Migration of contact binary cyclones and atmospheric river: Case of explosive extratropical cyclones in East Asia on December 16, 2014

Contact binary cyclones have been observed often in the marginal sea areas of East Asia; however, their explosive development and related moisture transport are not understood fully. This study investigated explosive extratropical binary cyclones that occurred near Japan on December 16, 2014, to elucidate both their explosive development and their efficient transport of moisture northward. The northernmost of the binary cyclones developed with a central pressure fall of 24 hPa during a 24-h period. It remained over the Japan Sea, where a trough and an area of high potential vorticity were located at 300 hPa. The southernmost of the binary cyclones developed explosively with a central pressure fall of 58 hPa during a 24-h period. It traveled rapidly around the northern low in a counterclockwise direction. The contact binary cyclones accompanied a tributary atmospheric river and moist delta, which aided the rapid development of the southern low. The tributary atmospheric river was formed by the regional convergence of humid air parcels that were modified by the surface moisture flux around the cold front (though the atmospheric river was extended from the tropical Pacific Ocean). As the southern low developed, northward moisture transport was predominant along the atmospheric river. In the explosive binary cyclones, the influence of latent heat release differed between the two contact lows. While the northern low was coupled with the upper-level trough over the Japan Sea, the southern surface low, which revolved around the northern low, was intensified both by the latent heat release and by the warm horizontal thermal advection and the vorticity advection along a front. Condensational latent heating did not influence substantially the pressure drop of the northern low, whereas it contributed considerably to the formation of the southern low and to the enhancement of the northward moisture transport along the atmospheric river.