Rapid merger and recyclogenesis of twin extratropical cyclones leading to heavy precipitation around Japan on 9-10 October 2001

The significance of twin extratropical cyclones (in contrast to twin tropical cyclones) has not yet been fully recognized in weather forecasting and analysis of heavy precipitation. This study investigated successive sets of twin extratropical cyclones over the Yellow and East China Seas (YECS) and over the Japan Sea and South Coast areas of the Japanese Islands (JSSC) on 9-10 October 2001. Although these twin cyclones were not strong, they led to heavy precipitation in Japan. The southern low of the YECS twin cyclones, which accompanied a moist tongue, rapidly merged with the northern cold low, resulting in the formation of a moist tongue across the Japanese Islands, causing heavy precipitation. This rapid merger is explained by a coupling of cold upper- and warm lower-level lows in a contact binary rotation system. Subsequently, the JSSC twin cyclones became newly organized. The northern deep low of the JSSC twin cyclones formed north of an upper-level jet by the merger of the YECS cyclones, while the southern shallow low newly formed around the occlusion point south of the upper-level jet. The sensitivity experiments of successive sets of the twin cyclones show that condensational heating and topographic effects contribute to the formation of the twin structures. The northern low of the JSSC twin cyclones is amplified by latent heating, while the southern low is hardly amplified. The topography of the Japanese Islands results in intensification of the upward flow and cyclonical flow via topographic convergence and blocking of horizontal flow.