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Dynamical processes in the tropopause region and their impact on the distribution of atmospheric trace gases

Final Report Summary - PROTRO (Dynamical processes in the tropopause region and their impact on the distribution of atmospheric trace gases)

PROTRO is a research project in atmospheric sciences that addresses tropopause related processes, which contribute to chemistry-climate coupling. The tropopause region is characterized by frequent air mass exchange between the upper troposphere (UT) and lower stratosphere (LS). It is a region of significant coupling between atmospheric dynamics, chemistry, and radiation. It is therefore particularly relevant to climate. The aim of PROTRO is to improve our ability to quantify the interaction of dynamical and chemical processes controlling the UTLS in a changing climate. The study makes use of a novel concept of the dynamical tropopause. This tropopause is identified by maximized potential vorticity (PV) gradients on isentropes. There is a close consistency of this PV gradient-based tropopause and the chemical discontinuity in trace gas concentrations near the jet streams suggesting that the novel tropopause concept well identifies the barrier for cross-tropopause transport. Further, preferred transport pathways in the UT/LS are revealed by studying regions where strong trace gas gradients and the PV gradient-based tropopause decouple. Using the novel tropopause concept, PROTRO investigates stratosphere-troposphere exchange (STE) on higher altitudes than hitherto common and aims at an improved quantification of trace gas fluxes across the tropopause. Feature-based climatologies of tropopause structures are produced using 30 years of ERA-Interim data. UTLS trace gas constituents associated with these tropopause structures are investigated using state-of-the-art numerical models and high quality observational trace gas data, which are further employed for model evaluation. In summary, this PROTRO provides new insight into fundamental and climate relevant UTLS processes.

Innovative results are achieved due to the use of a novel, dynamically motivated, tropopause definition, the focus on Lagrangian techniques, and the use of numerical models and observational data sets.
Dr. Anne Kunz extended the concept of the PV-gradient based tropopause on isentropes into the LS, i.e. between 350 and 500 K. This allowed a climatological study of the dynamically relevant PV contour representing isentropic transport barriers in the entire UTLS. She further revised existing classical Eulerian tools to identify PV streamers and cutoffs according to the PV-gradient based concept. Existing climatologies of PV streamers in literature are consequently extended on isentropes above 350 K. Trajectory calculations are performed and the dominant isentropic transport pathways related to the PV streamers are identified for the period 1979 to 2011 based on ERA-Interim reanalyses data.
A statistical analysis on the water vapor variability based on balloon-borne measurements in the UTLS from 1981 to 2014 at Boulder (CO, USA) is performed. Based on this data set water vapor trend analyses are extended into the tropopause region and the trend uncertainties and their influence on the radiative forcing are quantified. Dr. Anne Kunz further compiled a comprehensive aircraft-based water vapor climatology in the UTLS based on aircraft campaigns from 2001 to 2011. This data set is used to test the skill of numerical weather prediction models in simulating the global water vapor distribution in the UTLS.

The main results of PROTRO are published as three contributions in scientific journals and the following topics are discussed:

(1) Innovative results on trends and variability of water vapor in the UTLS and their influence on the radiative effects of water vapor: Kunz, A., R. Müller, V. Homonnai, I. M: Janosi, D. Hurst, A. Rap, P. M. Forster, F. Rohrer, N. Spelten, and M. Riese, Extending water vapor trend observations over Boulder into the tropopause region: Trend uncertainties and resulting radiative forcing, J. Geophys. Res., 118, 11,269-11,284, doi:10.1002/jgrd50831 2013.

(2) Comprehensive data set based on aircraft-based measurements reveals a global climatology of water vapor in the UTLS from 2001 to 2011. The numerical skill of numerical weather prediction models in simulating the water vapor distribution can be tested as shown in: Kunz, A., N. Spelten, P. Konopka, R. Müller, R. M. Forbes, and H. Wernli, Comparison of Fast In situ Stratospheric Hygrometer (FISH) measurements of water vapor in the upper troposphere and lower stratosphere (UTLS) with ECMWF (re)analysis data, Atmos. Chem. Phys., 14, 10803-10822, 2014.

(3) Novel insights into the temporal and spatial distribution of PV streamers and the related atmospheric transport pathways are finally presented here: Kunz, A., M. Sprenger, and H. Wernli, Climatology of potential vorticity streamers and associated isentropic transport pathways across PV-gradient barriers, J. Geophys. Res., under review, 2014.