From the scientific themes, the following scientific objectives and questions are deduced:
DYN-1: Coupling processes at the southern hemisphere tropopause
The overall question of scientific theme DYN-1 is which transport related processes control the concentrations of radiatively and chemically active species in the SH UTLS region and how do they differ from the NH.
- DYN-1_Q1: How significant is transport of aged vortex air into the mid-latitudes for the SH UTLS composition?
We will investigate the effect of the Antarctic vortex on the structure and composition of the SH UTLS in midlatitudes and will quantify time scales and air mass fractions. We will focus on the processes leading to the decay of vortex filaments and their role for the SH UTLS water vapor and ozone budget. A second focus will lie on potential impact on cross tropopause transport and downward transport into the SH troposphere.
- DYN-1_Q2: Which processes determine the chemical and dynamical structure of the tropopause region and the depth of the ExTL in the SH and determine particularly the seasonal cycle of water vapor abundance and ozone in the SH UTLS region and?
Since the water vapor seasonal cycle in the UTLS is seasonally asymmetric with its maximum in southern winter, the relative importance of the dynamical factors which determine the water vapor gradient in the southern hemisphere is substantially different from the northern hemisphere. This involves particularly STE processes in the SH associated with planetary and synoptic wave breaking during southern winter and spring which modify the tropopause structure and temperature. The upper bound of the ExTL coincides with the location of the TIL, which both show a seasonal asymmetry and with a smaller vertical extent in the southern hemisphere. This affects the vertical gradients of radiatively active species at the tropopause and thus radiation budget. We will quantify the vertical depth of the ExTL. Further we will focus on the processes and time scales which dominate the formation of the Tropopause Inversion Layer, since both features may be linked to each other. We will put a special focus on the impact of baroclinic life cycles on the evolution of the ExTL and TIL (Kunkel et al., 2016) under the influence of strong diabatic downwelling in the SH stratosphere in winter spring.
- DYN-1_Q3: Can we identify interhemispheric transport of northern hemispheric air into the southern hemisphere and what are relevant mechanisms?
Interhemispheric transport may occur during southern spring triggered by the westerly ducts over the Pacific. We will try to identify this process and asses its effect on the SH composition (e.g., for NH anthropogenic pollution transport into SH). The DFG partners will perform measurements of species with strong interhemispheric gradients at different altitudes in the SHUTLS to diagnose the fingerprint of northern hemispheric air in the SH-UTLS. To gain information on the relevance of such events, the data will be combined with CLaMS simulations to gain insight into the dynamical process, which triggers such events and respective time scales and frequency of occurrence.
DYN-2: Gravity waves in the southern hemisphere
- DYN-2_Q1: How large are the temperature amplitudes of mountain waves in the upper troposphere, stratosphere and mesosphere?
Here, mainly remote-sensing observations by GLORIA and ALIMA onboard HALO above South America and the Antarctic Peninsula will be conducted and analyzed. Additionally, highresolution flight level in situ data of wind and temperature from BAHAMAS are used to calculate vertical momentum and energy fluxes along the flight paths.
- DYN-2_Q2: Is it possible to prove experimentally the horizontal propagation of mountain waves from the Andes or Antarctic Peninsula into the PNJ? At which altitudes does the horizontal propagation occur? Can we detect trapped waves captured in the PNJ?
Extended flights into the predicted paths of obliquely propagating mountain waves will be conducted to use ALIMA sampling the wave signatures in the stratosphere and mesosphere. Long transects underneath the PNJ will be added to uncover trapped wave signatures.
- DYN-2_Q3: How persistent are large-amplitude mountain waves signatures in the lee of the Andes?
Here, the airborne ALIMA data are compared with the long-term ground-based CORAL measurements in Rio Grande which have been conducted since November 2017.
- DYN-2_Q4: What is the effect of gravity waves on mixing and the composition of the tropopause region?
The effect of gravity waves on the distribution and of chemical constituents and mixing in the tropopause region will be assessed by combination of fast response tracer measurements and remote sensing data of GLORIA as well as turbulence information from HALO to find signatures for gravity wave induced cross isentropic mixing.
CHEM-1: Impact of the Antarctic vortex on the SH-UTLS
- CHEM-1_Q1: What is the distribution of halogenated VSLS in the SH UTLS? Can we close the budget on brominated source and product gases?
While a number of observations of organic and inorganic brominated species exist in the Northern Hemisphere, the data base in the Southern Hemisphere is extremely small. A combination of organic and inorganic bromine observations can be used to compare the budget to that of the Northern Hemisphere and to modelling work.
- CHEM-1_Q2: What is the halogen induced ozone depletion in the SH UTLS?
Comparisons of observations of ozone and ozone-tracer relationships between observations and models including also passive ozone will be used to diagnose ozone depletion. Models will be initialized as close to the observations of halogen loading as possible to allow good quantitative comparisons.
- CHEM-1_Q3: How is UTLS water vapour affected by subsidence of vortex air?
Air subsiding in the vortex which is then mixed into the UTLS is extremely dry and further dried by dehydration processes occurring in the Antarctic polar vortex. This dehydration is not regularly observed inside the Arctic Vortex. This should result in differences in the distribution and seasonal variability of water vapour between the hemispheres, which will be quantified using high-resolution and high-accuracy in situ and remote sensing water vapour observations.
CHEM-2 Biomass burning and biogenic emissions in the southern Atlantic upper troposphere
- CHEM-2_Q1: What are the characteristics of biomass burning plumes in the SH UTLS?
Satellite observations have suggested that the chemical composition of the pollution belt associated with S-American fires, where rainforest burning is predominant, appears different from the part of the plume associated with southern African savanna burning. We will investigate this with in situ and remote sensing observations of ozone, NOx, PAN and other trace gases.
- CHEM-2_Q2: Can we identify ammonia and ammonium nitrate in the SH UTLS?
Ammonia (NH3) is emitted by biomass burning and together with its product ammonium nitrate may impact aerosol formation in the UTLS region. This has not been observed in the SH UTLS before. GLORIA observations will be analysed for HN3 and NH4NO3.
- CHEM-2_Q3: What is the source of OCS in the SH UTLS? Is there a significant biomass burning source of OCS?
Observations of OCS from remote sensing and in situ will be related to simultaneous observations of biomass burning tracers to determine whether or not OCS is enhanced in biomass burning plumes.
Kunkel, D., P. Hoor, and V. Wirth (2016). “The tropopause inversion layer in baroclinic life-cycle experiments: the role of diabatic processes”. In: Atmos. Chem. Phys. 16.2, pp. 541–560. DOI: 10.5194/acp-16-541-2016.