AN EVALUATION USING C-14 AND N2O SIMULATIONS OF 3-DIMENSIONAL TRANSPORT DRIVEN BY UNITED-KINGDOM METEOROLOGICAL-OFFICE AND GODDARD-SPACE-FLIGHT-CENTER ASSIMILATED WINDS

We have used two different sets of assimilated input wind fields (Unit ed Kingdom Meteorological Office (UKMO), NASA) to drive the transport in a three-dimensional global chemical transport model. In a first exp eriment the distribution of carbon 14 after nuclear bomb testing in th e early 1960s was simulated. The second, more comprehensive experiment consists of a comparison of simulated nitrous oxide (N2O) distributio ns for both input wind fields against CLAES satellite measurements. Bo th experiments show that the NASA winds are producing too much upward transport in the equatorial area. By analyzing the Eulerian-mean conse rvation equation for zonal mean mixing ratios, we were able to show th at these model differences are initially caused by differences in the mean residual circulations in the two different wind field sets but th at because the latitudinal gradients remain steeper in the NASA result s, differences in isentropic mixing rates are also evident. The NASA r esidual mean circulation in the equatorial lower stratosphere in the m odel is higher than the UKMO residual mean circulation by about a fact or of 2. However, there is also evidence that isentropic mixing rates are too large in the subtropics in both models, and there is evidence from the N2O comparisons of overly large amplitudes of wavenumbers 4 t o 7 in the tropics and subtropics in the models. Filtering the incomin g wind fields to eliminate wavenumbers larger than 10 resulted in only minor improvements in the model comparisons against CLAES observation s. Because of the significant discrepancies between the model simulati ons, using either one of the wind field sets, and the observations, we recommend care in using these assimilated winds for long-term model s tudies in general, Both wind field sets, however, produced good simula tions of strong intrusions of tropical air into midlatitudes of the up per stratosphere in September 1992.