On the Northward Ageostrophic Wind associated with a Tropical Cyclone

It is well-know that heavy rainfalls in Japan often occur when a typhoon or tropical depression exists on the sea far south of Japan. These phenomena, so-called 'pre-typhoon precipitation', are in many cases explained by the moisture transport over Japan from the tropical cyclone, however, the reason why the moisture is transported northward from the sea south of Japan has not been clarified. Ageostrophic winds are also sometimes observed over Japan in such the cases. The following possible causes of the ageostrophic winds can be listed: 1) The pressure gradient force by precipitation associated with the mesoscale convective system 2) The vertical transport of horizontal momentum by deep convection 3) Influence of orography 4) Ageostrophic wind toward the left direction of the local acceleration vector

In this study, first we show an example of significant ageostrophic winds observed over Japan on 6 October 2009, when typhoon Melor (200918) existed off the far south coast of Western Japan. This ageostrophic wind event was successfully reproduced by a numerical experiment using the JMA nonhydrostatic model with a horizontal resolution of 10 km. We review the relationship between ageostrophic wind and local acceleration vector, and show that the observed ageostrophic wind was mainly caused by the acceleration vectors in the northeast quadrant of the typhoon and in the south of the jet stream over Japan.