9:00 AM - 9:15 AM
[MIS23-01] Development of a physics-based source model for JMA tephra-dispersal forecasting
★Invited Papers
Keywords:Volcanic ash, Volcanic plume, Eruption column model, Gravity current model
To improve the accuracy of operational tephra-dispersal forecasting, we have developed a physics-based source model. Our model is based on recent studies on eruption plumes, such as plume dynamics model (e.g. Woods 1988; Bursik 2001; Suzuki and Koyaguchi, 2015) and sedimentation theory (e.g. Bursik et al. 1992; Koyaguchi et al. 2009). An early version of this model was developed at the University of Tokyo, thereafter, it was provided to the Japan Meteorological Agency.
Our model (NIKS-1D; Ishii et al. 2022) is defined as a combination of two models: an eruption column model below the NBL, and a downwind gravity current model around the NBL. The eruption column model describes the process of the plume rising as a result of the reduced density due to turbulent entrainment and thermal expansion of cold ambient air (e.g., Woods 1988; Burisk 2001), whereas the downwind gravity current model includes dynamics that express the crosswind spreading of a plume as a gravity current coupled with downwind advection (Bursik et al. 1992). In addition, the particle sedimentation theory (Martin and Nokes, 1988) is applied to both the models to describe the particle segregation process from the side of the eruption column and the base of the downwind gravity current (e.g. Bursik et al. 1992; Bursik 2001; Koyaguchi et al. 2009). The particle segregation distribution (referred to as SMD: Source Magnitude Distribution) can be used as a source for tephra-dispersal models. Our model reproduces the following features of the SMD for typical eruption conditions: (1) a significant amount of coarse particles are released from the rising eruption plume, whereas most of the fine particles are carried to the NBL, (2) in a downwind gravity current, the coarse particles tend to decrease more rapidly with distance from the vent compared to the fine particles, (3) the SMD from the downwind gravity current decreases with distance more slowly in a strong ambient wind than that in a weak ambient wind, and (4) the SMD from the downwind gravity current for eruptions with large mass eruption rates decreases with distance more slowly than that for eruptions with small eruption rates.
Our model includes a new parameter that connects the eruption column model and the downwind gravity current model. In order to combine the two models, the volumetric flux at the NBL calculated by the eruption column model is used as an initial condition for the downwind gravity current model. Generally, a significant amount of ambient air is entrained into the plume around the NBL due to complex interacting upward and downward plumes (e.g. Devenish and Cerminara 2018). This effect can be expressed by a new parameter μ, which is defined as a ratio of the volumetric fluxes of downwind gravity current and eruption column at the NBL. We determined the value of μ as a function of the mass eruption rate by using the meteorological satellite images of the downwind gravity current for two well-studied eruption events with different mass eruption rates.
Our model (NIKS-1D; Ishii et al. 2022) is defined as a combination of two models: an eruption column model below the NBL, and a downwind gravity current model around the NBL. The eruption column model describes the process of the plume rising as a result of the reduced density due to turbulent entrainment and thermal expansion of cold ambient air (e.g., Woods 1988; Burisk 2001), whereas the downwind gravity current model includes dynamics that express the crosswind spreading of a plume as a gravity current coupled with downwind advection (Bursik et al. 1992). In addition, the particle sedimentation theory (Martin and Nokes, 1988) is applied to both the models to describe the particle segregation process from the side of the eruption column and the base of the downwind gravity current (e.g. Bursik et al. 1992; Bursik 2001; Koyaguchi et al. 2009). The particle segregation distribution (referred to as SMD: Source Magnitude Distribution) can be used as a source for tephra-dispersal models. Our model reproduces the following features of the SMD for typical eruption conditions: (1) a significant amount of coarse particles are released from the rising eruption plume, whereas most of the fine particles are carried to the NBL, (2) in a downwind gravity current, the coarse particles tend to decrease more rapidly with distance from the vent compared to the fine particles, (3) the SMD from the downwind gravity current decreases with distance more slowly in a strong ambient wind than that in a weak ambient wind, and (4) the SMD from the downwind gravity current for eruptions with large mass eruption rates decreases with distance more slowly than that for eruptions with small eruption rates.
Our model includes a new parameter that connects the eruption column model and the downwind gravity current model. In order to combine the two models, the volumetric flux at the NBL calculated by the eruption column model is used as an initial condition for the downwind gravity current model. Generally, a significant amount of ambient air is entrained into the plume around the NBL due to complex interacting upward and downward plumes (e.g. Devenish and Cerminara 2018). This effect can be expressed by a new parameter μ, which is defined as a ratio of the volumetric fluxes of downwind gravity current and eruption column at the NBL. We determined the value of μ as a function of the mass eruption rate by using the meteorological satellite images of the downwind gravity current for two well-studied eruption events with different mass eruption rates.