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

Abstract

The Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) experiment has been flown on two space shuttle missions (STS 66 and STS 85). During these missions, global trace gas distributions have been measured with high spatial resolution. The first flight was performed in early November 1994 during a period of disturbed dynamical conditions characterized by relatively large wave activity and associated exchange of tropical and extratropical air. As a result, numerous small‐ and medium‐scale structures were present in the trace gas distributions measured by CRISTA. The detailed structure of the constituent distributions has been modeled with the National Center for Atmospheric Research Research on Ozone in the Stratosphere 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 agreement is found in the lower stratosphere of the northern hemisphere, with the exception of unrealisticly high variability of the modeled trace gas fields at equatorial and subtropical latitudes. In this paper, model results of two specific tracers, CFC‐11 and N2O, are compared to respective CRISTA measurements. For quantitative comparisons of trace gas transport in the ROSE model and trace gas transport associated with the measurements, CRISTA version 1 (level 2) data have been assimilated into the model by using a simple 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 mbar) calculated from the assimilated trace gas concentrations are in reasonable agreement with respective fluxes calculated from modeled trace gas fields without data assimilation. Both data sets indicate a large degree of temporal cancellation of eddy transport during the time period of the CRISTA measurements. The assimilated trace gas distributions represent a value‐added level 3 product, which has been used for transport and budget studies of CFC‐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 during the time period of the CRISTA measurements. Most interesting is a pronounced mixing event from midlatitudes into the tropics, which is also evident in measured and calculated zonal mean CH4 mixing ratio tendencies.