Geography Reference
In-Depth Information
Table 6.1
Characteristics of a Developing Baroclinic Disturbance
Physical
A
B
C
parameter
500-hPa trough
Surface low
500-hPa ridge
∂(δ)/∂t
Negative (thickness
Negative
Positive (thickness
(500-1000 hPa)
advection partly
(adiabatic
advection partly
canceled by
cooling)
canceled by
adiabatic warming)
adiabatic cooling)
w (500 hPa)
Negative
Positive
Positive
∂/∂t
Negative
Negative
Positive
(500 hpa)
(differential
(vortity
(differential
thickness advection)
advection)
thickness advection)
∂ζ
g
/∂t
Negative
Positive
Positive
(1000 hPa)
(divergence)
(convergence)
(convergence)
∂ζ
g
/∂t
Positive
Positive
Negative
(500 hPa)
(convergence)
(advection partly
(divergence)
canceled by
divergence)
becomes slightly subgeostrophic and experiences an acceleration across the iso-
bars toward lower pressure. This cross isobaric ageostrophic wind component is
responsible for the convergence spins up the vorticity in the upper troposphere
so that it adjusts geostrophically to the new geopotential distribution. In terms
of the momentum balance, the cross isobaric flow is accelerated by the pressure
gradient force so that the wind speed adjusts back toward geostrophic balance. In
the region of the upper level ridge, analogous arguments apply, but in this case
the ageostrophic flow leads to a divergent secondary circulation. In both cases,
as shown in Chapter 8, the ageostrophic flow toward lower pressure is associated
with conversion of energy from potential energy to kinetic energy.
PROBLEMS
α∂ ln θ
∂p may be written
=−
6.1.
Show that the static stability parameter σ
in terms of as
R
c
p
−
1
∂
∂lnp
−
∂
∂lnp
∂
2
∂p
2
1
p
∂
∂p
1
p
2
R
c
p
σ
=
−
=
6.2.
Show that for an isothermal atmosphere σ , as defined in Problem 6.1, varies
inversely as the square of the pressure.
6.3.
Suppose that on the 500-hPa surface the relative vorticity at a certain location
at 45˚ N latitude is increasing at a rate of 3
10
−
6
s
−
1
×
per 3 h. The wind
