Open AccessConstraints on the seasonal cycle of stratospheric water vapor using in situ measurements from the ER‐2 and a CO photochemical clock
Author(s) -
Weinstock E. M.,
Hintsa E. J.,
KirkDavidoff D. B.,
Anderson J. G.,
Andrews A. E.,
Herman R. L.,
Webster C. R.,
Loewenstein M.,
Podolske J. R.,
Bui T. P.
Publication year - 2001
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2000jd000047
Subject(s) - stratosphere , water vapor , atmospheric sciences , radiosonde , environmental science , mixing ratio , tropopause , altitude (triangle) , middle latitudes , air mass (solar energy) , climatology , troposphere , relative humidity , meteorology , boundary layer , physics , thermodynamics , geology , mathematics , geometry
We use in situ measurements of CO obtained in the tropics from 1995 to 1997 on the NASA ER‐2 aircraft and a simple photochemical model to calculate the elapsed time between the entry of air into the stratosphere and the observation, which we define as the photochemical “age” of the air. Assuming this age represents the transit time of the air mass from a boundary at 390 K to its measured altitude, we calculate boundary condition values of CO 2 derived from in situ measurements of this species from 400 to 480 K. We validate the approach by comparing these CO 2 boundary values with an independent representation of the boundary condition from observations of CO 2 in air that had recently entered the stratosphere (as indicated by simultaneous measurements of N 2 O, CO, and potential temperature). For five of the six flights, differences between CO 2 boundary condition values determined using the photochemical age of the air and those derived from independent measurements can be accounted for with isentropic mixing of midlatitude stratospheric air into the tropics. Having validated the photochemical ages of the sampled stratospheric air, we use the same analysis of in situ water data to provide water vapor boundary condition values that constrain the seasonal cycle of water vapor entering the stratosphere. On the basis of these constraints we evaluate the seasonal cycle of entry‐level water vapor derived from tropical tropopause temperatures from the radiosonde network between 10°S and 10°N. We conclude that while average saturation mixing ratios provide a suitable boundary condition for water vapor entering the stratosphere, the uncertainties in saturation mixing ratios derived from radiosonde temperatures and the lack of coverage prevents distinguishing between ascent preferentially occurring over the western equatorial Pacific or throughout the tropics. With the assumption that vertical diffusion and midlatitude mixing have a negligible effect on the calculated age, ascent velocities can be inferred from the photochemical ages. These ascent velocities show a seasonal cycle that is inconsistent with our current understanding of the dynamics driving the stratospheric circulation and with independent estimates of tropical vertical ascent rates.