The effects of convective clouds on vertical scalar transport using a numerically simulated flow field

In the planetary boundary layer (PBL), pollutants travel shorter distances and have shorter residence times relative to those in the upper troposphere. Without clouds, pollutants released near the surface are likely to stay within the PBL or deposit onto the ground if there is no deep convection to carry them upward. The objective of this study was to understand how deep convective cloud systems influence the transport of insoluble species to the upper troposphere using a Lagrangian approach. We used velocity field data from a synthetic flow field numerically generated from a 3-D large eddy simulation (LES) model of a deep convection system. The domain was large, consisting of 2048 x 2048 x 256 (or 109) grid points, so this LES is also known as a giga-LES. The results showed three interesting features: First, a peak in the particle concentration occurred around 8-9 km at 3, 6, and 12 hours after their release. Possible mechanisms explaining this include water loading and a larger areal coverage of downward motion than upward motion at that height. Second, approximately 25% of particles remained near the surface, the lowest level used for the particles. This is probably due to neglecting the subgrid-scale (SGS) motion in our Lagrangian approach. Finally, within the PBL, we found a sharp drop of pollutants. Possible explanations include (1) neglecting the SGS effect and (2) the use of a large time interval of the velocity field, 15 minutes. Our future work will include the SGS effect, and will use a 5-minute time interval of the flow field.