Causes of an East Asian Winter Cold Surge and Its Impact on a Flash Flood Event in Malaysiaia
This study analyzed the impact of a cold surge during the East Asian winter monsoon on an extreme flash flood event in Malaysia using satellite observations and reanalysis data including ERA5 from the European Centre for Medium-Range Weather Forecasts and data from the Global Data Assimilation System.The results indicate that,with the East Asian winter monsoon in the Northern Hemisphere,the redevelopment of the East Asian trough,the strengthening of surface cold high pressure,and the development of explosive cyclones over Japan and Northwestern Pacific jointly provided favorable large-scale circulation background for the cold surge formation.The impact of Tropical Cyclone Rai and the continuous influx of cold air from mid and high latitudes prolonged the duration of the cold surge.Interactions between the cold surge,the peripheral circulation of the tropical cyclone,the easterly airflow in tropical regions,and the topography of the western Maritime Continent led to the formation and maintenance of the Borneo Vortex,providing continuous dynamic conditions for heavy precipitation.The convergence between the cold surge and the equatorial easterly airflow,along with the interaction between the cold surge and the topography of the Malay Peninsula's coastline,further enhanced low-level dynamic conditions,which then led to the continuous influence of multi-stage dynamic convergence.The water vapor channel established by the cold surge from the South China Sea to the Malay Peninsula was the main source of precipitation moisture,with the easterly component contributing the strongest water vapor input,followed by the northerly component.This pathway created strong water vapor convergence conditions in the lower troposphere near the Malay Peninsula.As the cold surge moved southward,its temperature and humidity conditions are intensified by the vast ocean surface,particularly in the lower layers,leading to the generation and strengthening of unstable conditions.The warm,energetic,and highly humid environment in the lower troposphere was conducive to the development of mesoscale convective systems and the generation of efficient warm-cloud precipitation.
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