Effect of Upper-Air Moistening by Northward Ageostrophic Winds Associated with a Tropical Cyclone on the PRE Enhancement

As a complement work to the authors' previous studies, we examined the pre-typhoon rainfalls (PRE) ahead of typhoon T0918 (Melor) in October 2009. The influence of moistening in the upper atmosphere induced by the northward ageostrophic winds on PRE precipitation was examined by a sensitivity experiment using a cloud resolving model with a horizontal resolution of 2 km. The cloud resolving simulation showed a large impact of the water vapor in the upper atmosphere on the precipitation over western Japan. In the sensitivity experiment where the moisture in the middle and upper layers was reduced over the area off the south coast of western Japan, the water vapor reduction area was advected northward, and the snow in the middle and upper layers and the cloud ice in the upper layer decreased, reducing the rain below the melting level. The intrusion of drying air into the upper atmosphere reduced the thickness of the moist absolutely unstable layer (MAUL), and the maximum intensity of convective updrafts decreased by about 10% in the test experiment. In this case, the increase of rain in PRE was primarily caused by the deep northward water vapor transport which yielded a large amount of condensation in the middle and upper layers, and change of moist instability in the upper atmosphere subsidiarily enhanced the convective updrafts.