Transcript PPT - Colorado State University

Types of Lightning
• Intracloud: most frequent (can also have cloud to cloud)
• Cloud to ground
• Air discharge: from charged region to air
Intercloud
Types of Lightning
Bolt from Blue
Cloud-to-Air
Intracloud & Cloud-to-Ground
Spider
The Lightning Discharge: How does it occur?
Stepped leaders and Return Strokes (what your eyes see)
Charge in cloud
produces large
electric fields and
induces charge of
opposite sign at
ground
Attachment: The
downward moving
stepped leader reaches
an upward moving
leader from the ground
establishing a
complete channel of
ionized air – good
conductor of electricity
Breakdown/Stepped
leader development:
The stepped leader is
a series of short
current bursts (~50 m
long) that ionizes a
column of air as it
moves downward
Return stroke:
large current wave
follows ionized air
channel formed by
the stepped leader
and neutralizes
more charge in the
cloud
Subsequent Return Strokes
A dart leader quickly follows the return
stroke and re-ionizes the conducting channel
Average of 2-4 return strokes
per flash (this produces a
flicker that your eye detects)
The second return stroke follows the dart
leader’s arrival at the ground site
The second return stroke can discharge layers
of charge located in other regions of the cloud
Enough with these decades-old drawings,
what does all this look like in real life?
Negative CG with 11 return strokes and a 200 ms continuing current
High-Speed Video via Tim Samaras
Thunder
• Thunder is always produced by
lightning
• Lightning heats the air to
approximately 50,000 oF (Sun
is about 10,000 oF)
• Heating causes rapid expansion of
air around the lightning strike
• Thunder is a shock wave due to
this rapid heating
– Ear responds to this wave energy
© 1999 Steven L. Horstmeyer
Why, at times, is thunder not
heard after a lightning flash?
• Speed of sound slower than speed of light
– If strike is not right next to us, thunder will occur
later
– Count number of seconds between seeing
lightning and hearing thunder and divide by 5,
gives approximate distance to lightning in miles.
• Atmosphere may bend the sound wave
• Eddies may scatter sound waves
– Depending on how far away from the strike you are,
you may not hear the thunder
Audible zone
Since temperature decreases with height, the speed of a sound
wave moving near the ground is faster than a sound wave
above the ground. As a result, the sound wave is bent
(refracted) upward as it moves away from its source
(a lightning channel). Normally, thunder cannot be
heard more than about 15 miles, or 25 kilometers from a
lightning flash (storm).
How do we detect lightning?
• Lightning is an electromagnetic and optical phenomena- we take
advantage of these characteristics to both DETECT and LOCATE lightning
Electromagnetic methods
• Magnetic direction finding
• Time-of-arrival
• Electric field magnitude
Examples
National Lightning Detection Network
Optical methods
• Eyeballs (generally not as accurate)
• Photoelectric devices (sensitive to
optical wavelengths where lighting
exhibits maximum power)
Examples*
NASA Lightning Imaging Sensor
NASA Kennedy Space Center Electric
Field-Mill Network
NASA Optical Transient Detector
New Mexico Tech. University Lightning
Mapping Array
*these instruments are all deployed on
satellites
DMSP Optical Line Scanner
Cloud-to-Ground Lightning (U.S. and Front Range)
National Lightning Detection Network
Florida is the lightning
capital of the U.S.
1989-99
1995-99
0.01
0.02
0.04
0.08
0.16
0.32
0.64
1.28
2.56
5.12 10.24 14.5
2
Flashes/km /year
Fort Collins is in a relative “lightning hole” !
Maritime Continent Thunderstorm Experiment
MCTEX (Colorado State University)
Tiwi Islands, N. Australia, 1997
Electric Field Mill
Sign of charge in clouds
overhead; lightning Detection
E
Magnetic Direction Finder
(Electromagnetic Method)
Locating a cloud-to-ground lightning flash
Magnetic field
Antenna
Voltage  D magnetic Field*angle/D time
Angle
Faraday’s Law
With two or more DF antennas, the direction
(angle) and range to a flash can be determined
E  Charge/Area
Gauss’ Law
Lightning observed by Space Shuttle
NASA-TRMM Satellite
Optical Detection of Lightning
(note other instruments on the
satellite like a radar allow us to study
the structure of lightning producing
clouds)
Optical Sensor
Radar
Orbit altitude 220 miles
375 mile swath
Optical Sensor
NASA MSFC
Global Lightning Observed from the NASA-OTD
10-1000 times more lightning over land than Ocean
Global Flash Rate about 40 flashes per second
Intracloud to Cloud-to-Ground Lightning Ratio (IC:CG)
Integrated satellite observations with NLDN
LIS and OTD worked so well, why
not put a similar instrument on a
GOES satellite?
Band: 777.4 nm
Via Steve Goodman
Applications of Lightning Observations – Severe Weather Prediction
Lightning Jump
Severe weather (hail, winds, tornadoes) often occurs after a
convective surge and a pulse in updraft strength
Lightning flash rate is directly proportional to both updraft strength
and the amount of graupel in a thunderstorm
Williams et al. (1999)
VHF Lightning Mappers
LMA
LMA soon
LDAR
LMA charge structure methods
1) Initiation in max E-field between charge regions of
opposite polarity
2) Bi-directional breakdown
3) Negative breakdown is noisier at VHF
4) No charge structure w/o lightning
Most VHF
sources are
negative
breakdown
through region
of positive
charge!
Courtesy K. Wiens
29 June 2000 overview
Positive CGs
• Max reflectivity ~ 70 dBZ, Max updraft ~ 50 m/s
Classic supercell,
CG lightning
predominately
Positive (Xs)
29 June charge structure
(+CG supercell)
• “Inverted” tripole
in precipitation;
dipole in updraft
• Lower negative
charge present in
region of +CGs
Radar data time: 2325 UTC
NLDN data time: 2320-2330 UTC
Wiens et al. (2005)
LMA data time: 23:24:42-23:24:57 UTC
Mobile Electric Field Ballooning
DE/DZ  Charge/Volume
National Severe Storms Lab.
(Gauss’ Law)
E
Observing layers of electrical charge
Preparing to launch into a severe storm
Q = Charge
E = Electric field
V = Volume
Z = Height
Electric Field mill
Q
Q+
Q
Q+
Z
Archetypal charge structure in MCSs
Inferred via balloon soundings
Stolzenburg et al. (1998)
Lightning in MCSs
>90% in convective line; downward sloping into stratiform region
Most stratiform lightning flashes
initiate in the convective line and
show downward sloping into the
stratiform region
Lang et al. (2004, 2010)
Lang and Rutledge (2008)
In situ initiation
in the
stratiform
region
Nearly always
near melting
level
Lang and
Rutledge
(2008)
Transient Luminous Events (TLEs)
High-speed sprite video
Halo precedes actual sprite, visual appearance of different
sub-structures of sprite a function of altitude (air density)
Phantom HSI, 5000 fps, 11 July 2011 (Tom Warner)
Sprites
Caused by large charge moment change CG strikes, normally
associated with enormous stratiform lightning flashes (100s of km)
CMC = Q x Z (Wilson 1925) – Can be 1000s of C km
Stresses upper atmosphere to point of dielectric breakdown
Lang et al. (2011)
Gigantic Jets
• Discharges from the top
of the thundercloud
toward the ionosphere
• These and other jet
features caused by
charge imbalances in
thunderstorms
Krehbiel et al. (2008),
Lu et al. (2011)