|Problems to be solved|
|STACCATO is a comprehensive study of stratosphere-troposphere exchange (STE) processes and their effect on atmospheric chemistry. STE is a key factor controlling the budget of ozone, water vapour and other substances in both the troposphere and lower stratosphere. Earlier studies of STE have concentrated primarily on the flux of air or trace constituents across the tropopause alone. Shallow exchange events are indeed partially reversible in nature and only produce compositional changes in the tropopause region. However, deep STE events are largely irreversible and have a highly significant and lasting impact on atmospheric chemistry through a substantial body of the atmosphere, even down to the earth's surface. Up until now, the importance of STE for the ozone budget relative to photochemical ozone formation from natural and anthropogenic precursor emissions, including those from aircraft, has remained uncertain. A comprehensive description of STE, which STACCATO seeks to provide, is thus a vital component for understanding the chemical composition of the atmosphere and its consequences.|
|Scientific objectives and approach|
STACCATO is undertaking a first detailed investigation of STE
mixing of stratospheric and tropospheric air. Meteorological
processes under investigation include the creation of fine-scale
structures by chaotic advection, radiative decay of tracer filaments
and mixing through turbulence in the free troposphere. The non-linear
effect of this mixing on chemical processes is addressed with
a box model as well as with a global model. The impact of STE
on the oxidizing capacity of the troposphere, relative to other
factors, is examined with two global chemistry models coupled
to climate models. The fate of aircraft emissions is being addressed
using passive tracer simulations and including these in the chemistry
A new three-dimensional Lagrangian perspective of STE, focussing on deep exchange events, is being developed. The variability and recent trends of STE is being assessed, based on very high quality meteorological re-analysis data. Potential future changes to STE significance are being computed under scenarios of climate change obtained from simulations with two climate models. A major comparsion of seven methods and models used to calculate STE is being carried out to find strengths and weaknesses of each approach and to identify reasons for discrepancies. A measurement dataset is being created to validate model results and to provide an independent estimate of the strength of STE. This includes the first long-term monitoring of two radionuclides, beryllium-7 and beryllium-10.
Provision of an observational estimate of the strength of STE based on two years of radionuclide measurements.
Analysis of the strength of STE and its variability during the last 15 years, based on Lagrangian models set up on meteorological re-analyses.
Study of the possible changes in STE in a future climate
Assessment of the importance of STE for atmospheric chemistry relative to other factors.
Improved knowledge of the role of mixing.