Mapping the Impact of Aerosol-Cloud Interactions on Low Level Rainfall in Mountainous Regions Using a Column Model

Long-term observations and model simulations in the Southern Appalachian Mountains indicate the local fog and low level clouds (LLC), composed of very high number concentration of small droplets (< 0.2 mm), can significantly enhance the surface rainfall intensity by increasing coalescence rates and efficiency when interacting with incoming storm systems. However, the attribution of these low-level enhancement of precipitation to the cloud condensation nuclei (CCN) availability and local microphysical processes remains elusive.

In order to explore the impact of aerosol on the formation and evolution of the vertical structure of fog/LLC, Duke's Rain Microphysics Column Model that simulates the dynamical evolution of raindrop microphysics (e.g., bounce, coalescence and breakup mechanisms and seeder-feeder interactions among stratiform rainfall and LLC and fog) was coupled to an adiabatic cloud parcel model for describing the growth and evolution of a cloud droplet spectrum from a given population of aerosols. The performance of the newly implemented aerosol-cloud-rainfall column model is evaluated using the field data (aerosol, cloud and rainfall) from the Integrated Precipitation and Hydrology Experiment (IPHEx) toward detailed space-time mapping of aerosol-cloud interactions on the vertical structure of precipitation microstructure in the lower 2-3 km of the troposphere. Accurate ground-based measurements of the complete spectra of aerosol, CCN, and vertical profiles of cloud and rain droplets available at the Maggie Valley (MV) supersite from IPHEx will be utilized to perform modeling studies of the fog/feeder cloud formation with CCN activation included. Several new particle formation events were identified during IPHEx intensive operating periods (IOP), which can play a significant role in forming CCN and perhaps fog/cloud droplets. The multi-sensor IPHEx IOP observations at MV will be analyzed and integrated to study the microphysical processes that govern in-column hydrometeor evolution from CCN activation to fog/LLC formation and consequently precipitation. The model simulations will also be used to investigate the sensitivity of the aerosol variations to the microphysical properties of fog/LLC and precipitation development.