Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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The Realism of the ECHAM5.2 Models to Simulate the Hydrological Cycle
in the Arctic and Baltic Area
Klaus Arpe, Stefan Hagemann, Daniela Jacob and Erich Roeckner
Max-Planck-Institute for Meteorology, Bundesstr. 55, 20146 Hamburg, Germany; email: arpe@dkrz.de
A new version of the ECHAM model [ECHAM5; Roeckner
et al., 2003] with considerable changes compared to the
former standard version ECHAM4 [Roeckner et al., 1996] is
investigated with respect to the hydrological cycle in the
Arctic and Baltic area. The emphasis will be put on two
versions with different resolutions, i.e. a low-resolution
version resolving 42 waves in the horizontal (T42) and 19
levels in the vertical (L19) and a high horizontal and vertical
resolution (T106 L39).
The superiority of the high resolution ECHAM5.2 T106 L39
model compared to the other simulations can been shown in
many respects, especially on dynamical quantities and
precipitation. The year-by-year variability seems to be too
high in this simulation but it is hard to judge if this result is
statistically significant. Astonishing is the similarity in the
LHFX of all simulations and the ECMWF reanalysis.
Therefore the good results shown for 5T106 with respect of
precipitation do apply also for the simulated river discharge.
The low resolution ECHAM5.2 T42 L19 model is in many
respects inferior to the ECHAM4.5 model with the same
resolution, which is probably due to the fact that the latter
has been tuned over a long time. There is hope that also the
ECHAM5.2 can be improved by small adjustments after it
has been in service over a longer period.
An intermediate resolution model T63 L31 of ECHAM5
will be investigated as well. First results suggest similar
qualities as the high-resolution model.
Further, the influence of different horizontal (T42, T63,
T106, and T159) and vertical (L19 and L31) resolutions on
the simulated hydrological over the Baltic Sea catchment
and the Arctic Ocean catchment will be shown. Here, the
latter is represented by the catchment area (about 9.7 million
km 2 ) of its six largest rivers (Jenisei, Kolyma, Lena,
Mackenzie, Northern Dvina, Ob) that cover an area of about
65% of the whole Arctic Ocean catchment. For all
combinations of horizontal and vertical resolutions listed
above (except for T159 which was only used in combination
with L31), AMIP2-type simulations were conducted for the
years 1978-1999. In order to pay regard to spin-up of the
model, the first year is not considered in the evaluation of
the simulations.
Figure 1 shows the mean annual amounts of precipitation
simulated by ECHAM5.2 using the different combinations
of resolutions. The simulated values are compared to
observations of CMAP [Xie and Arkin, 1997] and GPCP
[Huffman et al., 1997]. Note that CMAP precipitation data
are not corrected for the systematic undercatch of
precipitation gauges, which is especially significant for
snowfall. For GPCP data, a correction has been applied
which is known to be too large by a factor of about 2
(Rudolf, personal communication, 2001) so that the actual
precipitation amounts are expected to be in between GPCP
and CMAP.
Precipitation [mm/a]
1000
900
800
700
600
500
400
300
200
100
0
1979-99: Annual Mean Precipitation in [mm/a]
6 largest Arctic
Rivers
Baltic Sea
catchment
ECHAM5 T42 L19
ECHAM5 T42 L31
ECHAM5 T63 L19
ECHAM5 T63 L31
ECHAM5 T106 L19
ECHAM5 T106 L31
ECHAM5 T159 L31
CMAP
GPCP
Figure 1: Annual mean precipitation over the Arctic and
Baltic Sea catchment for 1979-1999.
Figure 1 shows that the influence of the vertical resolution
on the precipitation is larger than the influence of the
horizontal resolution. ECHAM5 slightly overestimates the
precipitation over both catchments except for the L19
simulations over the Baltic Sea catchments where the
simulated precipitation is just in between the CMAP and
GPCP data. For both catchments, precipitation is larger in
the L31 simulations than in the L19 simulations. While for
the Arctic Ocean catchment the simulated precipitation
seems to slightly increase with finer horizontal resolutions,
it decreases with finer horizontal resolutions over the
Baltic Sea catchment.
References
Huffman, G.J., R.F. Adler, A. Arkin, A. Chang, R.
Ferraro, A. Gruber, J. Janowiak, R.J. Joyce, A.
McNab, B. Rudolf, U. Schneider und P. Xie, The
Global Precipitation Climatology Project (GPCP)
combined precipitation data set, Bull. Amer. Meteor.
Soc., 78, 5-20, 1997.
Roeckner, E., K. Arpe, L. Bengtsson, M. Christoph, M.
Claussen, L. Dümenil, M. Esch, M. Giorgetta, U.
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circulation model ECHAM-4: model description and
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Roeckner, E., G. Bäuml, L. Bonaventura, R. Brokopf, M.
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description, Max Planck Institute for Meteor., Report
349, Hamburg, Germany , 2003
Xie, P., und P. Arkin, Global precipitation: A 17-year
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satellite estimates and numerical model outputs, Bull.
Amer. Meteor. Soc , 78, 2539-2558, 1997.