During Baiu season, orographic line-shaped rainfall systems often appear in the west of Kyushu Island. They extend northeastward from an isolated island or a peninsula.
One of them is the Goto line: the line-shaped rainfall system extending from the Goto Islands locating to the west-northernmost coast of Kyushu Island. The line forms along the northwestern coast of Kyushu Island.This system has about 150 km length, although its appearance is not much.The northeastern end of the line brings heavy rainfall over Fukuoka, which is a major city about 130 km far from the Goto Islands. For an accurate prediction of the heavy rainfall, case studies on formation mechanisms of the Goto line are necessary. As few researchers have studied the Goto line, the purpose of my study is to clarify the structure and formation process of the Goto line with the maximum length of 200 km on 19 June 2001.
According to surface weather charts, the Goto Islands are located in the warm sector on the southern side of the Baiu front before the formation of this system. Upper-air sounding data in Fukuoka shows that the lower atmosphere has much moisture and nearly convectively stable stratification.There is a strong wind shear in the lower layer, with southerly winds around the surface and southeasterly on the low-middle level.
The JMA Fukuoka operational radar data at 2.0 km height indicates thatrainfall region extends northeastward from the northeastern side of the Goto Islandsand forms a line-shaped rainfall system about 150 km in length with many convective cells.The axis of this line-shaped system rotates counterclockwise graduallywith the shift of middle-level wind direction.
In terms of a generation of individual cell that makes up the line,there are not only convective cells generating near the northeast side of the Goto Islandsbut also convective cells generating on the coast and mountain peaksof northern Kyushu in the downstream of the Goto Islands.
On the former, it was found that the upslope forcingdue to isolated obstacles triggered convection from high Froude number (Fr=3.2).They are advected by middle-level winds.Some convective cells can re-develop or generateon the coast and mountain peaks of northern Kyushu.In this way, the Goto line keeps having about 150 km lengthand brings heavy rainfall at its northeastern end.
We will discuss factors for the appearance of such orographic line-shaped rainfall systems.