Three-dimensional simulation of stratospheric trace gas distributions measured by CRISTA

The Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRI STA) experiment has been flown on two space shuttle missions (STS 66 and ST S 85). During these missions, global trace gas distributions have been meas ured with high spatial resolution. The first flight was performed in early November 1994 during a period of disturbed dynamical conditions characteriz ed by relatively large wave activity and associated exchange of tropical an d extratropical air. As a result, numerous small- and medium-scale structur es were present in the trace gas distributions measured by CRISTA. The deta iled structure of the constituent distributions has been modeled with the N ational Center for Atmospheric Research Research on Ozone in the Stratosphe re and Its Evolution (ROSE) model, which is driven by assimilated winds and temperatures provided by the U.K. Meteorological Office. The modeled trace gas distributions capture much of the measured structures. Very good agree ment is found in the lower stratosphere of the northern hemisphere, with th e exception of unrealisticly high variability of the modeled trace gas fiel ds at equatorial and subtropical latitudes. In this paper, model results of two specific tracers, CFC-11 and N2O, are compared to respective CRISTA me asurements. For quantitative comparisons of trace gas transport in the ROSE model and trace gas transport associated with the measurements, CRISTA ver sion 1 (level 2) data have been assimilated into the model by using a simpl e sequential technique. The trace gas assimilation system interpolated the measured distributions of long-lived tracers onto the model grid and yields synoptic fields that are consistent with the CRISTA measurements, at each time step of the model (20 min). Horizontal eddy fluxes of CFC-11 (31.6 mba r) calculated from the assimilated trace gas concentrations are in reasonab le agreement with respective fluxes calculated from modeled trace gas field s without data assimilation. Both data sets indicate a large degree of temp oral cancellation of eddy transport during the time period of the CRISTA me asurements. The assimilated trace gas distributions represent a value-added level 3 product, which has been used for transport and budget studies of C FC-11 (31.6 mbar) and CH4 (4.6 mbar). The study indicates that irreversible transport processes are rather important at the 4.6 mbar pressure level du ring the time period of the CRISTA measurements. Most interesting is a pron ounced mixing event from midlatitudes into the tropics, which is also evide nt in measured and calculated zonal mean CH4 mixing ratio tendencies.