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Thunderstorms
AOSS 102:
Extreme Weather
18 November 2015
Atmospheric
Stability and
Instability
Stability of the atmosphere can be
determined when comparing the environmental lapse
rate with the dry adiabatic (10 K/km) and moist adiabatic
lapse rates (6 K/km)
6 K / km
10 K / km
Image Quiz
Altitude (km)
Thunderstorms develop in Absolutely Unstable or Conditionally Unstable Conditions
Conditionally unstable: stable for unsaturated ascent
and unstable for saturated ascent of an air parcel
Fig. 5.18, p. 135
Why do Thunderclouds (Cumulonimbus clouds) form in unstable conditions?
Air parcel
Fig. 5.10, p. 130
Image Quiz
Image Quiz
Annual Average Days with Thunderstorms
in the United States
Annual Average Days with Thunderstorms
Worldwide
Thunderstorm Categories:
Airmass Thunderstorm
(Ordinary Cell Thunderstorm)
Characteristics:
➡  can be located far from fronts
➡  forced by surface heating on sunny days, low-level
warming destabilizes the environment
➡  weak vertical wind shear is present
➡  three life cycle stages
➡  relatively short-lived system
➡  intense precipitation when mature, but rarely severe
➡  lightning
Airmass Thunderstorm:
Mature Stage
Airmass Thunderstorm:
Beginning Cumulus Stage
(Growth Stage)
Fig. 11.2, p. 299
Airmass Thunderstorm in its Mature Stage: Distinctive Anvil at the Top
Anvil: Spread-out ice cloud at the top
Fig. 11.1, p. 299
Airmass Thunderstorm:
Dissipating Stage
•  After thunderstorm enters mature
stage it begins to dissipate after
15-30 minutes
•  Downdrafts dominate, cutting off the storm’s fuel supply (rising
warm humid air)
•  All three stages might be over in
less than 1 hour
•  Lower-level clouds quickly evaporate
Third (Dissipating) Stage
Airmass Thunderstorm in its Dissipating Stage
“Weight” of a Thunderstorm
Assume thunderstorm is a cube 15 km wide,
deep and tall and contains about 1 gm/m3 of
water.
•  Volume = 3.4 trillion m3.
•  Mass = 3.4 trillion grams
or ~3.7 million tons of water.
Thunderstorm Categories:
Multicell Thunderstorms
http://www.youtube.com/watch?v=_cl0aw87LqA
https://www.youtube.com/watch?v=LYubHpEMTPM
Multicell Thunderstorms
—  Contain a number of convection cells, each in a different
stage of development
—  Moderate to strong vertical wind shear is present
—  Tilt: updrafts rides up and over the downdrafts
—  Overshooting top possible (intrusion into stratosphere)
—  The longer the storm persists, the more severe
—  Cold downdrafts under the storm reach the surface,
leading to sharp temperature drops
—  Leads to gust front and downbursts & microbursts
Characteristics of a Multicell
Thunderstorms
Updrafts and downdrafts are tilted, supplying additional energy.
Example of a Shelf Cloud
Example of a Roll Cloud
forming behind a gust front
Exam 2: Statistics
Mean:
74% Median: 75%
Exam 2: Statistics
Some rather big changes in the grades from exam 1 to exam 2
Exam 2: Observations
Multiple Choice section:
10) Porto, Portugal averages 51 °F in December. Why is it warmer than Ann Arbor even though they are at nearly the same latitude?
Image Quiz
Multiple Choice
Exam 2: Observations
13) What are some differences between El Niño and La Niña?
A) In La Niña, the trade winds strengthen, and in El Niño they slacken or reverse
B) The warm pool of ocean water is shifted to the east during La Niña
C) Upwelling along the coast of Peru can be "shut down" during a La Niña
La Niña
El Niño
D) All of the above
Sea Surface Temperature Anomaly
Exam 2: Observations
17.)
What makes wind flow across isobars at the surface
(while winds aloft generally flow parallel to them)?
A. Coriolis force
B. friction
C. the jet stream
D. gravity
E. pressure gradient force
Exam 2: Observations
19. The Coriolis force deflects the air motion in the
Southern Hemisphere ___ near a surface high pressure
system and ___ near a surface low pressure system.
A. to the left, to the left
B. to the left, to the right
C. to the right, to the left
D. to the right, to the right
Always to the left of the wind in Southern Hemisphere
Exam 2: Observations
28.) Direction of the PGF and Coriolis force at Mount Isa in Australia (no friction).
PGF
Coriolis
Coriolis force always to the left (right angle) of the wind in Southern Hemisphere
Exam 2: Observations
30) Direction and speed of the steering background wind
northeastward
10 mph
110 mph
-10 mph
110 mph
110 mph
110 mph
+10 mph
Exam 2: Observations
32.)
Position of the warm and cold front.
Warm
Cold