Geoscience Reference
In-Depth Information
400
7185
AREA A = AREA B
ENVIRONMENT
CURVE
500
5574
APPROXIMATE
CLOUD TOP
600
A
4206
700
3012
PATH
CURVE
B
850
1457
LEVEL OF FREE CONVECTION
SALR
LIFTING CONDENSATION LEVEL
DALR
1000
-10
0
T
d
10
Temperature (°C)
Figure 5.5
Schematic tephigram illustrating the conditions associated with the conditional instability of
an air mass that is forced to rise. The saturation mixing ratio is a broken line and the lifting condensation
level (cloud base) is below the level of free convection.
the surrounding air with it through
entrainment
will reduce its buoyant energy. However, the
parcel method is generally satisfactory for routine
forecasting because the assumptions approximate
conditions in the updraft of cumulonimbus
clouds.
In some situations a deep layer of air may be
displaced over an extensive topographic barrier.
Figure 5.6
shows a case where the air in the upper
levels is less moist than that below. If the whole
layer is forced upward, the drier air at B cools at
the dry adiabatic rate, and so initially will the air
around A. Eventually the lower air reaches
condensation level, after which this layer cools at
the saturated adiabatic rate. This results in an
increase in the actual lapse rate of the total
thickness of the raised layer, and, if this new rate
exceeds the saturated adiabatic, the air layer
becomes unstable and may overturn. This is
termed
convective
(or
potential
)
instability
.
Vertical mixing of air was identified earlier as
a possible cause of condensation. This is best
