Volcanic Aerosol Emission Impacts on Orographic Precipitation in Hawaii

In recent decades, multiple studies have reported a significant drying trend over the leeward side of the island of Hawaii, with many hypothesizing that the drying is associated with the volcanic aerosols emitted from the Kilauea Volcano. While it is clear that volcanic emissions, which include volcanic aerosols and gas phase sulfur dioxide (locally referred to as “vog”), can create hazardous air quality conditions to Hawaiian communities, the impact on rainfall is less clear. Here we investigate the hypothesis that vog may serve as cloud condensation nuclei (CCN) and affect localized cloud, convection and on-island rainfall through aerosol-cloud-interactions.

We utilize a daily high-resolution gridded rainfall map and the USGS observed daily SO₂ emissions record. Rainfall maps of high and low emission days are composited, and compared with emissions. Days with high SO₂ emissions are found to have less rainfall on the windward slope of the downstream mountainside. To investigate further, we use WRF with the aerosol-aware Thompson microphysics scheme to study the detailed physical processes. Sensitivity studies are simulated with different volcanic source amounts of aerosol emissions. Simulations with volcanic aerosol sources have large point sources of sulfate-like aerosol from the Kilauea vent locations. The added aerosols mix into the orographic convection where they modify the properties of the warm clouds: they increase the cloud droplet number concentration, reduce cloud droplet sizes, increase cloud water content and enhancing cloud evaporation. The volcanic aerosols also delay precipitation production, and modify the spatial distribution of rainfall on the mountainside. The impacts on cloud properties and precipitation are not surprising, but the modification of precipitation on an island has far reaching consequences. For this reason, we work to quantify the sensitivity of the orographic precipitation to volcanic aerosols, to understand the role of mountain meteorology in vog concentration at populated locations, and move beyond the observation of relationships to understanding of underlying processes.