Geoscience Reference
In-Depth Information
and free to rise. This is termed
conditional instability
;
the development of instability is dependent on the
airmass becoming saturated. Since the environmental
lapse rate is frequently between the dry and saturated
adiabatic rates, a state of conditional instability is com-
mon. The path curve intersects the environment curve
at 650 mb. Above this level the atmosphere is stable,
but the buoyant energy gained by the rising parcel
enables it to move some distance into this region. The
theoretical upper limit of cloud development can be
estimated from the tephigram by determining an area
(B) above the intersection of the environment and path
curves equal to that between the two curves from the
level of free convection to the intersection (A) in Figure
5.5. The tephigram is so constructed that equal areas
represent equal energy.
These examples assume that a small air parcel is
being displaced without any compensating air motion
or mixing of the parcel with its surroundings. These
assumptions are rather unrealistic. Dilution of an
ascending air parcel by mixing of the surrounding air
with it through
entrainment
will reduce its buoyant
energy. However, the parcel method is generally satis-
factory 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 about 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 illustrated
by use of a tephigram. Figure 5.7 shows an initial dis-
tribution of temperature and dew-point. Vertical mixing
leads to averaging these conditions through the layer
affected. Thus, the
mixing condensation level
is deter-
mined from the intersection of the average values of
saturation humidity mixing ratio and potential temper-
ature. The areas above and below the points where these
B'
(B')
DRY
ADIABAT
FINA
L
LAPSE RATE
(UNSTABLE)
Amount of
lifting
B
(B)
(DRY)
(A')
SATURATED
ADIABAT
A'
INITIAL
LAPSE RATE
(STABLE)
Condensation
Level
Amount
of lifting
Figure 5.6
Convective instability. AB
represents the initial state of an air
column; moist at A, dry at B. After
uplift of the whole air column the
temperature gradient A´ B´ exceeds
the saturated adiabatic lapse rate, so
the air column is unstable.
DRY
ADIABAT
A
(MOIST)
(A)
TEMPERATURE
