Darmstadt 2008 – scientific programme
Parts | Days | Selection | Search | Downloads | Help
UP: Fachverband Umweltphysik
UP 1: Atmosphäre und Klima
UP 1.4: Talk
Tuesday, March 11, 2008, 09:15–09:30, 3B
Does the Brewer-Dobson Circulation change? - Three Decades of Mean Age of Air Derived from Stratospheric SF6 Measurements — •T. Möbius1, A. Engel1, H. Bönisch1, E. Atlas2, S. Schauffler3, R. Borchers4, I. Levin5, and U. Schmidt1 — 1IAU Uni Frankfurt — 2RSMAS/MAC University of Miami — 3NCAR Boulder — 4MPI für Aeronomie — 5IUP Uni Heidelberg
Changes in the atmospheric greenhouse gas loading are known to lead to an increase of the radiative forcing of the atmosphere. Model studies, as referenced in the WMO O3 Assessment Report 2006, predict that this increase leads to a change in the behaviour of the overturning meridional circulation of the middle atmosphere, the Brewer-Dobson Circulation. The stratospheric mean age of air is a measure of the strength of this circulation. It can be derived from measurements of long-lived tracer, e.g. SF6. We present vertical profiles of SF6, measured in stratospheric whole-air samples. By reanalyses of stratospheric balloon samples a high quality data set of SF6 in-situ measurements, spanning three decades, was obtained on a consistent calibration scale. The SF6 data set was used to calculate the mean age of air of the stratosphere. The derived age of air trends show no decrease in contrast to the model predictions, but rather suggest a weak increase. To minimise the uncertainty we use fitting algorithms for the vertical age of air profiles and derive the mean age for specific altitude intervals. We studied the sensitivity of the derived trend by varying the input data set, to take into account the limited number of in-situ measurements and their seasonal and spatial distribution.