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Extreme rainfall events in Taiwan are often influenced by multiscale interactions between large-scale forcing, mesoscale process, and terrain effect, posing significant forecasting challenges. While orographic lifting is known to trigger convection, its role in modifying atmospheric stability, specifically through the formation of moist absolutely unstable layers (MAULs), remains underexplored. This study presents the first investigation demonstrating that terrain can induce MAUL and thereby enhance extreme rainfall in mountainous terrain in Taiwan, a mechanism not previously documented. Cloud-resolving simulations show terrain-driven moisture convergence and layer lifting promote deeper, more persistent MAULs. Removing terrain substantially limits MAUL development, resulting in weaker rainfall. Furthermore, MAUL volume rapidly increases prior to the most intense rainfall, highlighting its potential as a precursor for extreme precipitation. These findings highlight terrain's crucial role in modulating extreme rainfall through this thermodynamic pathway and underscore the importance of accurately representing orographic effects in numerical weather prediction.
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