Transcript Thunderstorms

Thunderstorms and Volcanic
clouds—analogies
Ashfall Grad Class Fall 2009
lecture #11
Thunderstorms
• Definition: a storm containing lightning and
thunder.
• Associated with midlatitude cyclones, localized
convection, orographic lifting and tropical
cyclones.
Thunderstorm Formation
• Ingredients
– warm, moist air (often mT)
– unstable (or conditionally unstable if lifting mech.)
– encouraged by diverging air aloft
Thunderstorm Life Cycle
Thunderstorm Life Cycle
• Towering Cumulus Stage
– Cumulus clouds build vertically and laterally, and surge upward to
altitudes of 8,000-10,000 m (26,000-33,000 ft) over a period of 10-15
minutes
– Produced by convection within the atmosphere
• Free convection – triggered by intense solar heating of Earth’s surface
– Generally not powerful enough to produce thunderstorms
• Forced convection – orographic uplift or converging winds strengthen
convection
– This is generally the cause of thunderstorms
– Latent heat released during condensation adds to buoyancy
– During the cumulus stage, the updraft is strong enough to keep water
droplets and ice crystals suspended
• As a result, precipitation does not occur in the cumulus stage
American Meteorological Society Education Program
5
Thunderstorm Life Cycle
• Mature Stage – maximum intensity
– Stage typically lasts about 10-20 minutes
– Begins when precipitation reaches Earth’s surface
– Features heaviest rain, frequent lightning, strong surface winds,
and possible tornadoes
– Weight of droplets and ice crystals overcome the updraft
– Downdraft created when precipitation descending through the
cloud drags the adjacent air downward
• Entrained dry air at the edge of the cloud leads to evaporative cooling,
which weakens the buoyant uplift and strengthens the downdraft
• At the surface, the leading edge of downdraft air resembles a miniature
cold front and is called a gust front
• Ominous-appearing low clouds associated with a gust front include a
roll cloud and a shelf cloud
American Meteorological Society Education Program
6
Thunderstorm Life Cycle
• Dissipating Stage
– Precipitation and the downdraft spread throughout the
thunderstorm cell, heralding the cell’s demise
– Subsiding air replaces the updraft and cuts off the
supply of moisture
– Adiabatic compression warms the subsiding air and
the clouds gradually vaporize
American Meteorological Society Education Program
9
Thunderstorm Classification
NOAA classification of thunderstorms, and the likelihood of
severe weather.
American Meteorological Society Education Program
10
Thunderstorm Classification
• Thunderstorms are meso-scale convective systems (MCS) and
are classified based on the number, organization, and intensity of
their constituent cells
• Single-cell thunderstorms
– Usually a relatively a weak system forming along a boundary within an air
mass (i.e., gust front)
– Typically completes its life cycle in 30 minutes or less
• Multicellular thunderstorms
– Characterizes most thunderstorms. Each cell may be at a different stage in
its life cycle, and a succession of cells is responsible for a prolonged
period of thunderstorm weather.
– Two types:
• Squall line
• Mesoscale convective complex
• Either can produce severe weather
American Meteorological Society Education Program
11
Thunderstorm Classification
A thunderstorm may track at some angle to the path of its constituent cells,
complicating the weather system motion. In the above idealized situation, the
component cells of a multicellular thunderstorm travel at about 20 degrees to the
eastward moving thunderstorm. As they travel toward the northeast, the individual12
cells progress through their life cycle.
Thunderstorm Classification
• Multicellular thunderstorm types
– Squall line – elongated cluster of thunderstorm cells that is accompanied
by a continuous gust front at the line’s leading edge
• Most likely to develop in the warm southeast sector of a mature extratropical cyclone, ahead of and parallel to the cold front
– