The relationship between the time-integrated equivalent radar reflectivity factor (S_Z) and the accumulated volcanic ash fall amount (S_A) S_A＝0.307×S_Z^0.504 (or S_Z＝9.70×S_A^2.12), where the relationship is applicable to S_A<2.0kgm^－2 (or S_Z<35mm⁶m^－3h), was derived by analyzing weather radar data for the volcanic eruption of Sakurajima, Japan, on August 18, 2013. To derive the S_Z-S_A relationship, we constructed the three-dimensional Constant Altitude Plan Position Indicator (CAPPI) data of S_Z. A total ash fall amount of 67,100 tons was estimated based on the relationship, and a total ash fall area of 102km² was estimated based on the weather radar data. The relationship between the equivalent radar reflectivity factor (Z) and the ash fall rate (R_A), R_A＝1.41×Z^0.350 was derived, assuming a trapezoidal model for the temporal change in R_A. The effects of the radar data sampling height, the horizontal advection of ash echo, and the shape of the trapezoidal model on the derivation of quantitative ash fall estimation formulae are examined and the results are presented herein. Both the S_Z-S_A and Z-R_A relationships that were derived in the present study are essential to quantitatively monitoring ash fall using weather radar data. Examples of ash fall monitoring, such as the accumulated ash fall distribution and temporal changes in the ash fall rate at specific points, are offered.

Many casualties have been caused by the collision of lapilli and blocks ejecta (ballistic ejecta) following the phreatic eruptions of Mt. Ontake in September 2014 and Mt. Moto-Shirane, which is the southern part of Kusatsu-Shirane volcano, in January 2018 in Japan. Therefore, studies on shelters that provide protection from ballistic ejecta are necessary. However, reinforced concrete and steel shelters are heavy and are not suitable for mountains with high altitudes where transportation is limited. Thus, it is expected that wooden buildings such as mountain huts, which are an existing facility, can play a role in providing shelter. In this study, we propose a staggered structure and a cross structure for roof, built using wooden materials, as a means to reinforce mountain huts. The specimens were made with Japanese cedar roofboards; one sheet with a thickness of 15mm (boundary energy with and without penetration: approximately 1200J) and another with a thickness of 18mm (boundary energy: approximately 1300J) were prepared. All structural reinforcement specimens comprised of the roofboard installed on a surface waterproof sheet and a Galvalume® steel sheet with 0.4mm thickness. A test of the impact of simulated ballistic ejecta was conducted with a cylindrical abrasive (2.66kg) using the pneumatic impact test apparatus in these structural specimens. We observed that the boundary energy increases due to an increase in the thickness of the roofboard, when the same type of structural reinforcement was being analyzed. On the other hand, the cross structure showed higher boundary energy than the staggered structure when roofboards of the same thickness were utilized. It was experimentally clarified that a cross structure using a roofboard with a thickness of 18mm could withstand a simulated ballistic ejecta impact of up to approximately 3000J. In conclusion, we recommend the usage of a cross structural reinforcement on the roof of a wooden building, constructed by stacking two cedar boards, as a simple method of impact resistance against ballistic ejecta.