In recent years, volcanic disasters have occurred repeatedly in Japan. In response, domestic efforts are underway to consider measures for dealing with large-scale eruptions. The Journal of Disaster Research has published several special issues that focus on volcanic disasters, featuring research outcomes related to the observation and investigation of volcanoes and disaster prevention measures. This special issue features papers that reflect the recent trends in volcanic disaster research. Topics include observational and analytical studies of volcanic ejecta, damage assessments based on volcanic ejecta, research on debris flows triggered by rainfall following ash and tephra deposition, and studies on aviation safety and evacuation planning for residents in the event of a large-scale eruption. Iguchi and Ishii proposed a forecasting system for ashfall amounts before eruptions around the Sakurajima Volcano by utilizing the statistical properties of ground deformation parameters associated with numerous eruptions. Kumaoka et al. proposed an airport alert system to address volcanic ash risks from large-scale eruptions at Sakurajima and analyzed the capacity of airports to accommodate evacuating aircraft. Shimizu et al. used 10 years of light detection and ranging observation data from Sakurajima to propose a threshold for distinguishing between clouds and volcanic ash. Otsuka et al. investigated small-scale spatial variations in ashfall in urban areas through numerical simulations, highlighting that wind disturbances caused by buildings significantly impact ashfall distribution, depending on the particle size. Nakada et al. demonstrated the utility of a method that automatically generates ashfall distribution contour maps and estimates the ash volume based on ashfall survey results from multiple domestic eruptions. Shimano et al. conducted colorimetric measurements on volcanic ash samples collected daily at Sakurajima and performed principal component analysis, showing that color measurement is a rapid and quantitative method suitable for monitoring volcanic activity. Takebayashi investigated river basins in the southwestern part of Sakurajima, where debris flows occur frequently, and clarified the relationships between permeability differences, gully formation rates, and sediment yield. Tsutsumi et al. examined the generation process of snowmelt-type volcanic mudflows through laboratory experiments. They revealed that when snow and gravel were mixed well, snowmelt accelerated, providing valuable insights into disaster prevention measures against such mudflows. Sugo et al. introduced a collaborative disaster preparedness effort between experts and residents in anticipation of a future large-scale eruption at Sakurajima that may cause heavy pumice and ashfalls in urban areas. They reported behavioral changes in both individual preparedness actions and communication-related activities involving others within the framework of this collaborative effort. We hope that the results of advanced research presented in this special issue will be utilized to prevent and mitigate volcanic disasters.

We developed a method for estimating the height and growth rate of volcanic eruption columns, at high-temporal resolution, by processing vertical cross-sectional images of areas around the crater obtained with a marine radar tilted on its side. We applied our method to 127 eruptions occurring at Sakurajima (Kagoshima, Japan) from June to December 2019 and successfully estimated the time-series height of the eruption column and its growth rate every 2.5 seconds. In 48 cases, we obtained the maximum height of the eruption column and confirmed that these results were consistent with those estimated using meteorological radar. Although the maximum height estimated with our method tended to be lower than that observed by monitoring cameras, results could be obtained even when observations were difficult due to cloud effects, etc.

In the present study, we analyzed the particle size distribution (PSD) of falling volcanic ash particles measured using optical disdrometers during six explosive eruptions of the Sakurajima volcano in Kagoshima Prefecture, Japan. Assuming the gamma PSD model, which is commonly used in radar meteorology, we examined the relationships between each of the gamma PSD parameters (the intercept parameter, the slope parameter, and the shape parameter) calculated by the complete moment method. It was shown that there were good correlations between each of the gamma PSD parameters, which might be one of the characteristics of falling volcanic ash particles. We found from the normalized gamma PSD analysis that the normalized intercept parameter and mass-weighted mean diameter are suitable for estimating the ash fall rate. We also derived empirical power law relationships between pairs of integrated PSD parameters: the ash fall rate, the volcanic ash mass concentration, the reflectivity factor, and the total number of ash particles per unit volume. The results of the present study provide essential information for studying microphysical processes in volcanic ash clouds, developing a method for quantitative ash fall estimation using weather radar, and improving ash transport and sedimentation models.

