East Asia and Western Pacific METEOROLOGY AND CLIMATE

290
perturbations in Mode I remains similar to the original Eady waves.
4> * and 0* for a 2 =1.25*rc (i.e., X, =1600 km) of Mode n are presented in Figs.
9-10 for 2a, and in Fig. 11-12 for 2e. The perturbations are more confined in the lower
atmosphere, especially for case 2e. The temperature slightly tilts eastward in the lower
layer, atop a transition layer, where it tilts westward drastically. The tilt of 9* is very
small in the middle and upper atmosphere. The structures of * and 0* for 2e are
similar to the short waves discussed by Nakainura ^1 Bretherton 15,16] argues that the
presence of the new unstable modes can be caused by the existence of a gradient of the
basic state potential vorticity (i.e, 3Q/3y ^ 0) in the interior of the flow, according to
Eq. (2.6).
The height of the disturbance may be measured by the Rossby depth of penetration,
h « c r /(V/H). h2a ~3 km for 2a, and h2e«2.5 km for 2e. Those heights are
comparable to the height of the transition level of 0 * in Figs. 10 and 12. The average
a0/3z=2.15 K km' 1 at z « 3 km for 2a; and 90/3z=2.63 K km" 1 at z « 2.5 km for 2e.
The effective stratification of 2a is smaller than that of case 2e, but the growth rate of 2e
is larger, as shown in Fig. 4. Therefore, the short waves are sensitive to the
stratification not only just in the lower atmosphere but also in the upper layer.
Comparing perturbations fields of 2a (Figs. 9-10) and 2e (Figs. 11-12), we can see that
decrease of I h. The change of w'0' across h is much smaller
in 2a than 2e, because motion in 2e is more confined in the lower layer and is able to