Role of convection in global-scale transport in the troposphere
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abstract
The role of convection in global-scale transport in the troposphere is studied by using climatological Green's functions of the mass conservation equation for a conserved, passive tracer. The Green's functions are calculated from long-term atmospheric trajectory calculations based on three-dimensional winds from a 19-year simulation by Version 3 of the NCAR Community Climate Model (CCM3). Two trajectory calculations are compared: one that includes the effects of the model's parameterized convection and one that neglects the convective transport. The global transport properties of the two cases are qualitatively similar. The global troposphere can be divided into three parts: the tropics, and the Northern and Southern Hemisphere extratropics. Transport within each region is rapid, while the exchange of air between the regions is comparatively slow. Semipermeable barriers to transport exist in the subtropics in both cases. Convection slightly suppresses transport across the subtropical barriers. Convective transport enhances the vertical dispersion of air parcels, particularly in the rising branch of the Hadley circulation. Localized differences between the two runs can reach 50-80%. The differences are largest in the upper troposphere. Interannual transport variations related to the El Nio - Southern Oscillation (ENSO) phase can be detected in both cases, with the vertical transport being enhanced in the presence of convection. Copyright 2006 by the American Geophysical Union.