This paper is submitted to accompany the article “Analyses of three-dimensional weather radar data from volcanic eruption clouds” [1]; it describes three-dimensional (3D) visualizations of the Sakurajima volcanic eruption clouds and the weather radar data used for analyses, as well as their availability and downloading procedures. The radar data were acquired by an operational X-band weather radar located approximately 11 km south of the Showa vent of Sakurajima in Kagoshima, Japan. The original raw radar data are available from the “XRAIN Precipitation Original Data search and Download System”, which is hosted on the website “Data Integration and Analysis System (DIAS)”. Animated images of the radar data shown here, which provide a visual explanation of the temporal evolution and the inner structure of volcanic eruption clouds, were created using the program “Analysis Tools of Three-dimensional Weather Radar Data (AN3D)”. The detailed methods of ANT3D are provided in the co-submitted article “Construction of three-dimensional weather radar data from volcanic eruption clouds” [2].

Analysis tools of three-dimensional weather radar data (ANT3D) was originally developed at the National Research Institute for Earth Science and Disaster prevention (NIED) to retrieve three-dimensional (3D) precipitation and wind fields for convective storms. In 2013, Kagoshima University significantly revised ANT3D for analyses of volcanic eruption clouds, mainly to improve the temporal and spatial interpolation of radar data and estimation of the advection vector, which is required for temporal interpolation. Detailed information pertaining to these algorithms is listed as additional information in this paper.•Procedures necessary for the construction of three-dimensional (3D) volcanic cloud weather radar data are described.•An algorithm based on temporal and elevation angle interpolation methods was used to create 3D constant altitude plan position indicator (3D CAPPI) data with high temporal and spatial resolution.•Two programs (ANT3D_GUI and the CAPPI viewer) are provided for readers interested in analyzing volcanic eruption cloud radar data.

Real-time monitoring of volcanic tephra fallout rate is an important factor to predict ash plume dispersion and to mitigate risk to air traffic. Ground-based weather radar has been one of the fundamental instruments to detect the plume and derive eruptive source parameters, such as the tephra fallout rate. The current work presents the use of two small and compact X-band Multi-Parameter (X-MP) radars for a new tephra fallout rate model development and the technical aspects of the system in Sinabung and Merapi Volcanoes. The new model estimates the tephra fallout rate using two radar parameters: the specific differential phase shift parameter and the reflectivity intensity factor. Total cumulated mass estimated from the radar-based tephra fallout rate model from the radar is compared with the plume height model and an empirical radar-based model. A volcanic eruptive index (VEI)-2 of Sinabung generated a plume exceeding 15 km, resulting in a maximum tephra fallout rate of 0.58 kg m-2 h-1 and a total tephra mass of 51 106 kg. The VEI 1 of Sinabung caused a plume height of 2.5 km, resulting in a maximum tephra fallout rate of 0.3 kg m-2 h-1 and a total cumulated tephra of 9 106 kg. In the last case, a VEI 1 eruption of Mt. Merapi produces a 6 km plume, resulting in a maximum tephra fallout rate of 0.28 kg m-2 h-1 and a total cumulated tephra of 35 106 kg. The sector range height indicator scan-mode strategy in the VEI 2 eruption of Mt. Sinabung ran at six degrees azimuth angles capturing only a partial volume of the plume. Thus, the total mass was only 22 % of the result from the empirical plume height model, even though the plume height was assumed to be equally the same with the maximum height scanned of radar at 7 km. In contrast, the volumetric scan by a plan position indicator strategy gave a total cumulated tephra mass, that matches better to the result of the empirical plume height model at 65-92%. Based on these results and the ability of the X-MP radar to capture the volcanic plume at the same reported onset time, we can confirm the importance of an X-MP radar for real-time tephra fallout monitoring during an eruption.

