Summary

Observations and model simulations over the last 15 years have greatly clarified the role of convection in transporting trace constituents in the atmosphere. It is now well established that some nonprecipitating cumulus clouds aid in venting the boundary layer. However, methods to determine the fraction of such clouds that actively transport trace gases to the free troposphere for a region on a given day still require further work. It is also well established that deep convection can transport large quantities of boundary layer gases to the middle and upper troposphere where they have a much longer chemical lifetime and can be transported large distances from their source region. Ozone production in the free troposphere can be enhanced by a factor of 4 or more as a result of deep convection. Downdrafts in convective storms can transport cleaner air from the midtroposphere down to the boundary layer. In remote regions low values of ozone and NOx can be transported to the upper troposphere, decreasing ozone and ozone production at these altitudes in such regions. In addition, convection induces downward transport of larger O3 mixing ratios into the remote boundary layer where photochemistry and surface deposition destroy O3. Storms that reach near or above the preconvective tropopause can induce exchange of trace constituents between stratosphere and troposphere.

Cloud-resolving models are the best tool for detailed studies of convective transport by individual storm systems. Air parcel trajectories and tracer transport calculations using the wind fields from such models are useful for understanding the flow patterns involved in the convective transport process. A cloud model is also useful in evaluating parameterized convective transport in regional or global models. Considerable uncertainty still exists in the output of convective parameterizations concerning the frequency, location, and magnitude of vertical transport, making convective transport one of the largest sources of uncertainty in regional and global CTMs.

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