DIABATIC CROSS-ISENTROPIC DISPERSION IN THE LOWER STRATOSPHERE

A significant contribution to vertical dispersion of tracers in the st ratosphere arises from the variability in the diabatic heating of air parcels. Air parcels starting on a given isentropic surface experience different time histories of diabatic heating, which causes vertical d ispersion across isentropic surfaces. We refer to this process as ''di abatic cross-isentropic dispersion,'' or ''diabatic dispersion'' for b revity. The present study investigates diabatic dispersion in the lowe r stratosphere by computing parcel trajectories initialized uniformly over the 500 K surface on January 1, 1993. Parcels are followed for 2 months using analyzed winds and diabatic heating rates computed from a nalyzed temperatures. Diabatic dispersion depends on the statistics of the large-scale horizontal eddy motion as well as on the spatial stru cture of the diabatic heating field. The trajectory statistics suggest that the polar vortex, surf zone, tropics, and extratropical summer h emisphere are, to varying extents, isolated from each other and that t he diabatic dispersion within each of these regions is different. In b oth the surf zone and southern hemisphere extratropics the dispersion is initially advective, with potential temperature variance <delta the ta(t)(2) > increasing as t(2) as time t increases. After about 1 month , the dispersion becomes diffusive in the sense that [delta theta(t)(2 )]similar to 2K(theta theta)t and with a diffusivity K-theta theta in the range 2-6 K-2 d(-1), roughly equivalent to K-zz similar to 0.1-0.2 m(2) s(-1). The emergence of a diffusive regime is discussed in terms of loss of memory of diabatic heating along parcel paths, as measured by the decay of the Lagrangian autocorrelation function. Diabatic dis persion within the tropics and polar vortex over the 2-month period is more than an order of magnitude smaller and is less clearly diffusive . The diabatic dispersion of parcels moving poleward out of the tropic s into either hemisphere is faster than either differential advection or diffusion, and in this case, [delta theta(t)(2)] increases as t(3) For the total ensemble of all parcels, the potential temperature varia nce increases as t(2), consistent with global-scale differential advec tion by the mean diabatic circulation. This is inconsistent, at least on the 2-month timescale considered here, with a one-dimensional diffu sive model of vertical dispersion.