We investigated the main characteristics of tephra fall from the eruptions at Sakurajima volcano in southern Kyushu, Japan, by using an optical disdrometer for simultaneous measurement of particle size and fall velocity. Measurements were carried out for 2 years at one station located 2.5 km south of the active crater. This made it possible to detect 76 tephra fall events, with the detection rate of the tephra events from these south-directed eruption clouds being about 25%. From the relationship between particle diameter and fall velocity, we were able to estimate morphological parameters of volcanic particles. Measurements of temporal change in the tephra fall for each minute revealed that both the tephra diameter and fall velocity gradually decrease with time during each event, where the ranges of the diameter and velocity for the events with south-directed eruption clouds were 0.336–2.04 mm and 1.40–6.24 m s−1, respectively. By using temporal changes in the diameter and velocity, we estimated travel time and distance for the tephra falling from a release point in the eruption cloud to the station, and found a positive correlation between the travel distance and column height for the south-directed clouds, with a determination coefficient of 0.498 for linear fitting. The comparison between tephra data and seismic data revealed that episodic increases in the tephra diameter and fall velocity were correlated with a drastic increase in amplitude of explosion earthquake or tremor. These results indicate that disdrometer measurements can provide valuable information about the quantitative features of the tephra fall and are useful for detecting changes in the magnitude of eruption if a number of suitably placed instruments are deployed.

Abstract. Information of aerodynamic parameters of volcanic ash particles, such as terminal velocity, axis ratio, and canting angle, are necessary for quantitative ash-fall estimations with weather radar. In this study, free-fall experiments of volcanic ash particles were accomplished using a two-dimensional video disdrometer under controlled conditions. Samples containing a rotating symmetric axis were selected and divided into five types according to shape and orientation: oblate spheroid with horizontal rotating axis (OH), oblate spheroid with vertical axis (OV), prolate spheroid with horizontal rotating axis (PH), prolate spheroid with vertical rotating axis (PV), and sphere (Sp). The horizontally (OH and PH) and vertically (OV and PV) oriented particles were present in proportions of 76 % and 22 %, and oblate and prolate spheroids were in proportions of 76 % and 24 %, respectively. The most common shape type was OH (57 %). The terminal velocities of OH, OV, PH, PV, and Sp were obtained analyzing 2-D video disdrometer data. The terminal velocities of PV were highest compared to those of other particle types. The lowest terminal velocities were found in OH particles. It is interesting that the terminal velocities for OH decreased rapidly in the range 0.52 mm. The histogram of canting angles followed unimodal and bimodal distributions with respect to horizontally and vertically oriented particles, respectively. The mean values and the standard deviation of entire particle shape types were close to 0 and 10∘, respectively, under calm atmospheric conditions.

This paper presents the major specifications and characteristics of the Ku-band high-speed scanning Doppler radar for volcano observation (KuRAD) introduced to Kagoshima University in March 2017 as well as the results of a test observation at Sakurajima. KuRAD is a Doppler radar for research with a wavelength of approximately 2 cm and uses a 45 cm diameter Luneberg lens antenna as a transmitting and receiving antenna to observe the development of a volcanic eruption column immediately following eruption at a maximum rotation speed of 40 rpm. The maximum transmitter power is 9.6 W and the maximum observational range is 20 km. Observed data includes radar reflectivity factor, Doppler velocity, and Doppler spectrum width. Another feature of KuRAD is an obtained radio station license for observation of a total of seven active volcanos in Kyushu. To assess the basic performance of KuRAD, we carried out test observations of volcanic eruptions at Sakurajima, Kagoshima Prefecture, Japan and collected a total of 87 eruptions (20 of which are explosive eruptions and 7 of which had 3,000 m or higher eruptive smoke from vents). From the eruption data of Showa vent on May 2, 2017, it was confirmed that KuRAD could monitor the three-dimensional internal structure of a volcanic eruption column immediately following eruption. Eruption data from Minamidake of Sakurajima on March 5, 2018, showed that KuRAD successfully observed the eruptive smoke reaching a height of 4,000 m, although the eruptive smoke was covered with clouds and could not be detected by optical instruments of the Japan Meteorological Agency.

