Elucidating the Microphysics of Aerosol-Shallow Cloud Interactions and the Reverse Orographic Enhancement of Rainfall in Complex Topography

In the Southern Appalachian Mountains (SAM), seeder-feeder interactions (SFI) among incoming storm systems and local low-level clouds and fog (LLCF), composed of very high number concentrations of small droplets (< 0.2 mm), lead to enhancement of the surface rainfall intensity up to one order of magnitude at low elevations and in inner mountain valleys. Indeed, similar phenomena can also be identified in middle mountains from Central America, the Himalayas, and the Andes. LLCF play a key role in governing plant water budgets, freshwater resources, and the terrestrial energy budget, and tend to be associated with biodiversity hotspots and biogenic aerosol maxima in regions of complex terrain. The objective of the research presented here is to elucidate the role of aerosol-cloud interactions in the formation and persistence of LLCF necessary to sustain SFI and high rainfall rates at low elevations.

To investigate the influence of aerosol properties (e.g., concentration, size distribution, and hygroscopicity) on LLCF formation, a cloud parcel model was implemented to describe key cloud microphysical processes including nucleation, condensation, collision-coalescence (with turbulence effects included), and lateral homogeneous entrainment. Ground-based measurements of aerosol and cloud condensation nuclei (CCN) spectra, and vertical profiles of cloud and rain droplets available from NASA’s Integrated Precipitation and Hydrology Experiment (IPHEx) in the SAM are utilized to perform and evaluate modeling studies of fog and feeder-cloud formation. Sensitivity assessment of the microphysical properties of LLCF to aerosol variations and initial thermodynamic conditions are also conducted in rigorous analysis of IPHEx case studies. To explore the impact of aerosol-shallow cloud interactions on SFI and subsequently quantify their contribution to low-level rainfall enhancement, the parcel model is coupled to Duke’s Rain Microphysics Column Model that simulates the dynamical evolution of raindrop microphysics (e.g., bounce, coalescence, and breakup mechanisms). Observations from IPHEx IOP (Intensive Observing Period) and previous IOPs in the SAM are used to elucidate the dynamics of local microphysical processes that govern in-column hydrometeor evolution from CCN activation to LLCF formation, and consequently low-level precipitation enhancement induced by SFI.