DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

conserves energy; if the column is conditionally
unstable and the humidity exceeds a specified
value, the column is adjusted to moist static
neutrality.
convective cloud A type of cloud that is
generated by convective activities in the atmosphere.
The main feature of it is its strongly
vertical development. Strong vertically developed
clouds are also called heap clouds.
convective heat transfer Transfer of heat
by mass movement, due to free or forced convection.
The latter case is also referred to as
advective heat transfer.
convective instability Stratification instability
caused by convective activities, i.e., the lower
layer has higher moisture and becomes saturated
first when being lifted, and hence cools
thereafter at a rate slower than does the upper,
drier portion, until the lapse rate of the whole
layer becomes equal to the saturation adiabatic
and any further lifting results in instability. In
general, use ∂θsw
∂Z
< 0 or ∂θse
∂Z
< 0 as the cri-
terion of the convective instability, where θsw
and θse are pseudo-wet-bulb potential temperature
and pseudo-equivalent potential temperature,
respectively.
convective scaling In the boundary layer,
pure convective turbulence depends only on the
thickness Hcon of the convectively well-mixed
layer and the boundary buoyancy flux J o b [Monin
and Obukhov, 1954]. Dimensional analysis provides
the scaling relations as a function of Hcon
and J o b by:
Length Lcs ∼ Hcon
Time τcs ∼ Hcon/wcs
Velocity wcs ∼ (HconJ o b )1/3
Diffusivity Kcs ∼ wcsHcon
Dissipation of turbulent
kinetic energy εcs ∼ J o b
If convection is driven purely by heat fluxes (i.e.,
J o b = gαFth/(cpρ)), further scaling relations for
temperature are as shown in the table on page 86.
convective turbulence If density increases
at the top of a fluid (i.e., cooling, evaporation
© 2001 by CRC Press LLC
convergent plate boundaries
of salty water at the surface, etc.) or if density
decreases at the bottom of a fluid (i.e., warming
from below, etc.), convective plumes (thermals
for temperature) will set in and mix a progressively
thicker boundary layer. Convective
scaling allows the quantification of the relevant
physical parameters of convection as a function
of the boundary buoyancy flux Jo b and the thickness
Hcon of the convectively unstable layer. See
also penetrative convection.
convergent boundary Plates are destroyed
or severely deformed at convergent boundaries.
Two types of convergent boundaries exist: subduction
and collision. A subduction convergent
boundary occurs when two plates composed of
oceanic crust (thin, basalt composition) or an
ocean plate and a continental plate (thick, more
silicic composition) meet. At the ocean-ocean
boundary, one of the oceanic plates dives down
under the other plate. At an ocean-continent
boundary, the oceanic plate is always subducted
under the continental plate. The location where
the first plate subducts under the second plate is
characterized by a deep trough, called a trench.
As the oceanic plate dives deeper into the Earth’s
interior, the temperature rises and sediments
which accumulated on the ocean floor begin to
melt. This magma rises towards the surface
and erupts on the overriding plate, creating the
very explosive volcanos called stratovolcanos
(or composite volcanos). Subduction boundaries
are characterized by this explosive volcanism
and earthquakes from a variety of depths
(down to about 200 km). Japan is an example
of an ocean-ocean subduction boundary, while
the Cascade volcanos (including Mt. St. Helens)
in the northwestern U.S. are an example of the
ocean-continent subduction boundary. If both
plates are composed of continental crust, neither
plate is subducted. Instead the two plates crumple
to form high mountain ranges, such as the
Himalayas. This type of convergent boundary is
called a collision boundary and is characterized
by earthquakes but no volcanism. Convergent
boundaries are believed to occur over the descending
portions of convection cells within the
Earth’s asthenosphere.
convergent plate boundaries See convergent
boundary.