The planetary-scale distribution of radiatively active trace species and aerosols in the UTLS is controlled by both transport and chemical/microphysical processes on varying spatial and temporal scales. These large-scale distributions in the UTLS have strong variabilities, both on short time scales, but also on seasonal and long-term time scales, all of which influence the radiative balance of the atmosphere. While the distributions of short lived chemical species are dominated by chemical processes and by faster small scale mixing processes, longer lived species are more strongly controlled by large scale transport processes, but also by microphysical processes in the case of water vapour.
Model studies have shown that changes in the UTLS region are expected to have a particular large impact on ozone and radiative forcing, but in addition are particularly uncertain with model predicted trends being on the same order as inter-model variability. Regarding chemistry transport coupling, model simulations indicate that the variability of ozone also modulates the circulation in the stratosphere with significant influences down to 300 hPa altitude, thus impacting the UTLS and via further couplings also near-surface climate. While the photochemistry of ozone in the stratosphere is probably one of the best understood phenomena in atmospheric chemistry, the long term evolution of ozone in the UTLS is highly uncertain, due to interaction between dynamics, chemistry, and radiation/temperature. Current trend estimates of ozone in the UTLS exhibit large ranges of uncertainty. This uncertainty in ozone trends and also in changes of stratospheric circulation feeds back on the amount of ozone transported downwards into the troposphere. Similar uncertainties as for ozone hold for the long-term evolution of UTLS water vapour and other radiatively important gases and aerosols.
Among the chemical processes with high uncertainty is the evolution and the role of halogenated very short lived substances (VSLS), which are also an important source of stratospheric halogens, and in particular for the halogen budget of the UTLS and thus for the budget of ozone in the lower stratosphere. Recent chemical transport model (CTM) simulations indicate that ozone loss from VSLS had a radiative effect nearly half of that from long-lived halocarbons in 2011 and has contributed a total of about −0.02 Wm−2 to global radiative forcing compared to pre-industrial times. However, the amount of bromine and chlorine in the stratosphere from VSLS remains highly uncertain, as also emission scenarios used in global modelling show considerable variability and partly fail to reproduce observations.
In summary, the distributions of radiatively active trace gases as well as aerosol particles in the UTLS impact the radiative forcing at the surface. The distribution in the UTLS are often characterised by large-scale features and controlled by large-scale processes. The resulting impact of UTLS changes on the troposphere and hence weather and climate can be substantial and call for improved understanding of distributions of trace gases and aerosols as well as the governing processes. On a shorter time-scale, the impact of the stratospheric dynamics on tropospheric weather and climate is also undoubtedly given. Again, the relevance of individual processes in this downward propagation of stratospheric circulation patterns is yet to be explored.
Research questions (C)
RQ-C1: What is the regional and temporal variability of trace gas and aerosols distribution on seasonal and multi-annual time scales in the UTLS?
RQ-C2: Which large scale processes dominate the concentrations of radiatively important trace gases and particles in the UTLS?
RQ-C3: Which (large scale) chemical, radiative and dynamical feedbacks impact the UTLS and regions beyond the UTLS, e.g. the surface climate and the stratospheric circulation, and how are these feedbacks modified by climate change?