CHARACTERISTICS OF ANNULUS BAROCLINIC FLOW STRUCTURE DURING AMPLITUDEVACILLATION

An investigation is made of the mechanics of amplitude vacillation in a numerically simulated rotating annulus flow system. Amplitude vacill ation is characterized by a periodic change of vertical wave structure in concert with growth and decay of wave amplitude and phase speed. T he temperature wave amplitude profile for the dominant component consi sts of three local maxima: (1) lower boundary layer, (2) upper half la yer and (3) lower half layer. The lower layer waves lead the time-depe ndent structural variation during vacillation. Two types of amplitude vacillation found in the experimental measurements (Buzyna et al., 198 9: J. Atmos. Sci. 46, 2716-2729) can be distinguished in the temperatu re wave by whether the lower layer waves split from and travel behind the upper layer waves by one wave period in each cycle of vacillation. Linear eigenvalue analyses with respect to the instantaneous axisymme tric state at various points in time are performed to elucidate the si mple interaction between the dominant wave and the zonal mean state, D uring the vacillation cycle, the zonal mean state is modified by the w ave, which causes a change in growth rate and vertical structure of th e linearly most unstable eigenmode. This, in turn, forces the actual c hanges of the nonlinear solutions. (C) 1997 Elsevier Science B.V.