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

Prather, Michael1

Published Jan 20, 2025; Updated Apr 15, 2025 on Dryad. https://doi.org/10.5061/dryad.pk0p2ngzt

Data files

Jan 20, 2025 version files 29.13 MB
Apr 15, 2025 version files 76.34 MB

Abstract

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.