CAST (Co-ordinated Airborne Studies in the Tropics): Overview and Highlights

Abstract:

We present an overview of the main field activities of the UK CAST (Co-ordinated Airborne Studies in the Tropics) campaign, which took place in the West Pacific in January/February 2014. The aim was to study the processes that influence the Tropical Tropopause layer (TTL), the major route for troposphere to stratosphere transport. The field campaign was based mainly in Guam (13.5^oN, 144.8^oE) and had three components: the ATTREX project organized by NASA and based around the Global Hawk; the NCAR-led CONTRAST campaign based around the Gulfstream V (HIAPER) aircraft; and CAST with the NERC FAAM BAe-146 research aircraft. Together, the three aircraft were able to make detailed measurements of atmospheric structure and composition from the ocean surface to 20 km. The BAe-146 made a total of 25 flights between 1 ^oS-14 ^oN and 130 – 155 ^oE. The 146 was used to sample at low altitudes (below 8 km) with much of the time spent in or close to the marine boundary layer. This strategy meant that the region of main inflow into the strong convection was sampled thoroughly. Concentrations of the following chemicals were measured in situ: CO, O3, NO2, NO, CO2, CH4, N2O, H2O, a range of halocarbons including CH2Br2, CHBr3 and CH3I; and reactive halogen species including BrO, BrCl, HOBr and IO. The CAST team also made ground-based measurements at the ARM site on Manus Island in Papua New Guinea during February 2014. Ozonesondes were deployed daily, and measurements of ozone, CH4, CO, CO2, H2O and halocarbons were made at the ground. Halocarbon measurements were also made at other West Pacific sites.