The effect of superparameterization on aerosol transport

The evolution and long-range transport of aerosols are strongly affected by vertical transport. Vertical transport can loft aerosols into the middle and upper troposphere, a region where the aerosols are much less subject to removal by precipitation and much more likely to be advected by the zonal jets. The focus of this paper is the vertical transport that occurs in convective systems. In conventional global chemical transport models, this transport is based upon highly parameterized treatments of sub-grid-scale vertical motion. The result is that the chemical state predicted by these models is strongly determined by the convective parameterization. This study applies the superparameterization (SP) framework in which a cloud-resolving model (CRM) is embedded in each grid cell of a global model to simulate explicit sub-grid processes. We use the SP framework in the NCAR Community Atmospheric Model (CAM). Compared to the conventional convective parameterization in CAM, SPCAM provides a more detailed treatment of the physical processes for dust, including small scale diffusion and plume-scale convection. Both CAM and SPCAM are operated in chemical transport mode so that the large-scale meteorological fields, including the large-scale velocities, are prescribed using the same meteorological reanalysis. Since the large-scale transports are identical between the two models, the only differences in the simulations arise from the differences between explicit versus parameterized treatments of convective transport.

Our focus is to investigate the effect SPCAM on aerosol distributions and their transport by comparing plots of dust distribution, cross section and vertical profiles. The preliminary results show the simulated dust distribution by SPCAM reveals a broader pattern, and high concentrations are in the source and nearby regions. The cross section of dust show similar patterns, with a spread-out dust feature simulated by SPCAM. The local dust concentration has a maximum at the surface and decrease with increase of height. The zonal average profiles of dust distribution reveal a peak at the latitude around 15N and a second peak at 42N. The first-order feature of using SPCAM on dust patterns is enhancement of vertical transport where dust is elevated to a higher altitude, as compared to CAM simulation. This may alter the atmospheric radiation distribution and will form the basis for future studies.