Toward Elucidating the Role of Daytime Aerosol–Cloud Interactions in the Diurnal Cycle of Orographic Rainfall

Topographic complexity favors the diurnal development of regional-scale convergence patterns that provide the moisture source for low-level clouds and fog (LLCF). Low-level moisture and cloud condensation nuclei (CCN) are distributed by ridge-valley circulations favoring LLCF formation that modulate the diurnal cycle of orographic rainfall, and in particular the mid-day peak. The objective of the research presented here is to elucidate the indirect effects of aerosols on the microphysical pathways of cloud formation and precipitation evolution in mountainous regions, and to unveil the interplay between microphysical and dynamical processes of clouds and precipitation in the warm season.

The Advanced Weather Research and Forecasting (WRF) model was used to investigate the CCN sensitivity of summer rainfall events during the Integrated Precipitation and Hydrology Experiment (IPHEx). CCN spectra estimated from the in situ measurements in the inner SAM during IPHEx were incorporated in the model microphysics scheme to quantitatively assess the impacts on cloud formation and precipitation development to changes in CCN characteristics. Further, numerical experiments were conducted with and without terrain features and with various microphysical parameterizations using the standard continental CCN spectrum in the model and the local CCN spectrum from IPHEx with high concentrations of fine aerosols. The simulated cloud droplet number concentrations using the local spectrum show better agreement with IPHEx airborne observations and better replicate the widespread low-level cloudiness around mid-day over the inner SAM region. Simulations with local aerosol spectra simulation also indicate an important role of early afternoon precipitation with enhanced ice processes tied to more vigorous vertical development of individual storm cells in conditioning the lower troposphere and strongly impact the storm structure and organization of late afternoon and evening rainfall. The findings in this study emphasize the importance of specifying regional-representative CCN to provide a realistic description of local cloud features and subsequently conduct detection and attribution studies of aerosol-cloud interactions on precipitation initiation and evolution.