East Asia and Western Pacific METEOROLOGY AND CLIMATE

515
EFFECTS OF VERTICAL WIND-SHEAR ON KELVIN WAVE-CISK MODES
Hock Lim, Tian-Kuay Lim and C.P. Chang
ABSTRACT
This paper consists of two parts : a basic study of the Kelvin
wave-CISK modes followed by an examination of the effect of vertical
wind shear on these modes with a view of interpreting the observed 30-
60 day oscillations in terms of such Kelvin wave-CISK modes.
Linear analyses of CISK have invariably led to the conclusion
that the instability mechanism strongly favours cumulus scale rather
than synoptic scale circulations. This is often considered a serious,
if not fatal, defect of the CISK theory. On the other hand, numerical
CISK models had experienced no great difficulty in generating
disturbances which have reasonably realistic structures. It is a
little surprising that this apparant contradiction between theoretical
prediction and simulation results has so far not been thoroughly
investigated.
Lau and Peng (1987) reported that when they used a linearized
CISK parameterization, the disturbance generated in their numerical
model indeed had its energy concentrated in the shortest wavelength
components. However, when they used a "positive-only" CISK where
latent heating was allowed in the ascending regions but cooling was
not allowed in the descending regions, the disturbance tended to take
on a wavenumber-one structure.
In this paper, we examine the dynamics of wave-CISK using a 5-
level spectral model in p-coordinate. The model has no damping
mechanism and conserves energy in the absence of CISK. In the
preliminary experiments with linearized CISK, short waves grow rapidly
to dominate the flow field, in agreement with theory. However, with
"positive-only" CISK, the strong nonlinear interactions due to the
CISK heating mechanism was found to lead to organised wave-CISK modes
which propagates forward steadily without change of shape. The wave-