Journal of the Meteorological Society of Japan. Ser. II

Tropical storm Bebinca (2006) was characterized by an abrupt termination due to the generation of an explosive convection to the north of its center. No convective clouds were observed near its center, nor did typical cloud patterns with spiral bands and eyewalls appear in the storm. Reanalysis data (ERA-Interim) was used in this study, and a set of sensitivity experiments conducted with a numerical model (NICAM) to examine the cause of the explosive convection.
The analyses of the reanalysis data elucidated mechanisms for excitation of deep convection, including a high convective available potential energy environment due to moisture convergence at lower levels, potential vorticity generation by a diabatic Rossby vortex mechanism, and dynamical forcing formation of upward motion induced by upper levels.
Moreover, two sensitivity experiments were conducted by modifying the vortex structure of Bebinca. The results suggested that Bebinca’s unusual characteristic, the weak low-level pressure gradient near the center was fundamental for excitation of the explosive convection in the northern area.

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On 20 October 2010, a heavy rainfall event occurred on Amami-Oshima Island, Japan, delivering a record 622 mm of rainfall in one day. To clarify the factors underlying this event, the supply mechanism and formation process of low-level humid air and the formation and maintenance mechanisms of the precipitation systems causing the heavy rainfall were examined using observation data, objective analysis data, and numerical simulation results. These investigations showed that low-level humid air, carried to Amami-Oshima Island during the rainfall event by strong east-northeasterly winds, originated more than 500 km to the east-northeast as low-level dry air on the northern side of a stationary front. This dry air was transformed into humid air on the way to the island by receiving large latent heat flux from the sea surface (air-parcel transformation). Warm sea surface temperatures around Amami-Oshima Island, about 2°C higher than the annual mean, contributed to this air-parcel transformation. At Amami-Oshima Island, the collision of the humid flows with a cold pool formed under earlier precipitation systems contributed significantly to the formation and maintenance of the precipitation systems, supplemented by topographic effects of the island.

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