Ground Observation of Tephra Particles: On the Use of Weather Radar for Estimating Volcanic Ash Distribution

Abstract

This paper reports a preliminary attempt to determine volcanic ash particle size distribution using the video drop size detector (VDSD) for estimating volcanic ash amount with X-band radar. The VDSD records an image showing the size and number of particles falling into the aperture by a charge coupled device camera. Size distribution spectra of a range of particles from fine ash to small lapilli were derived in discrete form from the VDSD observation. The parameterization of the particle size distribution following Gamma function was done using volcanic ash of eruptions at the Sakurajima Volcano between December 13–21, 2014. Three Gamma distribution parameters were determined analytically. The analytical results revealed a continuous distribution of particles characterized by shape, intercept, and slope. The distribution was used to determine volcanic mass concentration, ground deposit weight, and reflectivity. Verification of these results with X-band radar observations showed that the reflectivity obtained from analytical results is similar to that from radar observation. However, the ground deposit weight from analysis was overestimated, compared with the real weight of ash deposit on the ground. The algorithm proposed in this study is expected to provide a practical method for estimating ash distribution in the aftermath of a volcanic eruption using radar-reflectivity for cases where direct measurement at the location is not possible. An overview of the algorithm for volcanic ash retrieval from X-band radar observations is also presented.

A Near Real-Time Tephra Fallout Rate Model by a Small-Compact X-Band Multi-Parameter Radar

“…Ideally, the verification of radar-based tephra fallout retrieved should be done by comparing the radar-based model with a measurement from optical ground-based instruments such as a video drop size detector (VDSD) or parsivel disdrometer (Hapsari et al, 2019;Suh et al, 2019). However, these instruments were not available in our study areas.…”

“…Nevertheless, it has lower scanning ranges and less accurate measurements compared to those of S-and C-band radars (Antoninni et al, 2017). Despite this limitation, some studies have been able to apply X-MP radars to monitor volcanic cloud movement and retrieve the tephra concentrations and fallout rates (Oishi et al, 2016;Marzano and Vulpiani, 2012;Montopoli et al, 2011;Hapsari et al, 2019). The X-band system has greater sensitivity for the measurement of smaller particles of tephra and can be better in addressing the beam blockage problems of C-and S-band long-range radars in areas with complex terrain.…”

“…The applicability of multi-parameter radar in tephra fallout rate estimation from a volcanic plume is relatively unknown. Most of them have primarily relied on the radar reflectivity factor ZHH (Maki et al, 2016;Hapsari et al, 2019;Syarifuddin et al, 2019;Marzano et al, 2006 a,b ;2013), although this component was susceptible to attenuation (Bringi and Chandrasekar, 2001;Park et al, 2005).…”

Estimating the Velocity of Pyroclastic Density Currents Using an Operational Dual-PRF Radar

“…Increasing interest in applying weather radars to study volcanic plumes opens the possibility to investigate the dynamics of explosive eruptions and PDCs. Despite many studies in radar-retrieved plume dynamics (Montopoli, 2016;Freret-Lorgeril et al, 2018;Scharff et al, 2019;Donnadieu, 2012;Donnadieu et al, 2005Donnadieu et al, , 2016Marzano et al, 2006Marzano et al, , 2013Syarifuddin et al, 2019Syarifuddin et al, , 2020Hapsari et al, 2019), only one study has to date documented techniques to retrieve dynamics from a natural moving PDC. Scharff et al (2019) used a conventional Doppler radar to estimate the apparent average vertical velocity from PDCs that travelled down the flank of Volcán de Colima in Mexico at 30 m/s.…”