On the Northward Ageostrophic Winds Associated with a Tropical Cyclone

Distinct northward winds were observed at southern coastal areas of Japan when a developed tropical cyclone approached. These northward winds have large ageostrophic components, and the mechanism were examined by numerical experiments using the JMA nonhydrostatic model.

The northward winds are explained by the nature of the relation between the ageostrophic wind and the acceleration vector. When a typhoon approach a baroclinic zone from south, northeastward ageostrophic wind components are induced by southeastward acceleration vectors, and resultant composited winds become northward. Other causes (diabatic heating and orographic effect) are examined by sensitivity experiments. Diabatic heating by moist process acts to enhance the ageostrophic wind but its role is not primary. Orography has little effect on the observed ageostrophic wind.

Non-axisymmetric feature of the upper level divergence flow of a tropical cyclone is also elucidated by the similar mechanism of the ageostrophic winds.