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On 4 January 2018, an extremely rapidly developing extratropical cyclone ("superbomb" cyclone) appeared over the Gulf Stream. To clarify the key process of the intensification of the superbomb cyclone, we specifically examined the role of heat supply from the Gulf Stream by performing cloud-resolving numerical experiments. When the cyclone rapidly developed, sensible and latent heat fluxes from the warm current predominated around the cold conveyor belt (CCB) of the cyclone. These fluxes created a convectively unstable layer near the surface. When the instability was released around the cyclone center, latent heating (LH) occurred. The increased LH further intensified the cyclone and its associated CCB. These findings indicate that the positive feedback process between the CCB and LH played a key role in intensification over the Gulf Stream. In this way, the heat supply from the Gulf Stream significantly impacted the development of the superbomb cyclone.

Plain Language Summary Rapidly developing extratropical cyclones are called meteorological "bombs." In early January 2018, a "superbomb" cyclone appeared over the Gulf Stream, causing severe damage to the United States and Canada. To understand the key process of the development of the superbomb cyclone, we specifically studied the impact of the Gulf Stream. When the cyclone rapidly developed, the Gulf Stream supplied abundant moisture and heat to the atmosphere around the cyclone center. This moisture/heat supply led to precipitation intensification, because it destabilized the atmosphere near the surface. During precipitation formation, strong latent heating occurred owing to the condensation of water vapor. The latent heating caused a large drop in the central pressure of the cyclone, resulting in rapid intensification of the cyclone. In this matter, the moisture/heat supply from the Gulf Stream significantly impacted the rapid development of the superbomb cyclone.