the Target Temperature coefficients since 2002 added some MSU data from NOAA-14. The update only affected LS (T4). One of the coefficients barely exceeded the 40% explained variance threshhold this time (NOAA-11), so it was employed in the processing. This helped reduce the daily error variance and the difference in trends between NOAA10 v. 11 and NOAA11 v. 12. The net effect on the trend was about 0.02 C/decade (more positive)

We thank John Bates of NCDC for finding earlier MSU data from TIROS-N. As a result, version 5.1 daily data now begin at 16 Nov 1978 and the monthly grids now begin with Dec 1978.

Update 10 Jan 2003 *****************************

Roy Spencer has updated the ephemeris corrections which had been estimated for the past 9 months. Changes are minor with a few monthly global averages a few hundredths C warmer in 2002 than as shown last month for LT. With the completion of 2002, we will be recalculating the target coefficients and if the changes impact the trends much, we may up the version number to 5.1 from 5.0. This should be complete with the calculation of the January 2003 values. Coming in 2003 - diurnal drift corrections for the AMSUs on NOAA-15 and NOAA-16. There should be little impact. We will also be merging data from NOAA-17 this year. Also, the paper describing Version 5.0 of the microwave data will be appearing in J. Atmos. Oceanic Tech. this year.

Christy, J.R., R.W. Spencer, W.B. Norris, W.D. Braswell and D.E. Parker, 2003: Error estimates of version 5.0 of MSU/AMSU bulk atmospheric temperatures. 2003. J. Atmos. Oc. Tech.

Update 14 Aug 2002 *********************

I've noticed a noise problem with the gridded datasets in which the AMSU data, because of its much finer resolution is not being properly smoothed to match the old MSU spatial resolution. This creates spurious east-west oscillations of anomalies primarily from 40S to 40N where the stripping is most evident (see Christy et al. 1998 for complete discussion of this sampling problem.)

The scale of this problem is wave number 14 in the east-west dimension due to the fact there are 14 orbits per day per satellite. I'll be testing some better longitudinal downscale averaging techniques in the next few months to try and eliminate this feature in the anomalies. At present, I will likely smooth the anomalies of NOAA-16's AMSU to wave number 12 to greatly reduce the wave number 14 noise. The goal here is to reproduce an MSU-like spatial resolution from the AMSU high resolution data. This does not effect the global or even zonal average anomalies.

Update 9 May 2002 *********************

We are now using the non-linear correction for the diuranal drift of the satellites. The version will now have appended "_5.0" to indicate this. Note that the name of the file begins "tlt" rather than "t2lt" because channel 2 is not used from the new AMSU instruments, so no need to use the "2". "tlt" means "temperature lower troposphere".

Update 8 April 2002 **********************

Roy Spencer and I are in the process of upgrading the MSU/AMSU data processing to include a new non-linear approximation of the diurnal cycle correction (currently the approximation is linear). In preliminary results, the effect is very small, well within the estimated 95% C.I. of +/- 0.06 C/decade. In the products released today, some minor changes have been included (though not the new non-linear diurnal adjustment). The 2LT trend is +0.053 C/decade through Mar 2002. The difference in today's release vs. last month's is a slight warming of monthly data after 1998. Essentially, this release corrects an error in the linear diurnal adjustment and produces better agreement between the MSU on NOAA-14 and the AMSU on NOAA-15. The single largest global anomaly impact is a relative increase of +0.041 (April 2001) while most are within 0.02 of the previous values. The net change in the overall trend was toward a more positive value by +0.012 C/decade. Again, this is still an interim change, and we anticipate a final version ("E" or "5.0") next month. The current release will be designated t2ltmonamg.YYYYdx.

