Enhancing Volcanic Eruption Simulations with the WRF-Chem Model

Volcanic eruptions are one of the major natural hazards, exerting profound effects on the environment, economy, and infrastructure. The emissions associated with such eruptions pose substantial risks to terrestrial systems and public health, particularly through the induction of acid rain and air pollution. Additionally, these emissions impact the climate by releasing sulfur dioxide (SO₂), which subsequently undergoes conversion into sulfate aerosols due to oxidation by hydroxyl radicals (OH) and hydrogen peroxide (H₂O₂). Sulfate aerosols, SO₂, and volcanic ash influence extensive populations at distances reaching several thousand kilometers from the erupted volcano. In addition, information on ash concentration and the location of the volcanic cloud is crucial for air traffic control. Considering these aspects, accurate modeling of the transport and deposition of volcanic debris is essential. Among the available forecasting tools, the online WRF-Chem Eulerian model is distinguished for its capability to simulate the transport and deposition of volcanic debris.

Here, we enhance the existing and add new functionalities to the WRF-Chem code. In particular, we account for major sinks (wet and dry deposition of ash, sulfate, and chemical transformation of SO₂). We identified and rectified a bug in the subroutine for gravitational deposition of the ash. Due to this bug, the ash mass balance was violated. Furthermore, we established a mass balance for sulfate, SO₂, and ash by incorporating diagnostic variables into the model's output. Additionally, we corrected the deposition velocity of the coarse (>60 microns in diameter) ash particles and integrated gravitational settling for sulfate aerosols.

Emissions of sulfate along with water vapor, which are other (along with ash and SO₂) constituents of a typical volcanic eruption, were also added to the model code. Water vapor is an important greenhouse gas. The recent underwater eruption of the Hunga-Tonga volcano released approximately 150 Mt of water vapor, which affected the dynamics of the debris cloud as a result of the radiative cooling of the water vapor cloud.

The WRF-Chem code has been further enhanced to incorporate the direct radiative effects of ash and sulfate aerosols, acknowledging the substantial radiative forcing exerted by volcanic eruptions on the climate system. In particular, the ash released into the upper atmosphere can inhibit sunlight from reaching the Earth's surface for an extended period, cooling the surface and causing disruptions in ecosystems and agriculture. The stratospheric sulfate aerosol clouds can persist from a few months to a couple of years, reflecting solar radiation into space and causing global cooling.

In addition, we developed an open-source emission preprocessor written in Python. In comparison with the existing PREP-CHEM-SRC utility, our tool facilitates the workflow and adds flexibility in prescribing the volcanic eruption process given the eruption source parameters.

We demonstrate the effect of changes and additions implemented into the WRF-Chem code. The capabilities added to the code allow for significant advancement in volcanic debris forecasting and studies of the effects of volcanic eruptions on climate.