Understanding the chemical composition of the upper tropical troposphere - gas and aerosol particles - is of key importance to understand global climate. The upper part of the troposphere in tropics (or tropical tropopause layer TTL) behaves indeed as the base of the Brewer-Dobson circulation and can determine the composition for the global stratosphere. Additionally, the loading and nature of the aerosol particles present in the TTL affect directly and indirectly - through cirrus cloud formation - the radiative balance of the tropical troposphere.
The partitioning in the TTL between convective and non-convective transport is a function of height, horizontal location and season and remains a key quantitative uncertainty. To determine the role that deep convection in the tropics plays in transporting aerosol and chemical species from the planetary boundary layer to the upper troposphere, the ACTIVE (Aerosol and chemical transport in tropical convection) project has been conceived. Active is a consortium between the Universities of Manchester, Cambridge and York (UK); DLR and Forschungzentrum Julich (Germany); York University (Canada), Bureau of Meteorology (Australia), Airborne Research Australia, and NCAR (US). 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.
In the present work, we focus on the issue of how much and what kind of aerosol, reach the TTL via deep convection. We study Hector storms sampled in the first part of the campaign with a view to understanding the impact of the aerosol on the microphysics the the anvil region of the cloud.
In the pre-monsoon period (mid-November mid-December) and during monsoon breaks, convection is dominated by single isolated storms. Spectacular examples occur over the Tiwi Islands north of Darwin, due to convergence of sea breezes over the islands (Carbone et al, 2000). These are the Hector storms, which occur predictably over the islands at about the same time each day. These giant thunderstorms reach (and penetrate) the tropopause (at 17 km) thus directly injecting air into the TTL and TLS (tropical lower stratosphere). Their predictability and isolation also make them excellent laboratories for comparing the aerosol input and output since the task of modelling them is not so formidable as for extensive convection. During the first campaign, dedicated to Hector storms, the aircraft flights were designed to sample the inflow and outflow from convection. First, the Dornier aircraft flew in the boundary layer around the inflow, measuring the aerosol and chemical fields in the inflow. Then, the Egrett flew into the outflow of the storm to study the microphsycis of the anvil region. Both the Dornier and the Egrett aircraft carried instruments for aerosol, chemical and tracer measurements (see table 1). In addition, ozonesondes were launched to characterise the background atmosphere and to allow interpretation of the aircraft ozone measurements.
Table. 1 Instruments sampling aerosol particles, cloud microphysics and gas on the Dornier and Egrett Airplane.
Measurements Dornier Egrett AP concentration CPC CPC AP size distribution OPC, FSSP CAS, SP-2, CPI AP composition AMS, Filters SP-2 Gas Ozone, CO, VOC, Halocarbon Ozone, CO, VOC, Halocarbon, NO, NO2
It was found that in the core of the anvil CN concentrations were as high (concentration can reach 500 particles per cm3 much higher than in the surrounding air at that altitude. The was evidence that some of these particles were nucleating to form new ice crystals within the anvil, particularly in regions remote from the main storm where many of the larger particles ejected from the storm had fallen out. It was found that at distances of, 40 km from the exhaust of the storm, the average CO concentration was near 90 ppbv demonstrating the ability of the storm to transport polluted air from the boundary layer into the TTL region. The presence of insoluble precursors at the top of the cloud addresses the question of the role of deep convection in forming fresh ultra fine particles in the outflow (Raes et al., 2000).
Close to the storm it was found that large aggregates of ice crystals were present. Chain aggregates characteristic of the effects of electric fields on aggregation was detected.
Detailed modeling of the evolution of the storm was performed using the Met Office Cloud Resolving Model (CRM). This model enables us to predict the extent of the storm, the regions of detrainment from the storm and the ability of the storm to transport boundary layer air into the detrainment zones. The results of this modeling are compared to the observations. The results from the CRM model were used to initiate the Manchester Explicit Microphysics model (EMM) to simulate the microphysical processes within the anvil region. This model was used to investigate the roles of the aerosl transported from the boundary layer on the microphysics of the anvil region.
Preliminary analysis of data obtained during ACTIVE from the study of the deep convective clouds forming in the pre-monsoon phase over the tropical warm pool are efficient in transporting boundary layer aerosol and trace gases into the TTL region. Within the cloud anvil the microphysics is dominated by ice aggregates formed within the storm close to the detrainment region. Further out in the anvil, however, new nucleation of ice crystals on hygroscopic nuclei plays a key role in the microphysics of the anvil region. These conclusions will be extended and quantified in the presentation.