Update 8 Jan 2002 *******************

Revised values for the correction of orbital decay for NOAA-15 (Aug 1998) and NOAA-16 (Feb 2001) has been applied to the AMSU data. These corrections are based on a more analytical approach for the cross-scan biases that arise as the spacecraft falls in altitude. The net effect is minor, being a slight cooling of the early values and a warming of the more recent values observed by the AMSUs. The overall 23-year trend is essentially unchanged.

Update 2 Nov 2001*******************

The comparison between NOAA-14 and NOAA-15 had been superb up until about 21 Aug 2001. At that time NOAA-14 showed significant cooling relative to both NOAA-15 and NOAA-16. This is likely due to the diurnal drift of the spacecraft as it is now over 6.5 years old. The diurnal corrections are typically useful until the spacecraft drifts significantly (NOAA-14 is a p.m. bird, starting with a 2 p.m. orbit that is now in late afternoon.) I've determined that NOAA-14 for 2LT is likely in a situation where the diurnal corrections as well as instrument body temperature effects are not well characterized for this new solar shadowing regime. So, for now, I am eliminating the post 21 Aug 2001 NOAA-14 data in 2LT, relying on the excellent agreement between NOAA-15 and NOAA-16. As a result, the values for global anomalies for Aug and Sep (and now Oct) will be slightly warmer than previously reported (about 0.06 C). The trend has become more positive by only 0.002 C/decade as a result of this termination. We are investigating a fix for NOAA-14 so that the data may be used again. Please be advised that as the data from NOAA-16 are merged into the datastream there will be slight adjustments to the recent anomalies as we wait until a resolvable annual cycle is available. NOAA-16 has provided data since 1 Feb 2001.

Update 24 Aug 2001 *********************

I've discovered a Y2K error in the program which reads the diurnal corrections. The corrections for NOAA-14 were not applied after 1999. These will be applied when the August data are processed. Preliminary checks indicate the impact is less than 0.01 C/decade. Because the diurnal corrections were not completely applied, I recomputed the PRT coefficients to adjust for heating of the instrument. This impact is between .001 and .003 C/decade on the full trend, so it is tiny. The net effect of these corrections over the current dataset is to give a slight warming to MSU2 and a slight cooling to MSU2LT. As of July 2001, the trends in MSU2 and MSU2LT are virtually identical. This seems odd because the overall stratospheric trend is quite negative. However, the stratospheric component of MSU2 has "seen" a warming due to Mt. Pinatubo and a gradual warming since June 1996 in the stratosphere. Thus, relative to MSU2LT, MSU2 has seen more warming since 1996 and now expresses the same trend since 1979 as MSU2LT. This is not an artifact of NOAA-14, but is also present in the AMSUs on NOAA-15 and NOAA-16.

Update 9 May 2001 **********************

NOAA-14, with the last MSU, will soon be decommissioned. We will soon be phasing in data from NOAA-16. Through April, the dataset has been built from the MSU on NOAA-14 and the AMSU on NOAA-15. During the next several months, as we phase-in NOAA-16, there will likely be a little instability in the anomalies as it takes many months to accurately determine the bias between NOAA-16 and older satellites. Thus there may be changes from one month to the next for anomalies calculated for previous months.

Update 19 Jan 2001 *********************

A new algorithm for the NOAA-15 AMSU has been applied to match the NOAA-14 MSU2LT product. The new multi-angle method reduces the intersatellite noise from the previous method by 10 percent. The bias correction now is only 0.126 vs. 1.084. A few of the monthly values are changed by 0.01 C or so. The overall 79-01 trend under the new algorithm is identical to the old (0.044 C/decade).

Update 6 Oct 2000 **********************

Orbital decay values for NOAA-14 have been updated for 2000. This effect is very minor - a couple of hundredths of a degree on a monthly, global average. The global trend warmed up by +0.002 C/decade as a result.

Update 7 Mar 2000 **********************

We have now included the NOAA-15 AMSU data into the 2LT time series. After several experiments with multi-channel representations of the old MSU 2LT, we concluded that none quite reproduced the old temperatures. So, we are using a multi-angle retrieval from AMSU channel 5 which reproduces the multi-angle 2LT from MSU very closely. So, data in the 2LT time series since September 1998 now include AMSU.

