Abstract
Abstract. Volcanic eruptions eject ash and gases into the atmosphere that can contribute to significant hazards to aviation, public and environment health, and the economy. Several volcanic ash transport and dispersion (VATD) models are in use to simulate volcanic ash transport operationally, but none include a treatment of volcanic ash aggregation processes. Volcanic ash aggregation can greatly reduce the atmospheric budget, dispersion and lifetime of ash particles, and therefore its impacts. To enhance our understanding and modeling capabilities of the ash aggregation process, a volcanic ash aggregation scheme was integrated into the Weather Research Forecasting with online Chemistry (WRF-Chem) model. Aggregation rates and ash mass loss in this modified code are calculated in line with the meteorological conditions, providing a fully coupled treatment of aggregation processes. The updated-model results were compared to field measurements of tephra fallout and in situ airborne measurements of ash particles from the April–May 2010 eruptions of Eyjafjallajökull volcano, Iceland. WRF-Chem, coupled with the newly added aggregation code, modeled ash clouds that agreed spatially and temporally with these in situ and field measurements. A sensitivity study provided insights into the mechanics of the aggregation code by analyzing each aggregation process (collision kernel) independently, as well as by varying the fractal dimension of the newly formed aggregates. In addition, the airborne lifetime (e-folding) of total domain ash mass was analyzed for a range of fractal dimensions, and a maximum reduction of 79.5 % of the airborne ash lifetime was noted.
Highlights
Volcanic eruptions inject gases and ash particles of various sizes into the atmosphere, posing hazards to life, infrastructure, and aviation (Miller and Casadevall, 2000)
In an effort to study and predict volcanic ash aggregation effects using a fully coupled modeling system, where the fate of the airborne ash particles is coupled to the atmospheric environment, a volcanic ash aggregation scheme was incorporated into the Weather Research Forecasting with Chemistry (WRF-Chem) model (Grell et al, 2005)
The Brownian kernel became the major contributor to aggregation in the case of Df = 2.5, contributing to over 90 % of the aggregation. This agrees with parametric studies of varying fractal dimensions by Costa et al (2010), who noted this trade between ADS and AB when considering fine ash particles (< 63 μm)
Summary
Volcanic eruptions inject gases and ash particles of various sizes into the atmosphere, posing hazards to life, infrastructure, and aviation (Miller and Casadevall, 2000). FALL3D runs by ingesting offline meteorological fields from gridded atmospheric models, such as the Weather Research Forecasting (WRF) model, and calculating volcanic ash advection and sedimentation during the parent model output time step Another method of capturing volcanic ash aggregation is to initialize VATD models with PSDs that account for volcanic aggregation in the eruptive column by using initial plume models. In an effort to study and predict volcanic ash aggregation effects using a fully coupled modeling system, where the fate of the airborne ash particles is coupled to the atmospheric environment, a volcanic ash aggregation scheme was incorporated into the Weather Research Forecasting with Chemistry (WRF-Chem) model (Grell et al, 2005) This coupled system requires no temporal or spatial interpolations as it calculates interactions between the meteorology and ash at each modeling time step (on the order of seconds).
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