| This study describes key aspects of global chemistry-transport models and
their impact on stratospheric tracer transport. We concentrate on global
models that use assimilated winds from numerical weather predictions, but the
results also apply to tracer transport in general circulation models. We
examined grid resolution, numerical diffusion, air parcel dispersion, the
wind or mass flux update frequency, and time interpolation. The evaluation is
performed with assimilated meteorology from the "operational analyses or
operational data" (OD) from the European Centre for Medium-Range Weather
Forecasts (ECMWF). We also show the effect of the mass flux update frequency
using the ECMWF 40-year re-analyses (ERA40).
We applied the three-dimensional chemistry-transport Tracer Model version 5
(TM5) and a trajectory model and performed several diagnoses focusing on
different transport regimes. Covering different time and spatial scales, we
examined (1) polar vortex dynamics during the Arctic winter, (2) the
large-scale stratospheric meridional circulation, and (3) air parcel
dispersion in the tropical lower stratosphere.
Tracer distributions inside the Arctic polar vortex show considerably worse
agreement with observations when the model grid resolution in the polar
region is reduced to avoid numerical instability. The results are sensitive
to the diffusivity of the advection. Nevertheless, the use of a computational
cheaper but diffusive advection scheme is feasible for tracer transport when
the horizontal grid resolution is equal or smaller than 1 degree. The use of
time interpolated winds improves the tracer distributions, particularly in
the middle and upper stratosphere. Considerable improvement is found both in
the large-scale tracer distribution and in the polar regions when the update
frequency of the assimilated winds is increased from 6 to 3 h. It
considerably reduces the vertical dispersion of air parcels in the tropical
lower stratosphere.
Strong horizontal dispersion is not necessarily an indication of poor wind
quality, as observations indicate. Moreover, the generally applied air parcel
dispersion calculations should be interpreted with care, given the strong
sensitivity of dispersion with altitude.
The results in this study provide a guideline for stratospheric tracer
modeling using assimilated winds. They further demonstrate significant
progress in the use of assimilated meteorology in chemistry-transport models,
relevant for both short- and long-term integrations. |