線状対流系における水蒸気フラックス・降水粒子分布のマルチフラクタル特性

Abstract

マルチフラクタルは定量的なパターン解析手法として降水現象を始め様々な分野で応用されている．本研究では線状対流系の3次元的な自己組織化パターンを特徴づけるべく，水蒸気フラックスや降水粒子分布のマルチフラクタル解析を行った．2012年7月15日に発生した亀岡豪雨を対象とした再現実験を用いて対流系近傍の水蒸気フラックスを解析した．その結果，豪雨開始直前は対流不安定度の増加と対応してモノフラクタルに近づく傾向を見せたのに対し，対流系の形成とともにマルチフラクタル性が強まる傾向を見せた．またXバンド偏波レーダによる立体観測に基づき，氷相降水粒子の混合比を推定・解析した．帯状の強雨域が拡大し始め，地上降水強度の増加速度が増す時間帯と，霰・雪片混合比のマルチフラクタル性が大きく変化する時間帯が一致した．

Multifractal has been widely applied as the tool of quantitative pattern analysis. In this study, multifractal properties of water vapor flux and precipitation particles distribution associated with line-shaped rain bands were analyzed to identify the three-dimensional patterns of self-organization of convection systems. We analyzed water vapor flux obtained by the numerical simulation of the heavy rainfall event over Kameoka City on 15 July 2012. Before heavy rainfall occurred, water vapor flux approached to monofractal corresponding to increasing the convective instability at lower atmosphere. While during heavy rainfall, strong multifractality was shown due to the development of convection systems. Multifractal analysis was also conducted to characterize the multifractality of the mixing ratio distributions of ice-phase precipitation particles obtained by volume scans of X-band MP radars. The time period when the line-shaped rain band area began to expand and the regional averaged rainrate increased coincided with the one when the multifractality of the graupel and snowflake mixing ratio changed significantly.