Notes on Quasi-Geostrophic Processes for Short Waves (Ideal Case)
Geopotential tendency and omega equations
rising
Φ
produced by: anticyclonic
ζ
advection
warm advection decreasing with height
sinking
Φ
produced by: cyclonic
ζ
advection
cold advection decreasing with height
upward motion produced by: cyclonic
ζ
advection increasing with height
warm advection
downward motion produced by: anticyclonic
ζ
advection increasing with height
cold advection
Typical atmospheric conditions:
•
Temperature advection is strongest in the lower-troposphere and near-zero in the
midtroposphere.
•
Relative vorticity advection is strongest in the midtroposphere and near-zero in
the lower troposphere.
Typical growing extratropical cyclone:
•
The 500 mb trough is west of the surface low
•
Cold advection (b) under the trough (B) amplifies the trough
•
Warm advection (d) under the ridge (D) amplifies the ridge
•
Convergence (A) over the surface high (a) amplifies the surface high
•
Divergence (C) over the surface low (c) amplifies the surface low
Typical decaying extratropical cyclone:
•
The 500 mb trough is east of the surface low
•
Cold advection (b) under the ridge (D) reduces the trough
•
Warm advection (d) under the trough (B) reduces the ridge
•
Convergence (A) over the surface low (c) reduces the surface low
•
Divergence (C) over the surface high (a) reduces the surface high
Physical processes associated with a 500 mb short wave
West of upper-level trough (A)
•
Anticyclonic vorticity advection causes increasing anticyclonicity
•
Increasing anticyclonicity causes unbalanced inward Coriolis force (convergence)
•
Midtropospheric convergence increases atmospheric column mass and hence
raises surface pressure
•
Midtropospheric convergence with small surface change causes downward
motion in the lower troposphere
•
Downward motion adiabatically warms lower troposphere and causes 500 mb
geopotential to rise
•
Rising 500 mb geopotential causes ridge to propagate eastward
At upper-level trough axis (B)
•
No vorticity advection
East of upper-level trough (C)
•
Cyclonic vorticity advection causes increasing cyclonicity
•
Increasing cyclonicity causes unbalanced outward Coriolis force (divergence)
•
Midtropospheric divergence decreases atmospheric column mass and hence
lowers surface pressure
•
Midtropospheric divergence with small surface change causes upward motion in
the lower troposphere
•
Upward motion adiabatically cools lower troposphere and causes 500 mb
geopotential to sink
•
Sinking 500 mb geopotential causes trough to propagate eastward
At upper-level ridge axis (C)
•
No vorticity advection
TROUGH
RIDGE
A
B
C
D
Physical processes associated with surface low and high centers
At surface high center (a)
•
No temperature advection
West of surface low center (b)
•
Cold advection in lower troposphere causes 500 mb geopotential to sink
•
Sinking 500 mb geopotential causes unbalanced inward pressure gradient force
(convergence)
•
Midtropospheric convergence with small surface change causes downward
motion in the lower troposphere
•
Midtropospheric convergence increases atmospheric column mass and hence
raises surface pressure
•
Increasing surface pressure causes surface high to propagate eastward
At surface low center (c)
•
No temperature advection
East of surface low center (d)
•
Warm advection in lower troposphere causes 500 mb geopotential to rise
•
Rising 500 mb geopotential causes unbalanced outward pressure gradient force
(divergence)
•
Midtropospheric divergence with small surface change causes upward motion in
the lower troposphere
•
Midtropospheric divergence decreases atmospheric column mass and hence
lowers surface pressure
•
Decreasing surface pressure causes surface low to propagate eastward
a
b
c
d
L
H