We applied the three-dimensional chemistry-transport Tracer Model version 5(TM5) and a trajectory model and performed several diagnoses focusing ondifferent transport regimes. Covering different time and spatial scales, weexamined (1) polar vortex dynamics during the Arctic winter, (2) thelarge-scale stratospheric meridional circulation, and (3) air parceldispersion in the tropical lower stratosphere.
Tracer distributions inside the Arctic polar vortex show considerably worseagreement with observations when the model grid resolution in the polarregion is reduced to avoid numerical instability. The results are sensitiveto the diffusivity of the advection. Nevertheless, the use of a computationalcheaper but diffusive advection scheme is feasible for tracer transport whenthe horizontal grid resolution is equal or smaller than 1 degree. The use oftime interpolated winds improves the tracer distributions, particularly inthe middle and upper stratosphere. Considerable improvement is found both inthe large-scale tracer distribution and in the polar regions when the updatefrequency of the assimilated winds is increased from 6 to 3 h. Itconsiderably reduces the vertical dispersion of air parcels in the tropicallower stratosphere.
Strong horizontal dispersion is not necessarily an indication of poor windquality, as observations indicate. Moreover, the generally applied air parceldispersion calculations should be interpreted with care, given the strongsensitivity of dispersion with altitude.
The results in this study provide a guideline for stratospheric tracermodeling using assimilated winds. They further demonstrate significantprogress in the use of assimilated meteorology in chemistry-transport models,relevant for both short- and long-term integrations.
