Report 57: Uncertainties in Observationally
Based Estimates of Temperature Change in the Free Atomsphere
B.D. Santer, J.J. Hnilo, T.M.L. Wigley, J.S. Boyle, C. Doutriaux, M.
Fiorino, D.E. Parker, and K.E. Taylor
July 2000, 28 pp.
Uncertainties are quantified in atmospheric temperature
changes derived from satellites, radiosondes, and the renalyses of National
Center for Environmental Prediction and European Centre for Medium-Range
Weather Forecasts ( 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 stratosheric 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 1979-1993, ERA warms by 0.106C/decade, while NCEP cools by 0.028c/decade.
Applying the HadRT1.1 (radiosonde) data availability mask to NCEP improves
the agreement between these 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. If this were applied
without any additional adjustments, it 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.