Abstract. Radar variables of volcanic ash clouds are dependent on microphysical processes and can be expressed using physical parameters of volcanic ash particles, such as terminal velocity, axis ratio, and canting angle, which are necessary for quantitative ash-fall estimations. In this study, free-fall experiments of volcanic ash were accomplished using a two-dimensional video disdrometer under controlled conditions. Samples containing a rotating symmetric axis were selected and divided into five types according to shape and orientation, i.e., oblate and prolate spheroids with horizontally and vertically oriented axes and spheres. The horizontally and vertically oriented particles were present in proportions of 75.5 % and 21.6 %, and oblate and prolate spheroids were in proportions of 76.2 % and 23.8 %, respectively. The most common shape type was a horizontally oriented oblate spheroid (57.3 %). The terminal velocities were classified according to shape type. The terminal velocities of prolate spheroids (vertically oriented) particles were higher than those of oblate spheroids (horizontally). Terminal velocities were in the range 0.5 < volume–equivalent spherical particle diameter (D) < 1 mm for OH because of an increase in axis ratio and a sharp decrease in sample size from D < 0.7 mm. The axis ratios fell over a wide range, from 0 to 1.5, at D < 2 mm, but converged to 0.94 at D > 2 mm. The histogram of canting angles followed unimodal and bimodal distributions with respect to horizontally and vertically oriented particles, respectively. The mean values were close to 0° and the standard deviation for the entire particle shape types was close to that of raindrops (10°) under calm atmospheric conditions.

The Tokyo Metropolitan Area Convection Study for Extreme Weather Resilient Cities (TOMACS) began as a Japanese domestic research project in 2010 and aimed to elucidate the mechanisms behind local high-impact weather (LHIW) in urban areas, to improve forecasting techniques for LHIW, and to provide high-resolution weather information to end-users (local governments, private companies, and the general public) through social experiments. Since 2013, the project has been expanded as an international Research and Development Project (RDP) of the World Weather Research Programme (WWRP) of the World Meteorological Organization (WMO). Through this project, the following results were obtained: 1) observation data for LHIW around Tokyo were recorded using a dense network of X-band radars, a C-band polarimetric radar, a Ku-band fast-scanning radar, coherent Doppler lidars, and the Global Navigation Satellite System; 2) quantitative precipitation estimation algorithms for X-band polarimetric radars have been developed as part of an international collaboration; 3) convection initiation by the interaction of sea breezes and urban impacts on the occurrence of heavy precipitation around Tokyo were elucidated by a dense observation network, high-resolution numerical simulations, and different urban surface models; 4) an "imminent" nowcast system based on the vertically integrated liquid water derived from the X-band polarimetric radar network has been developed; 5) assimilation methods for data from advanced observation instruments such as coherent Doppler lidars and polarimetric radars were developed; and 6) public use of high-resolution radar data were promoted through the social experiments.

In this study, we propose a new methodology for analysis of the initial stage of localized convective precipitation. The developed algorithm was used to create three-dimensional constant-altitude plan-position-indicator (3D CAPPI) data, which are high-spatiotemporal-resolution volumetric data, using two X-band polarimetric radars that are located in the Kanto region of Japan. Advection vectors are estimated by applying the normalized cross-correlation method to observed successive precipitation echoes on the polar coordinate system at each antenna tilt angle. The estimated advection vectors are expressed by a linear regression model that depends on time and tilt angle. In addition, 3D CAPPI data utilize the mosaic method to obtain further precipitation information from radar observations. The algorithm produces the detailed 3D structure of rapidly developing convective cells and provides quantitative information on convective precipitation, such as the echo top height, maximum reflectivity, and appearance time and height of each cell. The 3D CAPPI mosaic also clearly shows the back-building process at the initial stage of convective precipitation.

The data presented in this article are related to the research article entitled “Three-dimensional analysis of the initial stage of convective precipitation using an operational X-band polarimetric radar network” [1]. The data presented were obtained using a three-dimensional constant-altitude plan-position-indicator (3D CAPPI), which was generated by a new method proposed by [1]. The data used to create the 3D CAPPI were derived from two X-band polarimetric radar installations in the Kanto region of Japan, Ebina (139.39°E, 35.40°N), and Shin-yokohama (139.60°E, 35.51°N). These data are superior to operational radar data in terms of their temporal and spatial resolution. These high resolution data can indicate a rapidly developing storm, such as localized precipitation. It is particularly important to understand the early stages of storms in terms of numerical and short-term models. These data show the time of appearance, life cycle, and evolution of each cell that constitutes a storm in three-dimensional detail.