Maintenance Mechanisms of Orographic Quasi-Stationary Convective Band Formed over the Eastern Part of Shikoku, Japan

Akira Nishii; Taro Shinoda; Koji Sassa

doi:10.51094/jxiv.758

##article.authors##

Akira Nishii Institute for Space-Earth Environmental Research, Nagoya University https://researchmap.jp/ankisho
Taro Shinoda Institute for Space-Earth Environmental Research, Nagoya University
Koji Sassa Faculty of Science and Technology, Kochi University

DOI:

https://doi.org/10.51094/jxiv.758

キーワード:

Mesoscale convective system、 Localized heavy rainfall、 Quasi-stationary convective band、 Orographic precipitation

抄録

This study examines the maintenance mechanisms of Muroto Lines (ML), a south-north oriented quasi-stationary convective band (QSCB) that appeared from the Muroto Peninsula in eastern Shikoku, Japan. The analysis area is characterized by complex orography, where many small-scale ridges are embedded in larger-scale ridges. We focused on two cases of ML that brought heavy rainfall: Case 1 (12-20 JST (Japan Standard Time; UTC+9 h) on July 3, 2018) and Case 2 (16-21 JST on August 15, 2018).

Atmospheric environments were characterized by warm-moist, and conditionally unstable lowest-level inflows (below 500 m in height) between east-southeasterly and south-southeasterly, and high humidity below the middle troposphere. The MLs exhibited back-building structures; convective cells were continuously generated at the southernmost tip of the MLs and advected northward by southerly wind 2-4 km in height. The convective cells in the MLs could be generated through two mechanisms: upslope lifting over a small-scale ridge oriented from south-southwest to north-northeast and convergence resulting from deflected flow at the ridge combined with undeflected flow at the eastern foot of the ridge. The former (latter) mechanism prevailed when the lowest-level wind direction was east-southeasterly (between southeasterly and south-southeasterly directions). Convergence at small-scale concave valleys and the lowest-level inflow with easterly components could further develop the ML. The vertical structures of the MLs showed that the heaviest rainfall in Case 1 (Case 2) was mainly due to relatively shallow (deep) convective cells, suggesting the importance of the collision-coalescence of raindrops (melting of graupel particles). Heavy rainfall in both cases was also caused by the development stage of convective cells by the collision-coalescence of raindrops in the southern part of the MLs. This study highlights the importance of orographic effects on the small-scale orography and cross-QSCB lowest-level inflow for the maintenance of heavy-rain-induced orographic QSCBs in warm and moist environments.

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The authors have no conflicts of interest.

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