09:15 〜 09:30
[SVC29-02] Reconstructing the 19 February 2018 Sinabung Eruption Column by a 3D Numerical Model
キーワード：3D numerical model, Eruption column, Doppler weather radar, Pyroclastic density currents, Column collapse
Recent developments in observation and numerical simulation have revealed the dynamics of explosive eruptions, such as the rising of volcanic plumes and the generation of pyroclastic density current (PDC). A Doppler radar can capture the onset of explosions and the sequential progress of eruptions (Syarifuddin et al., 2022). However, more studies in radar observation of eruption clouds are needed to translate the observables into physical properties and eruption conditions (i.e., the mass fraction of volcanic ashes and mass eruption rate). In contrast, numerical models reproduce the global features of eruption clouds observed in some volcanoes (Suzuki and Iguchi, 2019). To date, there has not been any study to compare the internal structures of the eruption cloud simulated by the model to the observations. To bridge the gap between the observation and simulation, we investigated the short-lived explosive eruptions by combining the observed ground-based weather radar data and the three-dimensional (3-D) numerical model.
We applied our combined approach to a VEI-2 eruption of Sinabung Volcano, Indonesia, on 19 February 2018 at 08:53, accompanied by one collapsing column pyroclastic density current (PDC), ten dome collapse PDCs, and the disintegration of a lava dome. A Doppler radar at 7.9 km SE captured the entire explosive event. The first sign of the eruption column occurred at 08:54:08 with a registered exit velocity w0 of 45 m/s. At 08:57:52, radar estimated a maximum exit velocity w0 of 184 m/s. The explosive phase lasted for 607 s, with a mean w0 of 121 m/s. We used the estimated w0 values as initial conditions in the numerical model. The 3-D numerical model aims to reproduce the injection of a mixture of pyroclasts and volcanic gas from a circular vent (25 m radius) at 2460 m height. The fluid dynamics model solves a set of partial differential equations describing the conservation of mass, momentum, and energy and constitutive equations describing the thermodynamic state of the mixture of solid pyroclasts, volcanic gas, and air. The calculation of tracers with different particle sizes reproduced the transport of volcanic ashes. The meteorological condition showed a little variation of wind below the tropopause, and the magmatic properties were based on the literature review.
We carried out six simulations of the eruption plume with the exit velocity w0 ranging from 20-170 m/s. The 3-D model was best initiated by w0 scenarios of 100 and 130 m/s, which produced 9.5 and 12.5 km plume height, respectively. These findings are consistent with the Tokyo VAAC report and satellite observation (10–12.5 km). As the exit velocity changed with the fixed vent radius from the plausible exit velocity, hence, the mass eruption rate was estimated to be 5.15 – 6.71x105 kg/s. Despite the consistency of plume height with the observation, the simulations produced narrower plume width than the radar observation.
All scenarios failed to reproduce the collapsing plume process reported in the event case, even at the lowest w0 scenario. According to the sequence images of radar echo, the first stage of the eruption was dome-collapsed PDCs, followed by an eruption plume collapse, which generated a different type of PDC. In our simulations, we assumed an ideal situation by ignoring the dome collapse and temporal variation of eruption conditions. Therefore, we propose a new mechanism for PDC generation. Essentially, a volcanic plume with a small mass eruption rate, such as Sinabung, easily gets buoyancy and does not generate collapsing plume PDCs. When the dome collapse and its PDCs precede, the following plume is surrounded by the co-ash clouds associated with the dome-collapse PDCs. Hence, the entrained fluid consists of the material of the lava dome plus already heated air, which cannot expand and get buoyancy as it is. The result is a partial collapse of the eruption column at the SE sector concurrent with a vertically rising plume at NW.
We applied our combined approach to a VEI-2 eruption of Sinabung Volcano, Indonesia, on 19 February 2018 at 08:53, accompanied by one collapsing column pyroclastic density current (PDC), ten dome collapse PDCs, and the disintegration of a lava dome. A Doppler radar at 7.9 km SE captured the entire explosive event. The first sign of the eruption column occurred at 08:54:08 with a registered exit velocity w0 of 45 m/s. At 08:57:52, radar estimated a maximum exit velocity w0 of 184 m/s. The explosive phase lasted for 607 s, with a mean w0 of 121 m/s. We used the estimated w0 values as initial conditions in the numerical model. The 3-D numerical model aims to reproduce the injection of a mixture of pyroclasts and volcanic gas from a circular vent (25 m radius) at 2460 m height. The fluid dynamics model solves a set of partial differential equations describing the conservation of mass, momentum, and energy and constitutive equations describing the thermodynamic state of the mixture of solid pyroclasts, volcanic gas, and air. The calculation of tracers with different particle sizes reproduced the transport of volcanic ashes. The meteorological condition showed a little variation of wind below the tropopause, and the magmatic properties were based on the literature review.
We carried out six simulations of the eruption plume with the exit velocity w0 ranging from 20-170 m/s. The 3-D model was best initiated by w0 scenarios of 100 and 130 m/s, which produced 9.5 and 12.5 km plume height, respectively. These findings are consistent with the Tokyo VAAC report and satellite observation (10–12.5 km). As the exit velocity changed with the fixed vent radius from the plausible exit velocity, hence, the mass eruption rate was estimated to be 5.15 – 6.71x105 kg/s. Despite the consistency of plume height with the observation, the simulations produced narrower plume width than the radar observation.
All scenarios failed to reproduce the collapsing plume process reported in the event case, even at the lowest w0 scenario. According to the sequence images of radar echo, the first stage of the eruption was dome-collapsed PDCs, followed by an eruption plume collapse, which generated a different type of PDC. In our simulations, we assumed an ideal situation by ignoring the dome collapse and temporal variation of eruption conditions. Therefore, we propose a new mechanism for PDC generation. Essentially, a volcanic plume with a small mass eruption rate, such as Sinabung, easily gets buoyancy and does not generate collapsing plume PDCs. When the dome collapse and its PDCs precede, the following plume is surrounded by the co-ash clouds associated with the dome-collapse PDCs. Hence, the entrained fluid consists of the material of the lava dome plus already heated air, which cannot expand and get buoyancy as it is. The result is a partial collapse of the eruption column at the SE sector concurrent with a vertically rising plume at NW.