210
W.B. Rossow
What atmospheric conditions produce deep convection in the tropics and what kinds of clouds
are produced? The simplest answer to the first question is that surface heating by sunlight produces buoyant parcels of moisture-laden air near the surface that, once condensation begins,
can rise all the way to the tropopause. This simple model predicts a concentration of convection over regions with high surface temperatures; but the actual distribution of such convective
systems contradicts this simple prediction in several ways. The hottest surface temperatures
appear in North Africa and other nearly cloud-free deserts where there is no convection. The
hottest ocean water is in the western Pacific and eastern Indian ocean, yet only the former is
persistently covered by convection. A closer look at the western Pacific shows that convection
does not occur everywhere, suggesting other factors that influence the occurrence of convection (Figure 9.15). Combining satellite observations of cloud properties with surface weather
observations shows that it is the large scale circulation and its influence on the boundary layer
humidity that sometimes inhibits convection over warm oceans and that the ocean temperature
is important for triggering convection because the tropical atmosphere is more nearly isothermal
than the ocean surface (Fu et al., 1990, 1994).
PACIFIC OCEAN(2X2)
20N
'"
ION
Q
::>
I0
...
I""
...l
l OS
20S
120£
14 0E
1 60E
180
IBOW
LONG I TUDE
Figure 9.15: Average distribution of sea surface temperature in °C (dashed contours) and
frequency of occurrence of deep convective cloudiness (solid contours) for January 1984 over
the tropical Pacific (from Fu et al., 1994).
The first survey of the cloud properties associated with deep convection shows two notable
features: strong concentration of convection over land (but not the hottest land areas) and the
strong variations of cloud properties with the size of the system (Machado and Rossow, 1993).
There is a particularly suggestive pattern to the size dependence: as the system size grows, so
does the area covered by convective towers. As they grow larger in horizontal dimension, they
extend higher into the atmosphere (lower cloud top temperatures in Figure 9.16). Surface and
aircraft observations have also correlated the size and vertical extent of convective towers with
the strength of the updrafts (e.g. Betts, 1973; Zipser and LeMone, 1980). The main change
in the stratoform cloud component of these systems as their size increases is that their optical
thicknesses or albedos increase (Figure 9.16). Since these stratoform clouds tend to have nearly
constant base and top locations (e.g., Gamache and Houze, 1982; Tollerud and Esbensen, 1985),
the albedo increase is equivalent to a water content increase, suggesting that precipitation also
becomes more frequent (Lin and Rossow, 1996).
W.B. Rossow
What atmospheric conditions produce deep convection in the tropics and what kinds of clouds
are produced? The simplest answer to the first question is that surface heating by sunlight produces buoyant parcels of moisture-laden air near the surface that, once condensation begins,
can rise all the way to the tropopause. This simple model predicts a concentration of convection over regions with high surface temperatures; but the actual distribution of such convective
systems contradicts this simple prediction in several ways. The hottest surface temperatures
appear in North Africa and other nearly cloud-free deserts where there is no convection. The
hottest ocean water is in the western Pacific and eastern Indian ocean, yet only the former is
persistently covered by convection. A closer look at the western Pacific shows that convection
does not occur everywhere, suggesting other factors that influence the occurrence of convection (Figure 9.15). Combining satellite observations of cloud properties with surface weather
observations shows that it is the large scale circulation and its influence on the boundary layer
humidity that sometimes inhibits convection over warm oceans and that the ocean temperature
is important for triggering convection because the tropical atmosphere is more nearly isothermal
than the ocean surface (Fu et al., 1990, 1994).
PACIFIC OCEAN(2X2)
20N
'"
ION
Q
::>
I0
...
I""
...l
l OS
20S
120£
14 0E
1 60E
180
IBOW
LONG I TUDE
Figure 9.15: Average distribution of sea surface temperature in °C (dashed contours) and
frequency of occurrence of deep convective cloudiness (solid contours) for January 1984 over
the tropical Pacific (from Fu et al., 1994).
The first survey of the cloud properties associated with deep convection shows two notable
features: strong concentration of convection over land (but not the hottest land areas) and the
strong variations of cloud properties with the size of the system (Machado and Rossow, 1993).
There is a particularly suggestive pattern to the size dependence: as the system size grows, so
does the area covered by convective towers. As they grow larger in horizontal dimension, they
extend higher into the atmosphere (lower cloud top temperatures in Figure 9.16). Surface and
aircraft observations have also correlated the size and vertical extent of convective towers with
the strength of the updrafts (e.g. Betts, 1973; Zipser and LeMone, 1980). The main change
in the stratoform cloud component of these systems as their size increases is that their optical
thicknesses or albedos increase (Figure 9.16). Since these stratoform clouds tend to have nearly
constant base and top locations (e.g., Gamache and Houze, 1982; Tollerud and Esbensen, 1985),
the albedo increase is equivalent to a water content increase, suggesting that precipitation also
becomes more frequent (Lin and Rossow, 1996).