Geography Reference
In-Depth Information
100
200
300
400
500
600
700
800
900
1000
-10
-5
0
+5
+10
+15
Divergence (10 -6 s -1 )
Fig. 11.5
Vertical profile of 4˚-square area average divergence based on composites of many equatorial
disturbances. (Adapted from Williams, 1971.)
achieve their maximum buoyancy. The deep convergence implies that there must be
substantial entrainment of midtropospheric air into the convective cells. Because
the midtropospheric air is relatively dry, this entrainment will require considerable
evaporation of liquid water to bring the mixture of cloud and environment air to
saturation. It will thus reduce the buoyancy of the cloud air and may in fact produce
negatively buoyant convective downdrafts if there is sufficient evaporative cooling.
However, in the large cumulonimbus clouds present in equatorial waves, the central
core updrafts are protected from entrainment by the surrounding cloud air so that
they can penetrate nearly to the tropopause without dilution by environmental air.
These undiluted cores constitute the “hot towers” referred to in Section 11.1.1.
Because the hot towers are responsible for much of the vertical heat and mass
transport above the boundary layer in the ITCZ, and the wave disturbances contain
most of the active convective precipitation areas along the ITCZ, it is obvious that
equatorial waves play an essential role in the general circulation of the atmosphere.
11.1.3
African Wave Disturbances
The considerations of the previous subsection are valid for ITCZ disturbances
over most regions of the tropical oceans. In the region of the North African
Search WWH ::




Custom Search