Meteorology 1010Supplement 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 StreamsMeridional 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.

Notice that the cold-front side is more vigorous than the warm front side (right side).

Cold front

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

Notice that in a mid-latitude 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.

12 3

4 5 6

Westerly winds

Warm, wet air rises above cool/dry air classic “frontal” storm.

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/dryhelps produce rising air, high wind, precipitation, hail, lightning.

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 air-mass 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

2013 tornado in Oklahoma probably produced 300 mph wind.

One Sunday tornado reached nearly 200 mphRegarded as EF4

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