A partial column perspective on ozone and water vapor in the lowermost stratosphere from satellite observations, reanalysis, and model data

Ozone and water vapor in the lowermost stratosphere (LMS) modify the Earth’s radiative budget, influence large-scale dynamics, and affect tropospheric air quality through exchange processes at the tropopause. Despite this importance, the variability and long-term trends of LMS ozone and, in particular, water vapor (WV) remain highly uncertain. This uncertainty is compounded by variability in the thermodynamic structure of the LMS itself: extratropical tropopause height, tropical tropopause temperatures, and the latitudinal width of the tropical tropopause have all shown systematic changes in recent decades.

In this study we present an LMS partial column framework that explicitly accounts for the variable LMS boundaries and explore the partial column as an UTLS diagnostics for global observations and models.

The LMS limits are defined from ERA5 reanalysis and partial columns of ozone and WV are obtained by integrating satellite measurements from the Microwave Limb Sounder (MLS) and the Atmospheric Chemistry Experiment – Fourier Transform Spectrometer (ACE-FTS) within these limits. Unlike conventional mixing-ratio analyses, the partial column approach also incorporates the density and area effects of the spherical atmosphere.

Our analysis shows that the extratropical LMS comprises a considerable amount of the total stratospheric mass, stratospheric WV mass, and stratospheric ozone mass. The spatial and seasonal variability of LMS partial column ozone and WV is largely influenced by variations in total LMS mass. LMS partial column ozone shows very good agreement across the data sets whereas LMS partial column WV exhibits a larger spread. Calculated long-term trends result from a complex interplay of LMS mass and mixing-ratio changes.