Wallace and Hobbs Ch 8
Cell Model
storms: tropical and3- extratropical
Upper
Level
wind
Jet streams unstable
ITCZ
phase speed 10 m/s
~u700 steering level
group speed much faster
5 Nov
time
25 Nov
A
B
• “Downstream
development”
B
C
B
• Dispersion of Rossby
waves
C
C
D
D
D
E
Rossby wave dispersion relation w(k)
Wikipedia:
phase
moves
westward
Rossby wave dispersion relation w(k)
10
8
6
4
2
 planetary wavenumber k 
0
Rossby wave dispersion relation w(k)
Upper level troughs and
extratropical cyclones (storms)
• That upper-level dynamical view (Rossby
waves in mean westerly flow) is just 2D
– ignores why we care: storms at surface
– ignores why there ARE vigorous waves on the jet
stream (baroclinic instability)
• Vorticity strip at 500
gets pooled,
‘induced’ flow
intensifies
– induces flow at sfc. too
• surface flow wraps
warm air north, cold
air south so dT/dx
increases near L
• this increases v500
by thermal wind,
intensifying trough
• 500mb trough is in
westerlies, catches
up to surface low
• trough ‘cuts off’ at
500mb by end
• case
• 1969 book
Vertical motion and
clouds
East-west section: omega
RH and model cloud water
East-west section: O3
East-west section: Ertel’s PV
cyclonic
(Trof)
cyclonic
Low
v on Temperature
warm
cyclonic
(Trof)
cold core =
low
thickness
warm
cyclonic
Low
Meridional T advection
cool air
flowing
south
warm air
flowing
north
Low
Veering/backing and T advection
TROF
winds
backing
with
height
cool
coolair
air
flowing
flowing
south
south
winds
veering
with
height
warm
warmair
air
flowing
flowing
north
north
Low
The omega equation
• Why is there upward motion ahead of these
upper level troughs/ PV cyclones?
cyclonic z
(Trof)
Thermal wind
balance prevails:
There is a Z trough
(trof) for
geostrophic
balance, with a
cold core beneath
it, supporting it
hypsometrically (in
hydrostatic
balance).
Unsheared advection of T, u, v, vort, PV:
no problem, whole structure moves
cyclonic
(Trof)
Sheared advection
breaks thermal wind balance
cyclonic z
(Trof)
Sheared advection
breaks thermal wind balance
Z Trof
Sheared advection
breaks thermal wind balance
Z Trof
L
Fronts:
Initially E-W
isotherms
get
wrapped
around
the low.
Deformation
causes
the
gradient
to
tighten.
• Secondary cold
front
• Topographic?
frontal ‘surface’ can be defined in the vertical too
cold front...2
warm front 5 UTC
Appalachian mountains
effect
delayed warm front 15 UTC
Pressure drops
precipitation reports
water vapor in upper troposphere
Cold front contains a very
narrow squall line
• Rain
band
(seen on
radar) is
so thin!
• Other
levels
C
W
W
C
C
W
W
C
warm
cold
cold
warm
topographic
Rossby waves
PV
PV
Convective storms
• 8.3 Deep Convection
http://weather.uwyo.edu/
upperair/sounding.html
ORDINARY
MULTI-CELL
SQUALL LINE: LIKE
A 2D VERSION OF
MULTI-CELLULAR
CONVECTION
SUPER-CELL
pairs
in strong
shear
L
L
nonhydrostatic
so it acts on w
Curved hodograph (not Coriolis force
acting on the storm) favors right-moving
(cyclonic) supercells in USA
Storm motion to right of
tropospheric mean wind
which steers ordinary
thunderstorms
• Coincidental
resemblance to
a baroclinic
frontal cyclone
but at much
smaller scale
Hurricanes – familiar at RSMAS