Journal of the Atmospheric Sciences, 57, 3-16.
(First JAS paper of the new millenium!)
Andrew Gettelman and
Adam H. Sobel
Department of Atmospheric Sciences, University of Washington, Seattle, Washington
This study discusses the direct diagnosis of stratosphere-troposphere exchange. The method introduced by Wei is applied to the GEOS assimilated data set. In many respects, the results generally agree with those of other studies using the same method and different data sets. However, sensitivity tests and theoretical considerations indicate that the instantaneous two-way exchange may be significantly exaggerated by the Wei method, because the method is rather sensitive to input data errors such as those that are invariably present in assimilated data sets. The method becomes somewhat better conditioned as the results are more heavily averaged, but this also reduces the method's ability to diagnose two-way exchange. Additionally, when the flux across various surfaces is averaged over the globe and the entire year, the result implies unrealistically large imbalances in the annually averaged mass budget of the stratosphere. This could be caused by modest biases in the model used to perform the data assimilation. Since pure model simulations have an internal dynamical consistency that is lacking in assimilated data sets, the analysis appears to explain the fairly large discrepancies between the two way fluxes obtained in studies using models and assimilated data sets. It may also explain the discrepancies between the net fluxes obtained by the Wei method and those obtained by other methods.