Tropopause Initiative (UTLS)

The SPARC project has, since its inception, tried to stimulate research into the dynamics, transport and chemistry in the Upper Troposphere/Lower Stratosphere  (UTLS) region. One success has been the organization of several multidisciplinary workshops on this topic, starting with the influential Cambridge workshop in 1993 that resulted in the seminal review by Holton et al. in 1995., and the 2009 UTLS workshop in Boulder, Colorado, that resulted in the recent review by Gettelman et al. (2011)., as well as the recent review of the tropical tropopause by Fueglistaler et al (2009).

The tropopause region and the UTLS is a transition region between the stratosphere and the troposphere. The UTLS includes the Tropical Tropopause Layer (TTL) and the extratropical upper troposphere and lower stratosphere (Ex‐UTLS).

The Tropical Tropopause Layer (TTL) is a transition layer in which the air has mixed stratospheric and tropospheric properties. The TTL has increasing levels of ozone with height, large lapse rates and near its base the net radiative heating changes sign from negative below to positive above. The heat, moisture and chemistry budgets of the TTL ultimately affect the properties of stratospheric air. These budgets are influenced by slow ascent within the upward branch of the Brewer-Dobson circulation and by overshooting deep moist convection, which is an effective transport method from the boundary layer. The relative importance of the contribution of deep moist convection to the heat, moisture and chemistry budgets of the TTL is still uncertain and subject of debate.

The TTL has received attention within the SPARC community from the perspective of its importance for processes in the tropical lower stratosphere, while research on modelling and understanding of deep convection in the tropics has received considerable attention within the GEWEX Cloud System Study (GCSS). The IGAC (International Global Atmospheric Chemistry) community is interested in the role of deep convection in transporting and processing chemical constituents and aerosols. Past activities have aimed at bringing these three groups together to understand the TTL and the role of deep convection in determining the composition of the TTL and the inputs into the stratosphere. The outcome of a workshop was a set of cloud resolving model experiments that aimed at modelling the TWP-ICE experiment over Darwin. Several CRMs were used to allow for a model intercomparison.

The Ex‐UTLS includes the tropopause, a strong static stability gradient and dynamic barrier to transport. The barrier is reflected in tracer profiles. This region exhibits complex dynamical, radiative, and chemical characteristics that place stringent spatial and temporal requirements on observing and modeling systems. The Ex‐UTLS couples the stratosphere to the troposphere through chemical constituent transport (of, e.g., ozone), by dynamically linking the stratospheric circulation with tropospheric wave patterns, and via radiative processes tied to optically thick clouds and clear‐sky gradients of radiatively active gases. A comprehensive picture of the Ex‐UTLS recognizes that thermal gradients and dynamic barriers are necessarily linked, that these barriers inhibit mixing and give rise to spe- cific trace gas distributions, and that there are radiative feedbacks that help maintain this structure.

The current SPARC Tropopause activity has helped facilitate workshops leading to these review papers, and continues to work with a network of scientists interested in the UTLS to further science in this area. Specific foci are to better understand the TTL and the ExUTLS to be able to better understand the impacts of the tropopause region on stratospheric chemistry and tropospheric climate.

Activity leaders:

Juan A. Añel
EPhysLab, Universidade de Vigo, SPAIN

Peter H. Haynes
DAMTP, Centre for Mathematical Sciences, UK

Andrew Gettelman
NCAR, Boulder, CO, USA

Published results:

Science papers:

ARM/GCSS/SPARC TWP-ICE CRM Intercomparison Study. Ann Fridlind, Andrew Ackerman, Jon Petch, Paul Field, Adrian Hill, Greg McFarquhar, Shaocheng Xie

Fueglistaler, S., A. E. Dessler, T. J. Dunkerton, I. Folkins, Q. Fu, and P. W. Mote (2009), The tropical tropopause layer, Rev. Geo- phys., 47, RG1004, doi:10.1029/2008RG000267.

Gettelman, A., P. Hoor, L. L. Pan, W. J. Randel, M. I. Hegglin, and T. Birner (2011), The extratropical upper troposphere and lower stratosphere, Rev. Geophys., 49, RG3003, doi:10.1029/2011RG000355.

Holton, J. R., P. H. Haynes, A. R. Douglass, R. B. Rood, and L. Pfister (1995), Stratosphere‐troposphere exchange, Rev. Geophys., 33(4), 403–439.

SPARC activity reports:

SPARC Newsletter No. 29 (2007), p. 14: A SPARC Tropopause Initiative

SPARC Newsletter No. 28 (2007) p. 7: Modelling of Deep Convection and Chemistry and their Role in the Tropopause Layer: SPARC-GEWEX/GCSS-IGAC Workshop

SPARC Newsletter No. 26 (2006) p. 8: Processes governing the chemical composition of the extratropical UTLS A report from the joint SPARC-IGAC Workshop

SPARC Newsletter No. 21 (2003) p. 2: Highlights from the joint SPARC/IGAC Workshop on Climate-Chemistry Interactions

SPARC Newsletter No. 17 (2002) p. 22: Convection in the Tropical Tropopause Region and Stratosphere-Troposphere Exchange