The numerical simulation for the heavy rainfall over a mountainous area in Japan caused by typhoon Meari (2004)

The heavy rainfall in a mountainous area in Japan caused by typhoon Meari (2004) is investigated using the Japan Meteorological Agency nonhydrostatic model (NHM). The targeted area is the east coast of the mountainous Kii peninsula. The typhoon initially tracked northwestward as it gradually intensified and reached its lowest minimum sea-level pressure (MSLP). The typhoon recurved to the northeast in the East China Sea and made landfall at Kushu, Japan, and moved almost parallel to the mountainous spine of Honshu.

We conduct a numerical simulation for the typhoon beginning after the recurvature. It is found that the track of the model typhoon is close to that of the observed storm before landfall although the former tracks to the west of the latter after the landfall. The MSLP of the model typhoon is 960 hPa at the initial and rises to 980 hPa over 24 h. The tendency for MSLP in the model storm agrees well with that in the observed one.

From surface weather maps around Japan, it is found that stationary front crosses Honshu and lies between the typhoon and high pressure systems. The horizontal gradient of sea-level pressure near the Kii eninsula, the targeted area, becomes greater due to the approaching typhoon and the intensified high pressure system.

12 h-accumulated rainfall amounts, ranging between 6 h before and 6 h after the landfall, show that heavy rainfalls started before the landfall along the east coast of the Kii peninsula, relatively far from the storm center, more than 500 km. In the middle part of the peninsula, the amounts are more than 400 mm. From radar observations, it is found that the recipitation systems are categorized as follows: A) the stationary system developed by the elimination of convective instability due to updrafts on steep mountains, B) the system along with the line of horizontal wind shear located at the boundary between the two areas characterized as easterly flow and south-eastery flow, and C) the moving system accompanying the typhoon. The categorized precipitation systems are also observed in the time evolution of 10-min accumulated rainfall amount measured with rain gauges located along with the coast.

Compared with the observed precipitation, the horizontal distribution of precipitation simulated by NHM successfully reproduces the heavy rainfall qualitatively. The area of stationary rainfall that stretches along the east coast of the Kii peninsula is well simulated. Band-like precipitation systems, whose axes are nearly perpendicular to the coast line, are also well reproduced. However, the amount of the rainfall is less than the real one. In particular, the amounts along the northern part of the coast are much less than the observed one.