The Role of Surface Friction in Tropical Cyclones

[摘要] In order to study the role of surface friction in a tropical cyclone described in a numerical experiment (Yamasaki, 1977) in which the effects of cumulus convection were not parameterized but explicitly calculated, numerical experiments are performed by changing drag coefficient concerning surface friction.It is shown that surface friction plays important roles to reduce the horizontal scale of the disturbance and to produce an eye and eyewall as observed in the tropical cyclone. A more important result is that a disturbance may develop even when surface friction is not taken into account. This result indicates that Yamasaki (1977)'s tropical cyclone (referred to as AF mode) is different from tropical cyclones (TC mode) described by Ooyama (1969) and Yamasaki (1968) in the sense that TC mode becomes neutral in the absence of surface friction.It is possible to interpret AF mode as a superposition of TC mode and NF mode, where NF mode is defined as a CISK mode whose instability does not depend on surface friction. That is, when surface friction does not exist, NF mode is dominant because TC mode is neutral. Also, even when surface friction exists, TC mode is not noticeable if the vorticity of the disturbance is too weak to produce appreciable frictional convergence. When the disturbance becomes strong, TC mode with pronounced frictional convergence becomes dominant. In this case AF mode is similar to TC mode.It is also suggested that tropical cyclones simulated by Anthes et al. (1971) and Kuri-hara and Tuleya (1974) are similar to AF mode rather than TC mode.

[学科分类] 大气科学