A Model of Typhoons Accompanied by Inner and Outer Rainbands

Abstract

Both hurricanes and typhoons are relatively circular storms with their winds at high levels blowing mostly outward. Assuming that the outflow is characterized by central motion under the influence of the pressure-gradient forces pointing toward the storm center, the absolute angular momentum of the outflow winds and radar echoes were computed. The upward motion inside these echoes plays an important role in the upward transport of the low-level angular momentum. Detailed analysis made by using satellite, radar and aerological data revealed that the absolute angular momentum of radar echoes near the eye wall is almost one order of magnitude smaller than that of the echoes in the outer rainbands. This fact leads to a suspicion that the radial distribution of the vertical velocity inside a storm would significantly alter the pattern of the outflow winds near the storm top. A numerical model, including the radial distribution of the vertical velocity, was thus established. Computations indicated that the vertical transport of momentum by outer rainbands alters the outflow wind pattern, resulting in the formation of a ring of high-cloud-free region which separates the inner cirrus shield from the outer cirrus decks. This ring sometimes appears as a clear area on a satellite photograph.