Abstract
Tephra plumes can cause a significant hazard for surrounding towns, infrastructure, and air traffic. The current work presents the use of a small and compact X-band multi-parameter (X-MP) radar for the remote tephra detection and tracking of two eruptive events at Merapi Volcano, Indonesia, in May and June 2018. Tephra detection was performed by analysing the multiple parameters of radar: copolar correlation and reflectivity intensity factor. These parameters were used to cancel unwanted clutter and retrieve tephra properties, which are grain size and concentration. Real-time spatial and temporal forecasting of tephra dispersal was performed by applying an advection scheme (nowcasting) in the manner of an ensemble prediction system (EPS). Cross-validation was performed using field-survey data, radar observations, and Himawari-8 imageries. The nowcasting model computed both the displacement and growth and decaying rate of the plume based on the temporal changes in two-dimensional movement and tephra concentration, respectively. Our results are in agreement with ground-based data, where the radar-based estimated grain size distribution falls within the range of in situ grain size. The uncertainty of real-time forecasted tephra plume depends on the initial condition, which affects the growth and decaying rate estimation. The EPS improves the predictability rate by reducing the number of missed and false forecasted events. Our findings and the method presented here are suitable for early warning of tephra fall hazard at the local scale.
Highlights
Tephra is the fragmented material produced during an explosive volcanic eruption.Once in the atmosphere, the mix of tephra, volcanic gases, and ambient air forms a volcanic plume
All the predictability rate parameters indicate the tendency of skill to decrease with time, we still highlight the importance of ensemble prediction system (EPS) in improving the accuracy of the tephra forecast, as it can maintain the critical success index (CSI) and probability of detection (POD) to be greater than 0.99 for entire prediction time-steps
This study has demonstrated the potency of an X-band multi-parameter (X-MP) radar to detect and forecast tephra plume in Merapi
Summary
Tephra is the fragmented material produced during an explosive volcanic eruption. The mix of tephra, volcanic gases, and ambient air forms a volcanic plume. Tephra is classified according to its size as volcanic bombs or blocks (D ≥ 64 mm or φ ≤ −6), lapilli (2 mm ≤ D < 64 mm or −1 ≥ φ > −6), coarse ash (64 μm ≤ D < 2 mm or 4 ≥ φ > −1), and fine ash (D < 64 μm or φ > 4), where D is the diameter of the particle and φ ≡ −log D (mm). Because of the significance of the hazards posed by a tephra fall, its timely detection and tracking in the atmosphere is very important. Because of the significance of the hazards posed by a tephra fall, its timely detection and tracking in the atmosphere is very important. 4.0/).
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