Heavy rainfalls often occur when a tropical cyclone (TC) exists on the sea off the south coast. These pre-typhoon rainfalls (PRE) is associated with the northward moisture transport ahead of the TC. In this paper, we examine the northward moisture transport by the ageostrophic winds associated with typhoon T0918 (Melor) and its impact on PRE. According to a numerical simulation conducted in the previous study (Saito 2019), we analyzed the northward moisture fluxes by reproduced geostrophic and ageostrophic winds. Although the southerly ageostrophic winds are dominant mainly in the upper levels, the ageostrophic winds contribute to enhance the poleward water vapor transport for the upper and middle levels above 3 km.

To see the impact of the ageostrophic moisture transport on PRE, we conducted a sensitivity experiment where the model moisture in middle and upper levels over the sea off the south coast of western Japan was reduced. Precipitation over western Japan was decreased about 30% when the contributions in moisture fluxes by ageostrophic winds were removed. This result suggests that the northward ageostrophic winds associated with a TC enhance PRE by moistening the middle and upper atmosphere.

当南海岸外海存在热带气旋（TC）时，经常会出现强降雨。这些台风前降雨（PRE）与TC之前向北的水汽输送有关。在本文中，我们研究了与台风T0918（Melor）有关的变质风向北的水汽输送及其对PRE的影响。根据先前研究（Saito 2019）中进行的数值模拟，我们通过再现的地转风和年龄变大的风分析了北向水汽通量。尽管向南的地转风主要在高层，但在3 km以上的上层和中层，地转风促进了极向水汽的输送。

为了观察年龄变化的水分运移对PRE的影响，我们进行了敏感性实验，减少了日本西部南海岸海域中上层和上层模型的水分。当去除了古营养风对水分通量的影响时，日本西部的降水减少了约30％。该结果表明，与TC相关的北向变质风通过润湿中高层大气来增强PRE。