Update 4 Feb 2000 ***********************

We have made a slight change to the t2lt files. The orbit decay correction had been determined as a function of the daily average orbit altitude. We have recalculated the orbit decay effect on a scan-by-scan basis (i.e. over 3000 times per day) to account for the alititude change within a particular orbit. The time series is virtually identical to the earlier version, with the trend becoming more positive by 0.013 C/decade. We shall have one more alteration to the t2lt time series in the next few weeks - that of merging NOAA-15 (AMSU) into the time series. T4 and T2 already include NOAA-15. (See below for earlier discussion). We are closer to a multi-channel solution now and hope to include that soon for 2lt.

Update 27 Dec 1999 ************************

The daily zonal files have now been made Y2K compatible with the value for year as four-digits. I am still working on the grid point files, some of which have four digits, and some which don't. This is a bit more tricky than it seems. NOAA-15 became operational in September 1998 carrying the new Advanced Microwave Sounding Unit. This instrument has 16 channels, 12 more than the MSU of TIROS-N, NOAA-6, 7, 8, 9, 10, 11, 12 and 14. The channel frequencies are not exactly matched to the MSU, but we are working on merging the new data into the MSU stream. For T2 (MSU channel 2) we are using AMSU channel 5. AMSU5 peaks slightly lower in altitude than MSU2 and has a mean bias of 2.55 K (AMSU5 warmer than MSU2). AMSU5 global mean annual cycle is about 0.15 K greater than MSU2. In terms of anomalies, this should not be a problem, and the intercomparison of MSU to AMSU anomalies looks as good as any other MSU to MSU comparison. Therefore, the T2 anomalies now have AMSU5 data merged. We shall continue to call this product T2 in honor of the MSU heritage. For T4 (MSU channel 4) we are using AMSU channel 9. AMSU9 has a slightly different weighting function than MSU4, but the global mean bias is only 0.02 K warmer than MSU4 but still has a global mean annual cycle which is 0.1 K in amplitude greater than MSU4. The MSU4 to AMSU9 anomaly comparison is excellent. Therefore, the T4 anomalies now have AMSU9 data merged since September 1998. We shall continue to call this product T4 in honor of the MSU heritage. We are still working on an AMSU retrieval which matches MSU 2LT. At present, we have generated a retrieval whose weighting function is similar to 2LT using AMSU channels 4, 5, 6, and 7. There is a global mean bias of about 1.3 K, but the distribution of the bias is non-uniform, being much greater near the poles (about 5 K) and less in the tropics (about 0.6 K). At present, we have not merged the AMSU data into the T2LT time series until a better match is determined.

Update 19 Nov 1999 ***********************

Data are now available for version D in both zonal and gridpoint. The NOAA-12 non-linear coefficients have been applied to both zonal and gridpoint files. The description of version D is contained in Christy, J.R., R.W. Spencer and W.D. Braswell, 2000: MSU tropospheric temperatures: Dataset construction and radiosonde comparisons. J. Atmos. Oceanic Tech. (in press). The gridded files will now be in this directory as the corrections have now all been applied and the version extension is now simply "d".

Update 13 July 1999***********************

Recent research has shown that the time series of t2lt is affected by orbit decay. This effect causes artificial cooling on the time series when examined in isolation. There are offsetting warming effects related to the time-drifting of the spacecraft due to east-west drift. The new designation for MSU 2R (lower troposphere) will be t2lt representing a channel 2 retrieval of the lower troposphere (hence t2lt). The third character in t2lt is an "l" as in "lower", not a one. Note that the base period for the mean annual cycle for t2lt is now 1979-1998, or 20 years instead of the previous 1982-1991 ten years.

John Christy.

Data can be ordered and questions addressed at

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