Geography Reference
In-Depth Information
Fig. 11.3
Time-longitude sections of satellite photographs for the period July 1-August 14, 1967 in
the 5-10˚N latitude band of the Pacific. The westward progression of the cloud clusters is
indicated by the bands of cloudiness sloping down the page from right to left. (After Chang,
1970. Reproduced with permission of the American Meteorological Society.)
longitudinal separation of the cloud bands is about 3000-4000 km, corresponding
to a period range of about 4-5 days for this type of disturbance.
Diagnostic studies suggest that these westward-propagating wave disturbances
are generally driven by the release of latent heat in their accompanying convective
precipitation areas. The vertical structure of a typical equatorial wave disturbance
is shown in schematic form in Fig. 11.4. Vertical motion in such disturbances is
proportional to the diabatic heating rate; thus the maximum vertical velocities occur
in the convective zone. By mass continuity there must be convergence at low levels
in the convective zone and divergence in the upper levels. Hence, provided that the
absolute vorticity has the sign of f, the divergence term in the vorticity equation
will induce cyclonic vorticity tendencies in the lower troposphere and anticyclonic
vorticity tendencies in the upper troposphere. The process of adjustment between
mass and velocity fields will then generate a low-level trough and an upper-level
ridge.
1
Thus, the thickness (or layer mean temperature) in the convective zone
must be greater than in the surrounding environment.
1
The terms “trough” and “ridge” as used by tropical meteorologists designate pressure minima and
maxima, respectively, just as in midlatitudes. In the easterlies of the Northern Hemisphere tropics,
however, the zonal-mean pressure increases with latitude so that the pattern of isobars depicting a
tropical trough will resemble the pattern associated with a ridge in middle latitudes (i.e., there is a
poleward deflection of the isobars).
