Geography Reference
In-Depth Information
z
y
Fig. 9.5
Relationship of the ageostrophic circulation (heavy curve with arrows) in two-dimensional
frontogenesis to the potential temperature field (long dashes) and absolute momentum field
(short dashes). Cold air is on the right and warm air on the left. Note the tilt of the circulation
toward the cold air side and the enhanced gradients of the absolute momentum and potential
temperature fields in the frontal zone.
9.3
SYMMETRIC BAROCLINIC INSTABILITY
Observations indicate that mesoscale bands of cloud and precipitation commonly
occur in association with synoptic-scale systems. These are often aligned with
fronts, but are separate entities. In common with fronts, such features are generally
associated with strong baroclinicity and have length scales parallel to the mean
wind shear that are much larger than the scales across the wind shear. A plausible
source for such features is a two-dimensional form of baroclinic instability known
as
symmetric instability
(also referred to as slantwise convection).
For typical atmospheric conditions, buoyancy tends to stabilize air parcels
against vertical displacements, and rotation tends to stabilize parcels with respect
to horizontal displacements. Instability with respect to vertical displacements is
referred to as hydrostatic (or simply, static) instability (see Section 2.7.3). For an
unsaturated atmosphere, static stability requires that the local buoyancy frequency
squared be positive (N
s
> 0). Instability with respect to horizontal displacements,
however, is referred to as inertial instability (see Section 7.5.1). The condition for
inertial stability (7.54) requires that F
2
as defined in (9.16) be positive.
If parcels are displaced along slantwise paths rather than vertical or horizontal
paths, it is possible under certain conditions for the displacements to be unstable
even when the conditions for ordinary static and inertial stability are separately
satisfied. Such instability can occur only in the presence of vertical shear of the
mean horizontal wind and may be regarded as a special form of baroclinic insta-
bility in which the perturbations are independent of the coordinate parallel to the
