Geography Reference
In-Depth Information
100
W
C
N
N
S
200
S
W
C
C
400
S
S
N
N
N
600
S
S
N
S
800
N
S
N
W
W
C
1000
0.25
0
Phase (cycles)
0.25
0.50
E
W
(Time or longitude)
Fig. 11.4
Schematic model for equatorial wave disturbances showing trough axis (solid line), ridge
axis (dashed lines), and axes of northerly and southerly wind components (dotted lines).
Regions of warm and cold air are designated by W and C, respectively. (After Wallace, 1971.)
In the convectively active areas of equatorial waves there is net upward motion,
and midtropospheric temperatures are warmer than average (although generally
by less than 1˚C). The correlations between temperature and vertical motion, and
between temperature and diabatic heating, are thus both positive, and the potential
energy generated by the diabatic heating is immediately converted to kinetic energy
[i.e., the
conversion balances R
in (10.62)]. There is, in this approxima-
tion, no storage in the form of available potential energy. The energy cycle of these
disturbances, therefore, differs remarkably from that of midlatitude baroclinic sys-
tems in which the available potential energy greatly exceeds the kinetic energy.
For latent heat release by cumulonimbus clouds to be an effective energy source
for large-scale disturbances there must be an interaction between the convective
scale and the large scale, as mentioned in Section 9.7.2. In such interaction large-
scale convergence at low levels moistens and destabilizes the environment so that
small-scale thermals can easily reach the level of free convection and produce deep
cumulus convection. The cumulus cells, in turn, act cooperatively to provide the
large-scale heat source that drives the secondary circulation responsible for the
low-level convergence.
A typical vertical profile of divergence in the precipitation zone of a synoptic-
scale equatorial wave disturbance in the western Pacific is shown in Fig. 11.5.
Convergence is not limited to low-level frictional inflow in the planetary boundary
layer, but extends up to nearly 400 hPa, which is the height where the hot towers
P
·
K
]
[
