Band Structure of Mesoscale Echo Clusters Associated with Low-Level Jet Stream

Abstract

A large echo band, whose length is about 1,000km, developed over the East China Sea adjacent to southwestern Japan on 7 July 1970. The three dimensional structure of the echo band is analyzed by using the data of the Experiment of Severe Rainstorms Research Project. The echo band is associated with strong precipitations and a remarkably developed ageo- strophic low-level jet stream. The analyzed moisture field around the jet stream coincides well with the model of the low-level jet's circulation proposed by Matsumoto and Ninomiya (1971). It is inferred about the mechanism of the echo band that the convective mixing of the horizontal momentum in the echo band maintains the ageostrophic low-level jet. The unbalanced solenoidal field caused by the ageostrophic wind field accelerates the vertical circulation and thus maintains the convective activities of the echo band. It is also found that the echo band is not a zone of uniformly distributed echo cells but a row of mesoscale echo clusters whose spacing is about 150km. Two other cases of large-scale echo bands, one over the Pacific coastal area and another over the Midwest of U. S. A., are also presented as similar examples. 1.