Impacts of Varying Concentrations of Cloud Condensation Nuclei on Deep Convective Cloud Updrafts-A Multimodel Assessment

This study presents results from a model intercomparison project, focusing on the range of responses in deep convective cloud updrafts to varying cloud condensation nuclei (CCN) concentrations among seven state-of-the-art cloud-resolving models. Simulations of scattered convective clouds near Houston, Texas, are conducted, after being initialized with both relatively low and high CCN concentrations. Deep convective updrafts are identified, and trends in the updraft intensity and frequency are assessed. The factors contributing to the vertical velocity tendencies are examined to identify the physical processes associated with the CCN-induced updraft changes. The models show several consistent trends. In general, the changes between the High-CCN and Low-CCN simulations in updraft magnitudes throughout the depth of the troposphere are within 15% for all of the models. All models produce stronger (similar to 15%-15%) mean updrafts from similar to 4-7 km above ground level (AGL) in the High-CCN simulations, followed by a waning response up to similar to 8 km AGL in most of the models. Thermal buoyancy was more sensitive than condensate loading to varying CCN concentrations in most of the models and more impactful in the mean updraft responses. However, there are also differences between the models. The change in the amount of deep convective updrafts varies significantly. Furthermore, approximately half the models demonstrate neutral-to-weaker (similar to 25% to 0%) updrafts above similar to 8 km AGL, while the other models show stronger (similar to 110%) updrafts in the High-CCN simulations. The combination of the CCN-induced impacts on the buoyancy and vertical perturbation pressure gradient terms better explains these middle- and upper-tropospheric updraft trends than the buoyancy terms alone.

Automated summary

The study presents results from a model intercomparison project on responses in deep convective cloud updrafts to varying cloud condensation nuclei (CCN) concentrations among different cloud-resolving models. While there are consistent trends, models also show differences in the magnitude of changes in deep convective updrafts and responses in the upper troposphere. The combination of impacts on buoyancy and vertical perturbation pressure gradient terms better explains middle- and upper-tropospheric updraft trends.