The intercomparison of different stratospheric analyses is an
important issue for the understanding of any trends, but also for
the evaluation of, e.g., the tropopause temperatures in the
tropics or the potential for the formation of "polar
stratospheric clouds" (PSCs), to name a few of the questions.
Over the years and with the advent of different satellite data and
recently the re-analyses by NCEP/NCAR and the data assimilations
by different groups, this problem has been studied by several
groups, dealing with different aspects. Here, we refer to a few
studies where "Berlin Data" have participated. But this is not a
complete list of such studies.
A quantitative comparison of six meteorological analyses is
presented for the cold 1999-2000 and 1995-1996 Arctic winters.
Using different analyzed data sets to obtain temperatures and
temperature histories can have significant consequences. The area
below a polar stratospheric cloud (PSC) formation threshold
commonly varies by about 25% between the analyses, with some
differences over 50%......
Freie Universität analyses are often colder than
others at temperatures below 205 K but warmer than others at
temperatures above about 210-215 K. The FUB analyses are closely
matched to radiosonde observations and consequently may capture
local variations in the vicinity of radiosonde stations that are
smoothed over in the other systems that also give weight to low
vertical resolution satellite data; however, away from the
radiosonde locations they are more poorly constrained than the
other analyses, and may miss or severely smooth temperature
variations that occur between observation locations....
(Manney et al., 2002)
As also other comparisons have shown, the hand analyses
done by experienced meteorologists are catching the extremes
usually better than the computer analyses. And if there are few
radiosonde stations, when interpolated into data sparse region,
they can go a long way. Figure 27 is shown here as one
example of the comparisons, and the much larger scatter of the
"Berlin Data" is consistent with the above.
Figure 27: Scatter plots of the difference between
temperatures from each analysis and the ensemble mean temperature
(average over all analyses at each grid point) as a function of
the ensemble mean, for all grid points on a
5° x 5° grid from
60 to 90°N, at 50 hPa, for January and February 2000 and
1996. The shaded region shows the area filled by the individual scattered
points. The solid triangles show the average difference (analysis
temperature - ensemble mean temperature) in each 1-K average
temperature bin. The thin line is the zero line. (Figure 1 in
Manney et al., 2002)
Under the auspices of the World Climate Research Program
(WCRP) the sub-program SPARC (Stratospheric Processes and Their
Role in Climate) initiated the STTA Group. The charter was (1) to
bring together all available data sets of stratospheric
temperatures and (2) to analyse the trends in a consistent manner.
The results of this work are published in Ramaswamy et al. (2001).
The "Berlin Data" participated in this major intercomparison and
one Figure is given here as an example, Fig.28. The report
deals carefully with all the problems arising from the various,
very diverse data sets and their trends.
Figure 28: Zonal-mean decadal temperature trends at
50 hPa over the 1966-1994 period from different radiosonde data
sets. Ramaswamy et al., 2001, Plate 2
"Two independent daily stratospheric data sets
are compared for 16 northern winters. The objective is to assess
the consistency of temperatures low enough for polar stratospheric
cloud (PSC) formation at 50 hPa. The first data set is the
subjective [historic] analysis [of temperatures and geopotential
heights] produced from the radiosonde network at the "Freie
Universität Berlin" (FUB), which is constrained by hydrostatic
and thermal wind balance. The second is the satellite-based
analysis of geopotential height, compiled from the TIROS
Operational Vertical Sounding system by the United Kingdom
Meteorological Office; temperatures are derived from the
hypsometric equation. The Stratospheric Sounding Units (SSU)
provide most of the stratospheric data in that system. The FUB
data are generally colder, particularly at low temperatures, but
there is a large dispersion about the mean difference. The
uncertainties of the values of the lowest temperatures are around
1 K and 2 K in the mean and rms, respectively. There may be a
geographical bias in the data sets. There is a clear relationship
between the vertical temperature gradient and the difference
between the two data sets, the satellite-derived values becoming
relatively colder when the temperature decreases at pressures
lower than 50 hPa. Regarding PSC formation: adequately low
temperatures occur more often in the FUB data, but on 25% of
winter days the Area A
where PSCs might form where larger
in the SSU data. Seasonally integrated values of A show a
fairly good agreement between the two data sets, the
satellite-derived values generally being smaller. Both systems
give stable and consistent estimates of the areas of low
temperature at 50 hPa. On the basis of data quality alone, it is
not possible to recommend either analysis system in preference to
the other for studies of the coldness of the polar stratosphere."
Out of this very detailed comparison only one figure is
shown here, (Fig.29), which is Fig.2 in the report. The
text describing the figure is cited here:
"Fig.2a shows a histogram illustrating the number of
recorded T values at all gridpoints poleward of 40°N in
the FUB analyses on all days in the 16 winters (November - March).
The data were binned into 1K wide intervals for this presentation. The
display is cut off at the lowest analyzed TFUB = 183K and at
220K, which is well above Tnat. There are few occurrences of
extremely low T but the number begins to increase quickly for T
larger than 189K, before levelling off near T = 212K. The second
histogram in Fig.2a shows the number of records of each TFUB
at times when the SSU data were available. Although the sample is
clearly smaller the missing days do not change the shape of the
distribution, so the SSU sampling is adequate for this
intercomparison. There is evidence of peaks at 5K intervals in the
number of records; this is presumably because the FUB analyses are
made with a contour interval of 5K and the digitizing mechanism
cannot extrapolate to local minima inside closed isotherms.
The rms difference in T remains remarkably constant,
close to 3K, at all TFUB (Fig.2b). There is some hint of a
temperature dependence in the bias, since the mean difference
[T] decreases slowly with increasing TFUB, from
about -1.5K near TFUB = 190K to around zero at
TFUB = 220K; the envelope of extreme differences moves
towards stronger positive and weaker negative values as TFUB
These differences must be attributed to the two
observation and analysis systems: they are consistent with the
"local" nature of radiosonde measurements and the "vertically
smoothed" character of temperatures derived from satellite-based
radiance measurements, at least with retrievals of the kind used
in the SSU data set."
Figure 29: Statistics for T at 50 hPa, binned into 1K
intervals, at all gridpoints poleward of 40°N, and for
T <= 220K. (a) Histogram showing the frequency of occurrence of
TFUB on all days (gray) and on days with coincident SSU data
(black); (b) Plot showing TFUB against T (K). The
envelope is shaded; the mean and rms differences are depicted by
thick solid and dashed lines. All days in November through March,
1979 until 1996, were used
Manney, G.L., J. L. Sabutis, S. Pawson, M. L. Santee, B.
Naujokat, R. Swinbank, M. E. Gelman, and W. Ebisuzaki, 2002: Lower
stratospheric temperature differences between meteorological
analyses in two cold Arctic winters and theirimpact on polar
processing studies. J. Geophys. Res., 107, in press.
Ramaswamy, V., M. L. Chanin, J. Angell, J. Barnett, D.
Gaffen, M. Gelman, P. Keckhut, Y. Koshelkov, K. Labitzke, J. J. R.
Lin, A. O'Neil, J. Nash, W. Randel, R. Rood, K. Shine, M.
Shiotani, and R. Swinbank, 2001: Stratospheric temperature trends:
Observations and model simulations. Review of Geophysics,
Pawson, S., K. Krüger, R. Swinbank, M. Bailey and A.
O'Neill, 1999: Intercomparison of two stratospheric analyses:
temperatures relevant to polar stratospheric cloud formation. J.
Geophys. Res., 104, 2041-2050.
Last modified: Fri Mar 14 16:17:51 MET 2003