Meteorology 1010
Supplement to Chapters 9 and 10
This PowerPoint is not a substitute for
reading the textbook and taking good
notes in class.
Compare the models of atmospheric circulation in
this PowerPoint to the graphics in Chapters 9-11.
This PowerPoint presentation supports Quiz #4,
Fall Semester, 2013.
Powerpoint presentations are not a substitute for
reading the class textbook. Instead, they should
be viewed as a study guide.
Jet Streams
Meridional vs Zonal
Flow
When the jet stream
provokes storms by
mixing cool/dry with
warm/wet storminess
can occur.
Mid-latitude storms
often begin with
mixing of cool/dry air
and warm/wet air as
the jet stream(s)
undulates more
north/south, helping
to mix differing air
masses.
High
Low
November 18 Salt Lake weather report indicated that
westerly flow was going to be more ‘zonal’ rather than
‘meridional’ so there would not be any increasing
intensity that otherwise might occur if ‘storm track’
flow dipped north or south to pick up greater moisture
and/or greater contrast.
Sunday’s tornadoes in Illinois were unusual because of
more-than-normal moisture and heat – November
usually moves toward just bumpy snow storms, with
some wind.
Notice that is eastward-moving storm is typical - counter-clockwise
air in the mid-latitudes
Frontal Weather
This one is a cold front. A warm front would
have similar features, but more gradual.
Cold front
Notice that the coldfront side is more
vigorous than the
warm front side
(right side).
Life of a Midlatitude Cyclone
Notice that the
cold front side
shows more
severe radar
returns – more
vigorous lifting
and more likely
severe weather.
Relatively
Cold Air
Relatively
Warm Air
1
2
3
Notice that in a midlatitude cyclone, cold and
warm air don’t mix at first.
As Coriolis force helps
turn the air, mixing begins
as warm, humid air lifts
over cooler, drier air that
is more heavy.
Rising air provokes
condensation,
precipitation and strong
winds.
At the end, warm air is
temporarily stable above
cold air below.
4
5
6
Here we see how the jet
stream (with storm track)
helps pull low and high
pressure cells toward each
other.
The difference between
warm/wet and cool/dry
helps produce rising air,
high wind, precipitation,
hail, lightning.
Warm, wet air rises
above cool/dry air
classic “frontal”
storm.
If you click quickly on the next six slides you can
see how a mid-latitude cyclone can develop.
Figure 9.18a
Figure 9.18b
Figure 9.18c
Figure 9.18d
Figure 9.18e
Figure 9.18f
Air-Mass Thunderstorms
Air-Mass Thunderstorms
• Occurrence:
– Mountainous regions, such as the Rockies and the
Appalachians, experience a greater number of airmass thunderstorms.
Severe Thunderstorms
• Severe thunderstorms:
– Heavy downpours
– Flash flooding
– Straight line wind gusts
– Hail, lightning
– Wind shear
– Can overshoot (enter stratosphere)
– Downdraft preceding (gust front)
Supercell Thunderstorms
Supercell Thunderstorms
• Supercells
– These storms can produce extremely dangerous
weather.
• They consist of a single, powerful cell that can extend to
heights of 20 km or more.
• The clouds can measure 20–50 km in diameter.
• Mesocyclone:
– Vertical winds may cause the updraft to rotate, which
forms a column of cyclonically rotating air.
– Tornadoes often form.
Supercell Thunderstorms
• Squall lines:
– Squall lines are narrow bands of thunderstorms.
– cT air is pulled into the warm sector of a
midlatitude cyclone.
– Mammatus skies sometimes precede squall lines.
– These can also form along a dryline, where there
is an abrupt change in moisture.
Supercell Thunderstorms
• Squall lines
Supercell Thunderstorms
• Mesoscale convective complexes (MCC):
– An MCC consists of many individual
thunderstorms.
– It is organized into a large oval to circular cluster.
– They cover an area of at least 100,000 km2.
– It is a slow-moving complex that may last for 12
hours or more.
– MCCs tend to form mainly in the Great Plains.
Tornadoes
• Tornadoes (twisters, cyclones):
– These are violent windstorms with a rapidly
rotating column of air, or vortex.
• Pressures within tornadoes can be as much as 10%
lower than immediately outside the storm.
– It may consist of single or multiple vortices.
Tornadoes
The Development and Occurrence
of Tornadoes
• Tornado development
The Development and Occurrence
of Tornadoes
• Tornado climatology:
– Squall lines
– Cold fronts
– Where cP and mT meet
– Midwest U.S.
The Development and Occurrence
of Tornadoes
• Profile of a tornado:
– Average diameter 150–600m
– Travels ~45 kph – 28mph (Sunday storms moved at 60+mph)
– Path about 26 km long
– Most travel to the NE
– Exist between < 3 min to > 3 hours
– Wind speeds between < 150 kph to > 500 kph (Sunday
storm winds estimated at 190 mph)
The Development and Occurrence
of Tornadoes
Based on this chart, can we use single
events to support the theory of global
warming?
Tornado Destruction
Tornado Destruction
• Tornado intensity
One Sunday
tornado reached
nearly 200 mph
Regarded as EF4
2013 tornado in
Oklahoma
probably
produced 300
mph wind.
Tornado Destruction
• Loss of life
Tornado Forecasting
• Tornado watches and warnings:
– Watches alert the public.
• Tornadoes are possible and conditions are favorable.
• They usually cover an area of about 26,000 km2.
• Watches can last 3 hours or longer.
– Warnings are issued when a tornado is actually
sighted or conditions are just right.
• There is a high probability of imminent danger.
• They are usually for a much smaller area.
• Warnings are in effect for a much shorter period,
usually 30–60 minutes.
Sunday storms were described as
“Predictions are saving lives.”
?
Oddly, hurricanes are big enough that we can
almost predict details, but in some ways too big to
really say what, where and when.
Oddly also, tornadoes are so small and transient
that we get precise results but great difficulty in
prediction.
We know exactly what homes got hit, but only
afterward.
Tornado Forecasting
• Doppler radar
– This radar measures the motion and speed of the
wind.
– Two or more units are optimal for more accurate
forecasting.
– Tornadoes have hooked-shaped echoes.
Tornado Forecasting
• Doppler effect