Geography Reference
In-Depth Information
cumulonimbus are observed to be much smaller than those in midlatitude thun-
derstorms.
9.5.5
Entrainment
In the previous subsection it was assumed that convective cells rise without mixing
with environmental air so that they maintain constant θ
e
during their rise. In reality,
however, rising saturated air parcels tend to be diluted by entraining, or mixing
in, some of the relatively dry environmental air. If the air in the environment is
unsaturated, some of the liquid water in the rising parcel must be evaporated to
maintain saturation in the convective cell as air from the environment is entrained.
The evaporative cooling caused by entrainment will reduce the buoyancy of the
convective parcel (i.e., lower its θ
e
). Thus, the equivalent potential temperature
in an entraining convection cell will decrease with height rather than remaining
constant. Similar considerations hold for any other conservable variable
3
in which
environmental values differ from cloud values; entrainment will modify the in-
cloud vertical profiles.
Denoting the amount of an arbitrary conservable variable per unit mass of air by
A, the vertical dependence of A in an entraining convective cell can be estimated
by assuming that to a first approximation the cell can be modeled as a steady-state
jet as shown in Fig. 9.11. Thus, in a time increment δt a mass m of saturated cloud
air with an amount of the arbitrary variable given by mA
cld
mixes with a
mass
Fig. 9.11
An entraining jet model of cumulus con-
vection. See text for explanation.
3
A conservable variable is one that is conserved following the motion in the absence of sources and
sinks (e.g., a chemical trace constituent).
