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
15 Nov
time
5 Nov
Rossby wave dispersion relation f(k)
in a resting atmosphere
net group speed (U + cg) >>0: fast eastward
frequency f
= 1/period
mean flow U: (U+cp)>0, crests move east slowly
group velocity
cg = df/dk
a bit eastward
short waves
large (negative)
wavenumber
wavenumber k = 1/wavelength
(negative: Rossby waves have both
phase and group speed
westward)
0
phase speed 10 m/s
~700mb steering level
group speed much faster eastward
15 Nov
“downstream
development
”
time
phase speed
5 Nov
• omega 500
v250 (upper level vorticity) reaches
down into the troposphere to make
weather
15 Nov
time
phase speed
5 Nov
Upper level troughs and
extratropical cyclones (storms)
• That upper-level dynamical view (Rossby
waves in mean westerly flow) is just 2D
barotropic theory
• ignores why we care: storms at surface
• ignores reasons why there ARE vigorous waves
on the jet stream (baroclinic instability)
• Vorticity strip at 500
gets pooled,
‘induced’ flow
intensifies
• at surface too
• surface flow wraps
warm air north, cold
air south so dT/dx
increases near L
• this increases v500
by thermal wind,
intensifying trough
• trough in mean u>0,
catches up to
surface low
• trough ‘cuts off’ at
500mb by end
• case
• 1969 book
Vertical motion and
clouds
RH and model cloud water
v on Temperature
warm
cyclonic
cold core
below = low
thickness
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
warm front 5 UTC
Appalachian mountains
effect
delayed warm front 15 UTC
Pressure drops
precipitation reports
water vapor in upper troposphere
Cold front collapsed to a
very narrow squall line
• Rain
band
(seen on
radar) is
so thin!
• Other
levels
warm
cold
cold
warm
topographic
Rossby waves
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
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