• Christy, J. R., R. W. Spencer, and E. S. Lobl, 1998: Analysis of the merging procedure for the MSU daily temperature time series. J. Climate,11, 2016–2041.

  • Hurrell, J. W., and K. E. Trenberth, 1997: Spurious trends in the satellite MSU temperature trends arising from merging different satellite records. Nature,386, 164–167.

  • ——, and ——, 1998: Difficulties in obtaining reliable temperature trends: Reconciling the surface and satellite MSU 2R trends. J. Climate,11, 945–967.

  • NOAA, 1997: NOAA Polar-Orbiter Data Users Guide. [Available from NOAA/NESDIS/NCDC/CSD/SSB, FOB, Room G233, E/C C33, 4700 Suitland Road, Suitland, MD 20746; or online at www2.ncdc.noaa.gov/docs/podug.].

  • Prabhakara, C., R. Iacovazzi Jr., J.-M. Yoo, and G. Dalu, 1998: Global warming deduced from MSU. Geophys. Res. Lett.,25 (11), 1927–1930.

  • Spencer, R. W., and J. R. Christy, 1990: Precise monitoring of global temperature trends from satellites. Science,247, 1558–1562.

  • Trenberth, K. E., and J. W. Hurrell, 1997: How accurate are satellite“thermometers”. Nature,389, 342–343.

  • Wentz, F. J., and M. Schabel, 1998: Effects of satellite orbital decay on MSU lower tropospheric temperature trends. Nature,394, 661–664.

  • View in gallery

    MSU channel 2 12-month running means, TAM and TPM, deduced from NOAA-10, NOAA-11, and NOAA-12 for (a) global ocean and (b) global land.

  • View in gallery

    MSU channel 2 deduced TPMTAM from NOAA-10, NOAA-11, and NOAA-12 for (a) global ocean and (b) global land.

  • View in gallery

    The T2 time series of MSU channel 2 temperature developed by CSL from 1979–97. Dashed line indicates the linear temperature trend from 1979–91 (0.088 K decade−1). The solid line indicates the linear temperature trend from 1979–97 (0.003 K decade−1).

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Comments on “Analysis of the Merging Procedure for the MSU Daily Temperature Time Series”

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  • 1 NASA/Goddard Space Flight Center, Greenbelt, Maryland
  • | 2 Raytheon ITSS Corporation, Greenbelt, Maryland
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Corresponding author address: C. Prabhakara, NASA/Goddard Space Flight Center, Code 913, Greenbelt, MD 20771.

Email: cuddapah@climate.gsfc.nasa.gov

Corresponding author address: C. Prabhakara, NASA/Goddard Space Flight Center, Code 913, Greenbelt, MD 20771.

Email: cuddapah@climate.gsfc.nasa.gov

Spencer and Christy (1990) showed in a pioneering study that it is possible to monitor the global temperature with observations made in channel 2 (53.74 GHz) of the Microwave Sounding Unit (MSU). Based on this idea, Christy et al. (1998; CSL) analyzed the MSU data, taken from sequential National Oceanic and Atmospheric Administration (NOAA) operational satellites (NOAA-6, -7, -9, -10, -11, -12, and -14), to find the global temperature trend in the period 1979–97. They find the tropospheric global temperature trend obtained from their “T2” time series is nearly zero (0.003 K decade−1), and the global trend over the same period in the lower troposphere given by their “T2LT” time series has a small negative value (−0.046 K decade−1). A critical element of the CSL time series of MSU channel 2 global temperature anomalies is the small correction applied to the NOAA-11 data to account for the orbital drift of that satellite. When they implement this correction, the anomalies in MSU channel 2 temperature after 1991 are decreased systematically by about 0.17 K. As a consequence, their inferred global temperature trends for the period 1979–97, instead of being on the order of 0.1 K decade−1, are decreased to the small values given above. From a study by Prabhakara et al. (1998) (PIYD), where an examination of the MSU channel 2 data of NOAA-6 through NOAA-14 is made with a.m., p.m., land, and ocean subsets, we find that there are nonnegligible drift related errors in the data of all the satellites. The CSL analysis of drift errors is not comprehensive because it highlights only the errors from NOAA-11. The purpose of this comment is to show vividly that these drift-related errors are not limited to NOAA-11.

Problems related to the development of reliable time series of global temperature and its trend from MSU data, especially during the period between 1991 and 1994, are addressed in some recent studies. The accuracy of the “satellite thermometers” is discussed by Trenberth and Hurrell (1997). Also, in studies by Hurrell and Trenberth (1997, 1998) spurious trends in the satellite MSU temperature record arising from merging different satellite records, and difficulties in obtaining reliable temperature trends that reconcile surface and MSU channel 2R trends, are addressed. Wentz and Schabel (1998) have also identified this period as one where the orbit of the satellites was greatly affected by atmospheric drag. The above investigations relied on the global temperature anomaly time series developed by CSL. In order to probe specifically into the satellite drift-related effects that are coupled to the diurnal temperature cycle, in our investigation we develop time series of the MSU channel 2 data directly from a.m., p.m., land, and ocean subsets of the satellite measurements.

In PIYD, an independent analysis is made of the MSU channel 2 nadir brightness temperature data, which have been derived by applying the NOAA (1997) calibration procedure to the MSU observations taken from NOAA-6, -7, -9, -10, -11, -12, and -14. The MSU brightness temperature data of NOAA-6 to NOAA-14 have been partitioned into a.m. and p.m. sets based on the local equatorial crossing time (LECT) of each one of the seven satellites. Then, in a given month, these data are averaged separately in grid boxes of 2° lat × 3° long between 75°N and 75°S to obtain a.m. and p.m. monthly mean temperatures in those grid boxes. Finally, we apply cosine weighting to these gridbox data so that area-average values of monthly mean temperature, TAM and TPM, can be obtained for any given region on the globe. In this manner, we get TAM and TPM separately for land and ocean areas (see PIYD).

