Note on Kuo′s Problem

For the eigenvalue problem of H. L. Kuo [J. Met.6 (1949), 105-122] governing the linear stability of a parallel zonal flow of an inviscid, incompressible fluid on a β-plane, it is shown that the wave velocity of neutral modes is bounded. Moreover, Pedlosky′s semicircle region for the range of the complex wave velocity of an arbitrary unstable mode is further reduced.