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Centre for Atmospheric Science

ACTIVE: Aerosol and chemical transport in tropical convection

Introduction

A Hector storm over the Tiwi Islands as seen from the air on approach to Darwin Airport.
A Hector storm over the Tiwi Islands as seen from the air on approach to Darwin Airport.

ACTIVE combines field measurements and a range of modelling tools at different scales to address the question of what determines the composition of the upper tropical troposphere (or tropical tropopause layer, TTL) - is it rapid upward transport in deep convection or slow large-scale transport?

The ACTIVE project was a NERC funded consortium project to study the role that deep convection in the tropics plays in transporting aerosol and chemical species from the planetary boundary layer to the upper troposphere. This issue is crucial for climate science (since aerosol serves as cloud condensation nuclei) and for the overall composition of the tropical tropopause layer (TTL) – the layer of the tropical atmosphere between about 13 and 17 km altitude. The aim of the project was to make direct measurements of a range of aerosol and chemical species in the low-level inflow and the high-level outflow of tropical thunderstorms, and relate these to model simulations of the storms and the wider environment in which they are embedded.

The consortium consisted of scientists from The University of Manchester, The University of York, Cambridge University and NERC ARSF. International collaborators from DLR and FZJ in Germany, NCAR in the USA, York University in Canada, ARA, and the Australian Bureau of Meteorology (BoM) were also crucial to the project. A full listing of project participants can be found here.

The measurement phase of ACTIVE was conducted in two field campaigns based in Darwin, Australia (12°S, 131°E), during November and December 2005 and January and February 2006. During the 2005 campaign isolated giant thunderstorms (known locally as Hector) which regularly reach the tropopause were studied. These occur almost daily over the Tiwi Islands north of the Darwin coast, triggered by convergence of sea breezes over the islands. During this period storms occurred in relatively polluted conditions, with winds being predominantly easterly bringing continental air to the region, and also there being significant amounts of biomass burning in the Northern Territory towards the end of the dry season. These storms provided the opportunity to study the direct injection of material from low level throughout the TTL.

The NERC ARSF Dornier 228 Aircraft during ACTIVE.
The NERC ARSF Dornier 228 Aircraft during ACTIVE.

The aim of the 2006 campaign was to study monsoon convection; such convection is more widespread, less penetrating and more typical of the Pacific warm pool. Studies during this period will be more readily generalised to the global scale. Monsoon convection occurs in much cleaner westerly conditions, and with much less biomass burning in the region. In practise while some measurements were made in monsoon conditions, the meteorology was such that a variety of conditions were experienced, including isolated Hector storms as in the previous campaign.

The ACTIVE measurements were made by two research aircraft, the NERC ARSF Dornier 228, and the ARA Egrett. The Dornier was equipped for aerosol and gas phase measurements and operated at low levels to measure inflow into the convective systems being studied. The Egrett was equipped for cloud microphysics, aerosol and gas phase measurements, and operated at high altitudes within and around anvil outflows from the convective systems. Radiosonde ascents were also used to measure temperature, relative humidity, ozone and wind profiles in the atmosphere, and were launched several times a day on measurement days.

The ARA Egrett Aircraft during ACTIVE.
The ARA Egrett Aircraft during ACTIVE.

In addition to the ACTIVE measurements, aircraft and ground stations from the SCOUT-O3 and TWP-ICE projects, which had some similar objectives to ACTIVE, were operated in the 2005 and 2006 campaigns respectively. Ground based polarimetric and Doppler radar measurements, 920 and 50 MHz wind profiler measurements and forecast products were available from the Australian Bureau of Meteorology who were collaborators in the ACTIVE project. Darwin also has a fully-instrumented Atmospheric Radiation Monitoring (ARM) site, with comprehensive radiation measurements, micropulse lidar and 35 GHz cloud radar.

The measurement phase of the ACTIVE project was followed by considerable data analysis and modelling studies, which are still ongoing.