Line-shaped rainfall systems often appear in the western Kyushu to the south of the Baiu front. There are four remarkable line-shaped rainfall systems, which are called the Goto line, the Nagasaki line, the Amakusa line and the Koshikijima line. The line systems extend northeastward from the Goto Islands, the Nagasaki Peninsula, the Amakusa Islands and Koshikijima Islands, respectively. These topography has northeast orientation and is less than 500 m in height. Previous studies suggested the influence of orography on the formation of the line-shaped rainfall systems. However, the airflow structure induced by topography has not been studied in detail. The present study has two main purposes. The first is to clarify the formation mechanisms of convective clouds on the lee side of small orography in the highly moist environment and the role of orography. The second is to examine the chacteristics of moist convection that changes depending on the stability of the atmosphere and water vapour vertical profile. We conducted three dimensional numerical simulations of the line-shaped rainfall system formed on the Koshikijima Islands as an example.

First, the influence of topography around the Koshikijima Islands to Koshikijima line was examined. The existence of the Koshikijima Islands is more important factor for Koshikijima line than that of Sendai Region, Amakusa Islands and Nagashima Island. The formation mechanisms of convective clouds on the lee side of small orography was analyzed. The Koshikijima Islands consist of Shimo-koshikijima Island and Kami-koshikijima Island. The southwest wind hits against the Shimo-koshiki Island. The Shimo-koshikijima Island induce the ascent on the upwind side and the descent on the lee side. These airflow form shallow convective clouds following the shape of the Shimo-koshikijima Island. However, these clouds did not make the rainfall and did not form the line-shaped rainfall system on the leeside. On the one hand, the ambient air flow forms a convergence reigion on the lee side of the Shimo-koshikijima Island. In the previous observation pointed out the existence of this convergece region on the lee side of the Koshikijima Islands. The convergence region in the lower layer forms ascent motion. Then, this flow forms the shallow convective clouds. Additionally, a positive buoyancy which is generated by condensation heating strengthens the vertical updraft. The convective clouds move leeward with developing, and then precipitation occurs on the lee side of the Koshikijima Islands. The line had about 100~km in length and 10~km in width. From the result of this study, the line-shaped rainfall systems is not caused by orographic forcing but by the convergence on the lee side of the small orography.

Next, the environmental factors which govern the characteristics of convergence region on the lee side of the small orography was examined. We conducted a number of experiments which changed the wind velocity and the stability of atomosphere. The strength of convergence depends on the stability of atmosphere and the location of the convergence region depends on the wind velocity.

We examined that the stability of atmosphere and water vapour profile govern the characteristic of convective clouds on the lee side of the small orography. If the atmosphere is less unstable or the environment is more humid, the orographic convective clouds are more intese. From the result of observation data of Japan Meteorological Agency, the same tendency was seen. These two environmental foctors govern the strength of convergence on the lee side of the small isolated orography. Consequently the charcteristics of moist convection on the lee side of small isolated Orography depends on the stability of atmosphere and water vapour profile.

This paper clarified the formation process of convective clouds induced by small isolated orography in the highly moist enviroment, and the chacteristics of moist convection depending on the stability of atmosphere and water vapour profile.