Sensitivity of Aerosol-Cloud Interactions to Subgrid-Scale Vertical Velocity in Convective Clouds

Aerosol-cloud interactions pose the largest uncertainties in using models to projecting future climate changes. One critical issue is related to parameterizing the subgrid-scale processes, in particular the vertical aerosol transport. Here we investigate the role of subgrid-scale vertical velocity variability on regulating aerosol-cloud interactions in convective clouds. In the study, the squall line case during the Intensive Observing Period 8 (IOP 8, 14-17 June) in Southwest Monsoon Experiment / Terrain-influenced Monsoon Rainfall Experiment (SoWMEX/TIMREX) in 2008 is adopted because of its clear convective cloud structure. We used a version of WRF model that includes a statistical-numerical semi-two-moment mixed-phase cloud scheme to conduct sensitivity simulations using several horizontal resolutions that generate different resolved vertical velocity fields. Fine-resolution simulation has better representation on squall line structure. Further analyses of simulations indicate that increased cloud droplet numbers and mixing ratio, and smaller effective radius are usually associated with stronger cloud-base vertical velocity. However, these features become weaker as vertical velocity variability decreases, especially for the cloud effective radius.