Effects of cloud condensation nuclei concentration on the evolution of severe convective storms

Abstract

Idealized three-dimensional storm simulations were created to investigate the effects of varying initial cloud condensation nuclei (CCN) concentrations (CCNCs) ahead of the severe convective storm using the Weather Research and Forecasting (WRF) model with the National Severe Storms Laboratory (NSSL) double-moment microphysics scheme. The simulations showed that increased initial CCNC ahead of the storm (CCN mainly entrains from the boundary layer) increased the rain- and hailfall intensities. Multiple strong centers of rain- and hailfall appeared at the surface along the moving direction of the convective storm. The increased initial CCNC ahead of the storm played an important role in the warm cloud processes. As the CCNC increased, a large amount of CCN enter the cloud and was activated, which decreased the equivalent sphere radius of the cloud droplets and increased the content of cloud droplets. Thus, the equivalent sphere radius of the raindrops increased due to the enhanced collection process of cloud droplets by raindrops. Furthermore, the collection processes of supercooled cloud droplets and raindrops by hail were also enhanced as the CCNC increased, resulting in an increase in the content of hail. These processes further influenced the cold cloud processes. The increase in CCNC ahead of the storm accelerated the downdraft by the cloud microphysical processes and formed a stronger cold pool and front outflow. These thermodynamic and dynamic processes were favorable for increasing the updraft and thus enhancing the cloud microphysical processes. Therefore, the increase in the CCNC ahead of the storm created a positive feedback mechanism to the thermodynamic and dynamic processes by the cloud microphysical processes, which promoted the development and maintenance of the storm and increased rain- and hailfall intensities at the surface.