VERTICAL VELOCITY, VERTICAL DIFFUSION, AND DILUTION BY MIDLATITUDE AIR IN THE TROPICAL LOWER STRATOSPHERE

Air passing upward through the tropical tropopause is ''marked'' by an annually varying water vapor mixing ratio much as a tape recorder mar ks a magnetic tape; as the air ascends in the tropical stratosphere, t hese marks are effaced by a combination of vertical diffusion within t he tropics and dilution of tropical air by sideways (isentropic) mixin g-in of midlatitude air. We represent these processes using a one-dime nsional advection-diffusion-dilution model, which we inverse-solve for the vertical profiles of three unknowns (vertical advection velocity, vertical diffusion coefficient, and dilution rate coefficient) after prescribing the vertical profiles of time mean methane [CH4] and of am plitude and phase of the annually varying tape recorder signal in 2[CH 4]+[H2O]. When tested on synthetic data generated by forward solving t he same model, the method for inverse solution proved to be well condi tioned and to give accurate results above 18 km. Applying the method t o 5 years of smoothed data from the Halogen Occultation Experiment, we find a vertical advection velocity with a minimum of about 0.2 mm s(- 1) near 20 km, and both dilution rate coefficient and vertical diffusi on coefficient with remarkably low minima near 22 km, 1/(6-7 year) and roughly 0.02 m(2)s(-1), respectively. Our derived profile of vertical advection velocity agrees well, between 18 and 24 km, with an indepen dent, radiatively derived, mass-budget-constrained transformed Euleria n mean calculation. Despite the relatively modest values of the diffus ion coefficient, vertical diffusion plays a significant role in attenu ating the tape recorder signal, according to our model. The minimum va lue of the dilution rate coefficient corresponds to a relaxation times cale of 6-7 years, much longer than the timescales found in other stud ies. The long relaxation timescale at 20-24 km is, however, consistent with (1) the minimum in vertical velocity, (2) a reduced attenuation rate in the tape recorder signal, and (3) a decrease, hitherto unremar ked, in the tropical vertical gradient of [CH4] there.