Article ID: 2025-014
This study examines the maintenance mechanisms of Muroto Lines, 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 Muroto Lines with differing depth of convective clouds: Case 1 (12-20 Japan Standard Time (JST; UTC + 9 h) on July 3, 2018) and Case 2 (16-21 JST on August 15, 2018).
In both cases, atmospheric environments were characterized by warm-moist and conditionally unstable lowest-level inflows (below 600 m in height) between east-southeasterly and south-southeasterly, and high humidity below the middle troposphere (600 hPa). Both cases exhibited back-building structures; convective cells were continuously generated at the southernmost tip of the Muroto Lines and advected northward by southerly wind 2-4 km in height. The convective cells were generated over a small-scale ridge oriented from south-southwest to north-northeast due to upslope lifting when the lowest-level wind was east-southeasterly. On the other hand, when the wind was southeasterly or south-southeasterly, convergence at the eastern foot of the ridge, resulting from deflected flow at the ridge combined with undeflected flow, could trigger the convective cells. Convergence at small-scale concave valleys and the lowest-level inflow with easterly components could further develop the Muroto Lines. Vertical structures of the Muroto Lines showed that the strongest rainfall in Case 1 (Case 2) was primarily caused by relatively shallow (deep) convective cells, suggesting the importance of the collision-coalescence of raindrops (melting of graupel). Intense rainfall was also produced in the developing stage of convective cells through the collision-coalescence of raindrops in both cases. This study suggests the importance of small-scale orographic effects and cross-QSCB lowest-level inflow for the maintenance of orographic QSCBs in warm-moist environments.
