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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, D18110, doi:10.1029/2007JD008645, 2007

Chemical behavior of the tropopause observed during the Stratosphere-Troposphere Analyses of Regional Transport experiment

L. L. Pan

National Center for Atmospheric Research, Boulder, Colorado, USA


K. P. Bowman

Department of Atmospheric Sciences, Texas A&M University, College Station, Texas, USA


M. Shapiro

Office of Weather and Air Quality, NOAA, Boulder, Colorado, USA


W. J. Randel

National Center for Atmospheric Research, Boulder, Colorado, USA


R. S. Gao

Chemical Sciences Division, Earth System Research Laboratory, NOAA, Boulder, Colorado, USA


T. Campos

National Center for Atmospheric Research, Boulder, Colorado, USA


C. Davis

National Center for Atmospheric Research, Boulder, Colorado, USA


S. Schauffler

National Center for Atmospheric Research, Boulder, Colorado, USA


B. A. Ridley

National Center for Atmospheric Research, Boulder, Colorado, USA


J. C. Wei

QSS Group, Inc., Lanham, Maryland, USA


C. Barnet

National Environmental Satellite Data and Information Service, NOAA, Camp Springs, Maryland, USA


Abstract

During the Stratosphere-Troposphere Analyses of Regional Transport (START) experiment in December 2005, the behavior of the extratropical tropopause was examined under a variety of dynamical conditions. Using in situ measurements of ozone and water vapor, on board the new NSF/NCAR research aircraft Gulfstream V, and data from large-scale meteorological analyses, we address issues of the tropopause definitions and sharpness. Comparisons of the data from two flights show that the sharpness of chemical transitions across the tropopause varies with the sharpness of the static stability change across the tropopause. Using tracer correlations, air masses of mixed stratospheric and tropospheric characteristics are identified. The mixed air mass does not form a uniform mixing layer near the tropopause, but rather shows strong spatial variation. A depth of mixed air (∼5 km in vertical distribution) is found on the cyclonic side of the polar jet, where the thermal gradient is weak and significant separation occurs between the thermal and the dynamical tropopause. Away from the jet or on the anticyclonic side of the jet, where the stability gradient is strong, the chemical transition across the tropopause was much more abrupt and shows minimum mixing. In both cases (either significant or minimal mixing), the thermal tropopause is shown to be approximately at the center of the mixing layer, and the altitude relative to the thermal tropopause is found to be an effective coordinate for locating the chemical transition. To further understand the role of the thermal and dynamical tropopause as a chemical transport boundary, tracer correlations are used to examine the chemical characteristics, and the trajectory calculations are used to infer the fate of the air mass between the thermal and dynamic tropopauses in the region of significant separation. The tracer correlation analysis shows that the air mass in this region is a mixture of stratospheric and tropospheric air but predominantly of tropospheric characteristics. Trajectory model calculations show that a significant fraction of the air parcels in this region ended in the mid to lower troposphere, which suggest the irreversible nature of the observed stratospheric intrusion.

Received 9 March 2007; accepted 27 June 2007; published 26 September 2007.

Keywords: extratropical tropopause; stratosphere-troposphere exchange; aircraft in situ measurement.

Index Terms: 0341 Atmospheric Composition and Structure: Middle atmosphere: constituent transport and chemistry (3334); 0368 Atmospheric Composition and Structure: Troposphere: constituent transport and chemistry; 3334 Atmospheric Processes: Middle atmosphere dynamics (0341, 0342); 3362 Atmospheric Processes: Stratosphere/troposphere interactions; 3364 Atmospheric Processes: Synoptic-scale meteorology.

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Citation: Pan, L. L., et al. (2007), Chemical behavior of the tropopause observed during the Stratosphere-Troposphere Analyses of Regional Transport experiment, J. Geophys. Res., 112, D18110, doi:10.1029/2007JD008645.