Exploratory MPAS Sensitivity Experiments on Rainforest Biogenic Salt Aerosols, Tropical Rainfall, and Poleward Moisture Transport

Brian Lue

Rainforest ecosystems emit biologically influenced aerosol particles, including potassium-rich and other hygroscopic components that may affect warm-rain microphysics. While tropical biogenic aerosols have been studied extensively, their sensitivity within coarse-resolution global models remains incompletely characterized, particularly under differing background aerosol states. Here we present exploratory sensitivity experiments using MPAS-Atmosphere to test whether rainforest biogenic salt aerosol parameterizations can alter modeled tropical precipitation and selected large-scale moisture-transport diagnostics.

Using 30-day single-member simulations at 240 km and 120 km resolution, we examined ten alternative implementations ranging from simple autoconversion perturbations to a more explicit giant-cloud-condensation-nuclei (GCCN) lifecycle treatment with activation, condensational growth, coalescence, and wet scavenging. Across these implementations, diagnosed latent heat transport at 30°N varied substantially (range −211 to +153 TW; standard deviation ~126 TW — comparable in magnitude to some published transport estimates), indicating strong sensitivity of some circulation metrics to aerosol microphysical assumptions within these experiments.

A more robust and interpretable result emerged locally: the sign of the modeled Amazon rainfall response depended on baseline aerosol loading. Under a polluted baseline aerosol state (MPAS default, ~4,400 nwfa/cm3), added hygroscopic K-salt particles suppressed local rainfall (−17.1 mm/30-day). Under a pristine low-CCN baseline informed by published Amazon observations (~150 cm-3; Pöhlker et al. 2012), the same perturbation enhanced local rainfall (+5.4 to +5.6 mm/30-day). This regime dependence is qualitatively consistent with established aerosol-cloud theory in which precipitation responses vary with background cloud condensation nuclei concentrations.

These simulations are not intended as detection or attribution evidence and are limited by coarse resolution, short integrations, single realizations, and convective parameterization uncertainty. Rather, they demonstrate that rainforest bioaerosol assumptions can materially influence modeled tropical rainfall responses and some transport diagnostics. The results motivate targeted follow-up using convection-permitting ensembles, observational constraints on rainforest aerosol emissions, and higher-fidelity cloud microphysics. We release all code, bug-fix patches, Docker builds, and analysis scripts at github.com/bluesaltbarrier/blue-salt-barrier.