Thursday, 13 January 2000
Brian Soden, NOAA/GFDL and Princeton Univ., Princeton, NJ
Considerable debate exists regarding the ability of both surface (SFC)
and Microwave Sounding Unit channel 2 (MSU2) satellite observations to
accurately monitor the global variations in atmospheric temperature and
to detect long-term trends. In contrast to the surface record, the
MSU channel 2 measurements are sensitive to the temperature averaged over
a layer of the atmosphere several kilometers thick, centered in the
lower troposphere. This difference in vertical sampling is a key source
of uncertainty that has complicated previous comparisons of these
measurements. In an attempt to understand the differences between SFC and
MSU2 global temperature trends, both records are compared against an
objectively-analyzed global radiosonde network (RAOB). The radiosonde
measurements offer the unique ability to resolve the vertical
structure of the atmospheric temperature variability and are therefore
directly comparable with both the deep-layer satellite measurements and
the near-surface air temperature records. An initial comparison for the
period of 1979-1994 indicates that the global-mean MSU2 temperature
anomalies agree remarkably well with the corresponding channel 2 temperatures
computed from the RAOB network. The high level of consistency between the
two records (~0.03 K rms difference) provides confidence in both records.
Likewise, the global-mean SFC temperature anomalies also agree well with
temperature anomalies derived from the 1000 hPa level of the RAOB analysis.
Thus, the RAOB temperature record was consistent with both the MSU2 and
SFC temperature records over this period. However, the SFC and MSU2
temperature anomalies differ from each other by more than
0.25 K at two periods in the record (near 1980 and 1993), resulting in
long-term trends which differ by more than an order of magnitude. Moreover,
these differences in global temperature anomalies are also noted between
the RAOB-calculated channel 2 temperature anomalies and near surface
(1000 hPa) temperature anomalies, suggesting that the largest discrepancies
in between the MSU2 and SFC temperature anomalies results from differences
in the vertical sampling of the instruments and not from measurement
or sampling errors within the individual temperature records themselves.
Finally, the observed SFC, RAOB and MSU2 temperature records are compared
to those obtained from a GCM forced with observed SSTs. It is shown that the
GCM simulates a stronger coupling between the SFC and MSU2 records than
is observed. The implications of this comparison for monitoring and prediction
of global warming will be presented at the conference.
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