Water Contents Estimations of Volcanic Ash Particles Using 2DVD and Weather Radar

WATER CONTENTS ESTIMATIONS OF VOLCANIC ASH PARTICLES USING 2DVD AND WEATHER RADAR

Sung-Ho Suh¹, Dong-In Lee¹*, Masayuki Maki² and Masato Iguchi³

E-mail: suhsh1215@pukyong.ac.kr

Abstract

Weather radar is one of the useful instruments for measuring volcanic ash clouds and its inner structure. However, fundamental physical properties of volcanic ash particles are required for the quantitative analysis by a weather radar data in natural condition. Volcanic ash column contains lots of water contents and they are condensed on the particle. This feature would affect the value of radar reflectivity and dual-pol radar variables.

The present study was analyzed using the one of the biggest eruption case (18th Aug. 2013) for Mt. Sakurajima, Japan. The eruption was occurred from 1630 to 1730LST and its column moved toward the west with 5m s^-1 of the ground wind (AWS). The whole case was detected by Tarumizu X-band multi-parameter radar(130.695 °E, 31.488 °N) and 2D-Video disdrometer(130. 601 °E, 31.589 °N).

The entire volcanic ash particles were classified as five types based on shape and orientation: Horizontal Oblate (OH), Vertical Oblate (OV), Horizontal Prolate (PH), Vertical Prolate (PV), and Sphere (Sp). Dominant particle shape was OV (41.47%) and the next was PH (30.27%). The 41.19% data are concentrated on D < 0.5mm. It should be noted that these distributions were quite different from the results of laboratory experiment (LE) in NIED, Japan.

The water contents ratio (W_r) is estimated by the particle density (ρ_s in g cm^-3) with the theoretical terminal velocity (V_T in m s^-1): ρ_s(D) = 2.2tanh(3.4D-2.9)+2.15. Based on the result, The relationship of W_r was found as W_r(D) = 2.2exp(-3.8D). The result was applied on the T-matrix and compared to the radar data. The observed Z_H at the lowest elevation angle (1.7°) was larger up to around 5dBZ than the case of dried particle and its difference was not constant with time. However, the result considering the water contents ratio was shown quite similar distribution to the observed value.

These results will be utilized to develop QAE methods, to detect volcanic ash column and predict its activity.

Key words: wet volcano, weather radar, 2DVD, QAEs.