Atmospheric data used for calibrating the tropopause in global chemistry-climate or chemistry-transport models

We divide the atmosphere into distinct spheres based on their physical,
chemical, and dynamical traits.  In deriving chemical budgets and
climate trends, which differ across spheres, we need clearly defined
boundaries.  Focusing on atmospheric mass and greenhouse gases,
our primary spheres are the troposphere and stratosphere (~99.9 % by
mass), and the boundary between them is the tropopause.  The
standard method of locating the tropopause is based on the World
Meteorological Organization’s lapse rate tropopause (LRT) defined from the
vertical temperature gradient as observed by radiosonde balloons.
 Every global climate-weather model has one or more methods to
calculate the LRT. Both involve subjective choices: expert judgment given
meter-scale variability of sonde temperature profiles; methods for
calculating gradients from km-thick model layers. Further, LRT and similar
methods are consistent only in regions where gradients are primarily
vertical (core tropics and midlatitudes) and fail in others (sub-tropical
jets and polar regions). Age-of-air tracers clock the effective
time-distance from the tropopause, allowing unambiguous separation of
stratosphere from troposphere in the chaotic jet regions.  We
apply a global model with synthetic tracer e90 (90-day e-folding),
focusing on ozone and temperature structures about the tropopause using
ozone sonde and satellite observations. We calibrate an
observation-consistent tropopause for e90 using tropics-plus-midlatitudes
and then apply it globally to calculate total tropospheric air-mass and
tropopause ozone values.  Such calibration can identify weak
tropospheric mixing rates.  The concept of calibrating an
age-of-air tropopause can be readily applied to other models and even to
observed age-of-air tracers like sulfur hexafluoride.