UTLS Chemical Structure, ExTL Summary of the talks –Data sets –Coordinates –Thickness of the ExTL (tracers based) Outstanding questions Discussion

Key results (1/2) Hegglin et al., ACE satellite measurements of of ozone versus CO and ozone versus H2O in UTLS region are consistent with SPURT aircraft measurements, providing a broader global picture of the ExTL related to mixing between stratosphere and troposphere. –deltaZ from Tp Mackenzie et al.,: Enhanced ozone observed by TES at latitudes near 50 degrees North over Central Asia are consistent with large mixing layer depth at these latitudes. Using the GEOS-Chem model for analysis, up to 45% of ozone can be attributed to stratospheric air over Central Asia. –deltaZ from Tp Manney et al.: Ozone distributions in UTLS are related to jet structure. Transport and mixing processes related to the jets are captured in many instances by MLS. –Jet Core coordinates

Key results (2/2) Brioude et al.: MOZAIC data are used to characterized ozone and CO distributions in the mixing layer and relate them to PV within upper level troughs. Seasonal and regional differences are assessed. MOZAIC CO over Asia is much higher in UTLS region with peak values in the spring. –PV coordinates Pisso et al.: Research aircraft data. Increased thickness of the Extratropical Tropopause Layer with increasing latitude, from 0 at 20° to 30 K at 60°. –Equivalent latitude and delta theta Pan et al.: Theta coordinates and equivalent latitudes well describe stratospheric processes but do not characterize tropospheric ones. Disadvantage: Equivalent latitude coordinates are derived from re-analysis which do not have the same fine- scale resolution as the observations analyzed this way. Neu et al. : Problem: thermal, dynamical, and ozone tropopause definitions cannot simply be used for comparison with numerical simulations It was proposed to use “E90” tracer which defines the tropopause as a mixing barrier to better define the tropopause height in CTM’s. This tracer leads to good comparisons of ozone at extratropics but the model has problem with ozone in tropics.

Thickness of the ExTL ? Upper and lower boundary of the ExTL as distance from the thermal tropopause derived from ACE-FTS H 2 O-O 3 and CO-O 3 correlations. The ExTL in the Southern Hemisphere is thinner by ~1 km. The ExTL depth increases with latitude. The ExTL depth varies when using different tracer-pairs. - Hegglin : dz from Tp

Thickness of the ExTL Pisso : O3, CO,CO2, H20  0K at 20°, 30K at 60-70° Brioude : O3-CO  2-6 pvu (up to 8 pvu in summer) for NH mid-latitudes in 2003 MacKenzie : O3-CO  4-6 km Mixing Width [km] Zonal Mixing Layer Widths

Are authors happy with the 1-2 summary sentences ? If no, come to see me … If yes, but want to complete, come to see me as well …

Outstanding questions? - Are we all understand the mixing layer and ExTL the same way (common understanding ? Physical boundaries ?) - Is there a unified view of how to define the mixing layer (observational data set, coordinates, tracers based, units for thickness) and exchange processes between stratosphere and troposphere (vertical vs isentropic mixing) ? The answer probably depends on what are such analyses made for … Needed : - What is the impact of stratosphere on tropospheric ozone budget / How do we quantify reversible versus irreversible transport (horizontal advection) ? - Validation of CCM’s for examining impacts of climate variability on chemistry and their feedbacks. - Process studies for how extra-tropical UTLS is related to jet structure. (dynamical issues … today’s session)

Integral view ? Chemical structure of the lowermost stratosphere: - transition layer which extending around the tropopause - the vertical depth: 2-4 km relative to the tropopause K above 2 PVU tracer dependency, tropopause - chemical transition layer marks a change in transport time from the lower troposphere to the LMS - strong seasonality above the transition layer: - 'young' air in summer and autumn - rapid 'flushing' from late spring to summer - strong coupling to the TTL and tropical lower stratosphere

Challenges Which dynamical processes are most important for the observed chemical structure, particularly the strong coupling to the tropics? What is the role of transport and mixing in maintaining the dynamical structure of UTLS? Are there feedbacks between dynamical structure and composition ? How robust are the estimated budgets? Do we have enough data ? What kind of data and where ? High resolution, polar, SH, …