Aerosol Transport and Removal in Deep Convection
Aerosol particles have an important role in the climate system by absorbing and/or scattering radiation as well as by changing cloud reflectivity (albedo), cloud lifetime, and precipitation. The aerosol effects depend in part on their concentration and vertical distribution, which is influenced by wet removal (by rain/snow) and vertical transport (how they move within the atmosphere). A team supported by the Atmospheric System Research program studied wet scavenging of aerosols (examining how aerosol particles are removed from clouds) by continental deep convective clouds for a supercell storm complex observed over Oklahoma during the Deep Convective Clouds and Chemistry campaign. The team developed a new passive-tracer-based transport analysis framework to characterize convective transport using vertical profiles of several passive trace gases. The new analysis framework is used to estimate the efficiency of aerosol wet scavenging and to evaluate cloud-resolving simulations made with the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem).
Compared to the new observation-based analysis, WRF-Chem greatly underestimates aerosol scavenging efficiencies by 32% and 41% for aerosol mass and number, respectively. Adding a new treatment of secondary activation, which allows aerosols to form cloud droplets not only at the cloud base but also above cloud base, significantly improved the simulations, producing results that are only 7% and 8% lower than observation-based estimates. This finding emphasizes the importance of secondary activation (above the cloud base) for aerosol wet removal in deep convective storms. This study provides a framework that can be extended to different types of storms and could be used to evaluate the diverse parameterizations of convective transport and wet scavenging used in global models.
Yang, Q., R. C. Easter, P. Campuzano-Jost, J. L. Jimenez, J. D. Fast, S. J. Ghan, H. Wang, L. K. Berg, M. C. Barth, Y. Liu, M. B. Shrivastava, B. Singh, H. Morrison, J. Fan, C. L. Ziegler, M. Bela, E. Apel, G. S. Diskin, T. Mikoviny, and A. Wisthaler. 2015. “Aerosol Transport and Wet Scavenging in Deep Convective Clouds: A Case Study and Model Evaluation Using a Multiple Passive Tracer Analysis Approach,” Journal of Geophysical Research Atmospheres 120(16), 8448–68. DOI: 10.1002/2015JD023647.