A Computational Methodology for the Calibration of Tephra Transport Nowcasting at Sakurajima Volcano, Japan

Abstract

Ground-based remote sensing equipment have the potential to be used for the nowcasting of the tephra hazard from volcanic eruptions. To do so raw data from the equipment first need to be accurately transformed to tephra-related physical quantities. In order to establish these relations for Sakurajima volcano, Japan, we propose a methodology based on high-resolution simulations. An eruption that occurred at Sakurajima on 16 July 2018 is used as the basis of a pilot study. The westwards dispersal of the tephra cloud was ideal for the observation network that has been installed near the volcano. In total, the plume and subsequent tephra cloud were recorded by 2 XMP radars, 1 lidar and 3 optical disdrometers, providing insight on all phases of the eruption, from plume generation to tephra transport away from the volcano. The Weather Research and Forecasting (WRF) and FALL3D models were used to reconstruct the transport and deposition patterns. Simulated airborne tephra concentration and accumulated load were linked, respectively, to lidar backscatter intensity and radar reflectivity. Overall, results highlight the possibility of using such a high-resolution modelling-based methodology as a reliable complementary strategy to common approaches for retrieving tephra-related quantities from remote sensing data.

Seasonal Variations of Volcanic Ash and Aerosol Emissions around Sakurajima Detected by Two Lidars

“…Differences between the peaks in the extinction coefficient near the SVO lidar and the density of ash fall at Kagoshima observatory imply differences in the size distribution of ash particles erupted during the study period. The results of our analysis of the optical properties of transported particulates around Sakurajima could be combined with estimates of the amount of tephra emitted during each eruption to simulate the advection and sedimentation of aerosols by using the transport model of Poulidis et al [19]. Extinction coefficients observed by lidar can thus be used to validate such future numerical modeling results.…”

New insights into real-time detection of tephra grainsize, settling velocity and sedimentation rate

“…Indeed, our measurements were performed under small tephra plumes produced by relatively weak Vulcanian explosions (max plume heights of about 2 km above sea level) and under similar weather conditions. This is why comparing both disdrometer estimates with other methods and for different eruptive/atmospheric conditions (e.g., under rainfall and/or extreme ash fallout) would help to test and verify their capacity for operational monitoring and the calibration of other remote sensing systems such as Doppler radar 8 , 9 , 12 or dispersal models 10 , 11 .…”

“…During the last two decades, weather-dedicated radar and optical disdrometers have been introduced in volcanology to determine the size and velocity of tephra falling at various volcanoes such as Etna 5 , Eyjafjallajökull 6 , Sakurajima 7 – 11 and Stromboli 12 – 14 . The optical disdrometers have the advantage of determining size and velocity simultaneously, allowing for a better characterization of tephra particles, whereas radar disdrometers require additional backscatter models to provide particle sizes 5 , 6 , 15 .…”

“…As the OTT Parsivel 2 (called PS2 hereafter), the relatively low-cost LPM (i.e., currently twice cheaper than PS2 with prices < 4 kUSD) is one of the most used optical disdrometer for meteorological studies 25 and monitoring liquid and solid hydrometeor precipitation. The PS2 has been increasingly used in volcanology to monitor tephra fallout 7 – 9 , 11 ; however, in addition to the low cost that could increase the potential number of instruments deployed in the field, the LPM has some additional advantages in comparison to PS2. Firstly, data acquired on each measured particle can be accessed (i.e., “drop by drop” mode; see “ Materials and methods ” section).…”

“…A network of 12 PS2 disdrometers (at the time of the field campaign; Fig. 1 a,b) has been installed around the Sakurajima peninsula since 2017 to monitor tephra fallout 11 , 22 , 31 , 32 , 34 . Our main objective is to explore the potential of LPM to provide the size distribution of individual volcanic particles in real time and mitigate the impact of particle aggregation.…”