Observations of a deep tropopause fold over Darwin during the ACTIVE aircraft campaign– Implications for tropical convection

Factors affecting the intensity and frequency of deep tropical convection are important in determining the tropical radiation budget and troposphere-to-stratosphere exchange rates. This work discusses observations of an unusually deep tropopause fold associated with a breaking Rossby wave on the Southern Sub-tropical Jet, which penetrated the tropics as far north as -10oS. Ozone and radiosondes launched from Darwin, Australia, between 23 November 2005 and 27 November 2005 as part of the Aerosol and Chemical Transport in tropIcal conVEction (ACTIVE) aircraft campaign, show deep layers of characteristically stratospheric ozone-rich, low-humidity (less than 10%) air extending to 700 hPa over this period with ozone concentrations of up to 100 ppbV within such layers; exceeding climatological averages by a factor of three. Back trajectory and ECMWF analyses show these layers to be associated with the passage of a tropopause fold, with MTSAT-1R water-vapour and TOMS satellite ozone fields showing a resultant dry, ozone-rich pool residing over a large area of North West Australia during this period. Analysis of MTSAT-1R-derived cloud statistics and land-based rain gauges over the same area show markedly reduced precipitation and cloud cover during this time, with remaining convection confined to coastal zones. This reduction in convective activity may be due to the entrainment of very dry air into developing convective cells, inhibiting droplet formation and further latent heat driven convection. The frequency of such tropopause fold events and their potentially important impact on tropical convection is currently poorly characterised. These observations illustrate this importance and highlight the need for such processes to be considered in studies of tropical deep convection and tropical tropospheric chemistry.