Kritz, M. A., S. W. Rosner, E. F. Danielsen, and H. Selkirk (1991), Airmass Origins and Troposphere-to-Stratosphere Exchange Associated with Midlatitude Cyclo-genesis and Tropopause Folding Inferred from Be-7 Measurements, J. Geophys. Res., 96, 17,405-17.