The temperature difference (TPMTAM) reflects the amplitude of the diurnal cycle and is expected to be large over land in comparison to ocean. This is found in a gross fashion by PIYD in the MSU channel 2 data. Furthermore, when we consider this difference as a function of time, we expect in the presence of satellite orbital drift that the time derivative of (TPMTAM) on average will be significantly larger on land than on ocean because of the nature of the diurnal cycle. This property allows us to diagnose the drift-related changes in the MSU data.

For this note, we are limiting our analysis specifically to the MSU channel 2 data taken from NOAA-10, -11, and -12 to address the drift-induced effects in the data of the NOAA 11 satellite. During its life span, NOAA-10 had a drift in its LECT of about 20 min (0730/1930 to about 0710/1910 LT), while NOAA-12 had a drift of about 75 min (0730/1930 to about 0615/1815 LT). NOAA-11 had a much longer drift in its LECT of approximately 3 h and 45 min (0130/1330 to about 0515/1715). To assess the importance of these drifts, TAM and TPM from each satellite are calculated and then plotted in Figs. 1a and 1b as a function of time for global ocean and land, respectively. The variables TAM and TPM represent 12-month running means of a.m. and p.m. temperatures. The running mean is used to eliminate the annual cycle present in these data. Additionally, in Figs. 2a and 2b, we show TPMTAM as a function of time respectively for ocean and land. From Figs. 1b and 2b, we see on land that NOAA-10 and NOAA-12 data reveal a time dependence in TPMTAM, which suggests there are some drift-related effects in these data. Furthermore, the observations made by NOAA-11 indicate a much weaker drift-related error on land, while this error on the ocean is much larger. These observations do not support the assumption made by CSL that the data from NOAA-10 and NOAA-12 can be treated as a standard, that is, “backbone,” with respect to which the NOAA-11 data can be adjusted.

The drift-related effects on the MSU channel 2 time series may be appreciated from the data shown in Fig. 3. The CSL T2 time series of MSU channel 2 temperature anomalies shown in this figure yields a global warming trend of about 0.088 K decade−1 from 1979 through 1991.1 After 1991, this warming trend is eclipsed by the NOAA-11 drift-related corrections made by CSL, which affect the entire time series beyond 1991.

The results shown above indicate that effects due to satellite drift are evident in NOAA-10, -11, and -12 data. Therefore, we find the assumptions that lead CSL to make drift-related corrections are not realistic. Finally, we contend that further analysis of MSU data is needed before the results of CSL can be accepted.

REFERENCES

  • Christy, J. R., R. W. Spencer, and E. S. Lobl, 1998: Analysis of the merging procedure for the MSU daily temperature time series. J. Climate,11, 2016–2041.

  • Hurrell, J. W., and K. E. Trenberth, 1997: Spurious trends in the satellite MSU temperature trends arising from merging different satellite records. Nature,386, 164–167.

  • ——, and ——, 1998: Difficulties in obtaining reliable temperature trends: Reconciling the surface and satellite MSU 2R trends. J. Climate,11, 945–967.

  • NOAA, 1997: NOAA Polar-Orbiter Data Users Guide. [Available from NOAA/NESDIS/NCDC/CSD/SSB, FOB, Room G233, E/C C33, 4700 Suitland Road, Suitland, MD 20746; or online at www2.ncdc.noaa.gov/docs/podug.].

  • Prabhakara, C., R. Iacovazzi Jr., J.-M. Yoo, and G. Dalu, 1998: Global warming deduced from MSU. Geophys. Res. Lett.,25 (11), 1927–1930.

  • Spencer, R. W., and J. R. Christy, 1990: Precise monitoring of global temperature trends from satellites. Science,247, 1558–1562.

  • Trenberth, K. E., and J. W. Hurrell, 1997: How accurate are satellite“thermometers”. Nature,389, 342–343.

  • Wentz, F. J., and M. Schabel, 1998: Effects of satellite orbital decay on MSU lower tropospheric temperature trends. Nature,394, 661–664.

Fig. 1.
Fig. 1.

MSU channel 2 12-month running means, TAM and TPM, deduced from NOAA-10, NOAA-11, and NOAA-12 for (a) global ocean and (b) global land.

Citation: Journal of Climate 12, 11; 10.1175/1520-0442(1999)012<3331:COAOTM>2.0.CO;2

Fig. 2.
Fig. 2.

MSU channel 2 deduced TPMTAM from NOAA-10, NOAA-11, and NOAA-12 for (a) global ocean and (b) global land.

Citation: Journal of Climate 12, 11; 10.1175/1520-0442(1999)012<3331:COAOTM>2.0.CO;2

Fig. 3.
Fig. 3.

The T2 time series of MSU channel 2 temperature developed by CSL from 1979–97. Dashed line indicates the linear temperature trend from 1979–91 (0.088 K decade−1). The solid line indicates the linear temperature trend from 1979–97 (0.003 K decade−1).

Citation: Journal of Climate 12, 11; 10.1175/1520-0442(1999)012<3331:COAOTM>2.0.CO;2

1

We would like to point out that the time series of CSL includes another adjustment, a decrease of about 0.05 K for the year 1984, to compensate for satellite drift of NOAA-7. This adjustment is localize to the year 1984 and does not propagate beyond. The reason for this is that in the procedure of CSL NOAA-7 and -9 are linked directly with NOAA-6. The linkage of NOAA-9 to the time series is not done through NOAA-7 because of very short (∼50 days) overlap between these two satellites. This localized adjustment to the data of 1984 has a minor impact on the trend.

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