Geoscience Reference
In-Depth Information
the dry adiabatic lapse rate, so the potential temperature of the overlying air is
greater than the air in the ABL. In general (and in Fig. 3.3), virtual potential
temperature is typically just a few degrees greater than potential temperature,
the difference being directly related to the specific humidity of the atmosphere,
see Equation (2.8).
Atmospheric stability
If a parcel of air moves up or down in the atmosphere (perhaps in response to
atmospheric turbulence) it is likely that it will find itself in a new environment
whose temperature and density differs from that which would have resulted from
adiabatic warming or cooling of the parcel. Any resulting density difference
between the parcel and the air that surrounds it will give rise to a buoyancy force
which will influence further vertical movement of the parcel.
If after moving, the parcel has a higher temperature and is less dense and lighter
than the air it displaces, it is said to be unstable in its new location and further
upward movement will be encouraged and downward movement inhibited. If
after moving the parcel has a lower temperature and is more dense and heavier
than the air it displaces, it is said to be stable in its new location and further upward
movement will be inhibited and downward movement encouraged. If after
moving, the parcel has the same temperature and density as the air it displaces, it
is said to be neutral with respect to the environment in its new location and further
movement is neither encouraged nor inhibited.
Static stability parameter
From Archimedes principle, if a parcel of air with density
r
a
′
moves vertically and
displaces the same volume of surrounding air with density
r
a
, it will be subject to
a force such as to cause a buoyant acceleration,
a
p
, given by:
(
)
ρρ
ρ
′−
a
a
a
g
=−
(3.20)
p
a
Assuming the pressure of both parcels is the same, substituting for densities using
Equation (2.11) this last equation becomes:
(
)
TT
T
−′
v
v
a
g
=−
(3.21)
p
where
T
v
' and
T
v
are the virtual temperatures of the parcel and the surrounding air,
respectively. If vertical air movement is viewed in terms of small displacements,
