気象レーダによる降灰量推定

—2013年8月18日桜島噴火のZ-R_A 関係式—

抄録

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.