Uncertainties in "observational" estimates of temperature change in the
free atmosphere
B. D. Santer, J. J. Hnilo, J. S. Boyle, C. Doutriaux, M. Fiorino, K. E.
Taylor
Program for Climate Nodel Diagnosis and Intercomparison, Lawrence
Livermore National Laboratory Laboratory, Livermore, CA
T. M. L. Wigley
National Center for Atmospheric Research, Boulder, Colorado
D. E. Parker
Hadley Centre for Climate Prediction and Research, United Kingdom
Meteorological Office, Bracknell, England.
Uncertainties are quantified in atmospheric temperature changes derived from
satellites, radiosonds, and the National Center for Environmental Prediction and
European Centre for Medium-Range Weather Forecasts reanalyses (NCEP and ERA). To
facilitate intercomparison, we compute from the reanalyses and radiosonde data
deep layer temperatures equivalent to those estimated from the satellite-based
Microwave Sounding Unit (MSU). Equivalent MSU temperatures generated using
global mean weighting functions and a radiative transfer code give similar
results. NCEP's pre-1979 global mean lower stratospheric temperature anomalies
diverge markedly from radiosonde data. A smaller offset occurs in the
midtroposphere. These differences are attributed to a likely warm bias in the
tropical lower stratosphere in the temperature retrievals used by NCEP from
November 1978 onward, and changes in the error characteristics of the
assimilation model's simulation of the lower stratosphere. In the lower
troposphere, ERA and NCEP show different global mean trends due to differences
in assimilation strategy, input observational data, quality control procedures,
and model physics. Over 1979-1983, ERA warms by 0.106°/decade, while NCEP
cools by 0.028°/decade. Applying the HadRT1.1 (radiosonde) data availability
mask to NCEP improves the agreement between those data sets. Neglecting coverage
differences can yield misleading results in MSU-radiosonde trend comparisons.
Substantial trend uncertainties also arise from coverage differences between
various radiosonde data sets. Version c of the MSU lower tropospheric
temperature retrieval fails to adjust explicitly for orbital decay. Application
of this adjustment without any additional adjustments would resolve an important
discrepancy: in MSUc the lower troposphere has cooled in relation to the
midtroposphere, while the reverse is the case for both reanalyses and for the
radiosonde data examined here.
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Hongjun Zhang:
zhangho@ucar.edu