Cross tropopause transport induced by deep tropical convection

The characteristics of the Upper Troposphere/Lower Stratosphere (UT/LS) region are intrinsically determined by those of both the troposphere and the stratosphere. The balance of processes that regulate its dynamical, radiative and chemical characteristics is at the heart of the debate on UT/LS exchanges. In particular, the quantification of the respective role of convective overturning in the troposphere and radiative and/or diabatic overturning in the stratosphere in determining the entry of atmospheric constituents into the stratosphere is still a topical issue for air quality and climate applications.

The predictable and isolated nature of Hector thunderstorms that forms over the Tiwi Islands, north of Australia, makes them ideal case studies for investigating the role of isolated deep convection over the Tropics in driving UT/LS exchanges. An intense Hector thunderstorm observed during the ACTIVE (Aerosol and Chemical Transport in tropical ConVEction) field campaign in late 2005, was simulated by the ARW [Advanced Research core of the Weather Research and Forecasting (WRF) model] and UK Met Office UM (Unified Model) models. Online chemistry was included in the ARW simulation, which was performed using the WRF/Chem (fully coupled online chemistry within WRF) model.

Using a horizontal resolution as fine as 1 km, the numerical simulations reproduce the timing, structure, and strength of deep convection fairly well. Results of the simulations are found in good agreement with field campaign observations from research aircrafts, radiosondes and precipitation radars. UM slightly underestimates the intensity of Hector when compared with observations and WRF-ARW, while its timing and structure are well captured.

Several overshooting updraughts penetrating the tropopause are produced in the simulations. The penetration of rising towering cumulus clouds into the Lower Stratosphere (LS) maintains the entrainment of air from the LS down into the troposphere.

Stratosphere-Troposphere Exchanges (STE) resulting from this entrainment process leads to an increase of total water in the LS.  Conversely, ozone (O3) increases in the Upper Troposphere (UT). The vertical transport of H2O (upward) and O3 (downward) across the UT/LS region was quantified. The divergence of the vertical fluxes of mass diagnosing STE are not negligible, confirming that isolated deep convection over the Tropics can have a significant impact on the redistribution of atmospheric constituents within the UT/LS region. During the mature stage of the thunderstorm and at the scale of the islands, turbulent transport is found to overcome the large-scale transport above the zero net radiative heating level.

Acknowledgements & collaborations:

This work was conducted in collaboration with Maria Russo and John Pyle at the Centre for Atmospheric Science, University of Cambridge. We appreciate the tremendous efforts of the ACTIVE PIs as well as all those who made the intensive observation periods so successful. We acknowledge the Bureau of Meteorology for the use of the BMRC/NCAR C-Pol radar data. We wish to thank the UK Met Office and NCAS/CMS for helping on UM-related issues. We appreciate the collaboration with Georg Grell on the use of WRF/Chem.