Geoscience Reference
In-Depth Information
z
Fig. 2.4
Conditional instability in the
atmosphere. A partly saturated
air particle, which is raised,
initially undergoes a rate of
temperature decrease
Γ
d
(dashed
line) which is larger than that of
the environment,
Γ
. As it becomes
saturated at
z
C
, the rate of
temperature decrease is reduced
to the saturated adiabatic lapse
rate
Γ
s
(dashed line). Above
z
F
,
the free convection level,
conditions become unstable.
z
Γ
s
Γ
z
Γ
d
AIR
PARTICLE
Temperature
The quantity on the right-hand side of (2.24) is called the saturated adiabatic lapse rate,
s
. Since normally
dq
0, the saturated adiabatic lapse rate must be smaller than
the dry adiabatic lapse rate. Moreover, (
L
e
/
/
dz
<
dz
) depends on the temperature. At
high temperatures, near the Equator, it is of the order of
c
p
)(
dq
/
s
≈
.
d
, whereas at lower
0
35
d
,
i.e. 9.8
◦
Ckm
−
1
. In the lower layers of the atmosphere it is on average of the order of
5.5
◦
Ckm
−
1
. If in a rising air mass the condensed moisture is being removed (e.g. through
precipitation), the rate of temperature decrease is called the pseudo-adiabatic lapse rate.
However, under most conditions the heat loss by the removal of this condensed water is
fairly negligible, and the saturated adiabatic lapse rate is a satisfactory approximation.
Thus, for saturated air, one has the following stability criteria:
−
30
◦
C, it has approximately the same value as
temperatures, for example around
>
s
unstable
=
s
neutral
<
s
stable
2.2.3
Conditional instability
It often happens that the actual lapse rate in the atmosphere is intermediate between the
dry and the saturated adiabatic lapse rate, that is
s
<<
d
; this case is referred to as
conditional instability. When a partly saturated parcel of air is raised in such an atmo-
sphere, it will initially change its temperature at the dry adiabatic rate in accordance
with Equation (2.8), and thus remain colder than the surroundings (see Figure 2.4).
