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The Tropical Tropopause Layer

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If you have a question about this talk, please contact Dr Amanda Maycock.

An afternoon CCfCS meeting on ‘The Tropical Tropopause Layer’.

1430 Neil Harris – The influence of convective transport on TTL composition

1500 Alison Ming – The structure of the upwelling and diabatic heating in the tropical lower stratosphere

1530 Tea

1600 Stephan Borrmann – Results and questions arising from recent in-situ measurements

1645 Amanda Maycock – The future of the TTL as simulated in global models

1715 Discussion

1730 Close/wine reception

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The influence of convective transport on TTL composition

Neil Harris (University of Cambridge)

Passage of air through the Tropical Tropopause Layer (TTL) is the major route for troposphere to stratosphere transport. The UK CAST campaign took place in the West Pacific in January/February 2014. The field campaign was based mainly in Guam (13.5˚N, 144.8˚E) and had three components: CAST with the NERC FAAM B Ae-146 research aircraft; the NASA ATTREX project based around the Global Hawk; the NCAR -led CONTRAST campaign based around the Gulfstream V (HIAPER) aircraft. Together, these aircraft made detailed measurements of atmospheric structure and composition from the ocean surface to 20 km. An overview of the campaigns will be given along with the results of high resolution global Unified Model studies and NAME trajectory calculations to look at the transport of air into the TTL in convective systems over the Maritime continent and West Pacific. The focus will be on the transport of air from in and around the boundary layer and will assess the possible importance of natural and anthropogenic emissions for TTL composition.

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Results and questions arising from recent in-situ measurements

Stephan Borrmann (Johannes Gutenberg-Universität Mainz and MPI -Chemistry)

In-situ measurements were performed up to 21 kilometers altitude within the SCOUT -O3 and the AMMA -SCOUT framework in the tropical and mid-latitude UT/LS utilizing the Russian high altitude research aircraft M-55 “Geophysica”. Results of ultrafine particle measurements are discussed from three tropical campaigns in Brazil, Northern Australia and Burkina Faso. The emphasis here is on new particle formation processes in clear air, within anvils of mesoscale convective systems, and within convective outflows from clouds. With respect to clouds results from measurements in cirrus over West African MCS systems and Cb overshoots in Australia are discussed as well as thin and subvisible cirrus cloud events.

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The structure of the upwelling and diabatic heating in the tropical lower stratosphere

Alison Ming (University of Cambridge)

The upward velocity in the Brewer-Dobson circulation in the lower stratosphere (~70 hPa) has a double-peak structure, with the maximum upward velocity at about 20 N and 20 S and a minimum over the equator. This structure is evident in diabatic heating rates and vertical velocities calculated from ERA -Interim data and also in many model simulations. We analyse the different contributions to the localized maxima in diabatic heating rates and find an important contribution to the double-peak from ozone absorption, with secondary contributions from temperature structure and from latitudinal variations in upper tropospheric clouds.

As required by the angular momentum balance, the double-peak structure in upwelling corresponds to a certain latitudinal structure in wave driving. However we argue that the structure in the wave driving cannot be considered to be the primary cause of the double peak and that the cause is instead some component of the diabatic heating field. We look at the consequence of imposing a double peak structure in the heating in an idealized general circulation model and find that the circulation responds in such a way to produce two corresponding peaks in the upwelling with a temperature change that has a weak meridional gradient. There is a corresponding change in the wave driving but this is part of the dynamical response to the imposed heating.

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The future of the TTL as simulated in global models

Amanda Maycock (University of Cambridge)

Ozone and water vapour are particularly powerful greenhouse gases in the upper troposphere/lower stratosphere. Thus changes in TTL composition may affect how surface climate responds to future anthropogenic greenhouse gas emissions. However, climate models often struggle to capture the present day structure of the TTL , raising questions about the reliability of future projections in this region. This talk will review some features of the TTL in global models under present day conditions and discuss simulated changes under climate change with reference to possible implications for surface climate projections.

This talk is part of the Cambridge Centre for Climate Science series.

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