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Abstract:
The sensitivity of a general circulation model to changes in resolution is studied using the
Max Planck Institute for Meteorology (MPI) 19-level model, ECHAM4. Simulations
extending over a period between 10 and 15 years, with observed sea surface temperatures
as lower boundary conditions from 1979 onward, have been performed using four different
horizontal resolutions, T21, T30, T42 and T106. The atmospheric time-mean state and the
intraseasonal variability are compared to the European Centre for Medium Range Weather
Forecasts (ECMWF) reanalyses and a few other observational datasets.
The T30, T42 and T106 simulations are similar in many respects, whereas the T21 simula-
tion is qualitatively different. Several effects related to model resolution could be identi-
fied, such as increasing tropical upper tropospheric warming with increasing resolution.
This is due to more vigorous tropical convection, larger ice water content and, hence,
increasing cirrus cloud greenhouse effect. Associated with this increasing warming at
higher resolution is a poleward expansion of the zonally averaged circulation regime. On
the other hand, the zonally asymmetric component of the circulation, i.e., the stationary
waves, improve with higher resolution. Also, higher resolution has a positive impact on
regional precipitation patterns which are affected by orography such as the summer mon-
soon precipitation over India.
Intraseasonal variability has been analyzed only for the higher resolution models, T42 and
T106. Compared to the ECMWF reanalyses, both models are able to simulate the intrasea-
sonal geopotential height variability, eddy fluxes of heat and momentum, and eddy kinetic
energy with reasonable accuracy. This applies to transient eddies in both the bandpass and
lowpass regime and to the stationary eddies as well.
Some biases can be identified which are more or less independent of resolution. These
include the mislocation of the Azores high and the overestimation of its intensity, a cold
bias in the polar upper troposphere and lower stratosphere and the poleward and upward
displacement of the maxima of geopotential height variability, momentum fluxes and eddy
kinetic energy.
An important finding is that the operational ECMWF analyses, which have been widely
used for model validation, considerably overestimate the lowpass variability, as compared
to the reanalyses, due to frequent changes of the forecast model and data assimilation
scheme. This implies that the results from our investigations are not directly comparable
to previous investigations that used operational analyses for validation.
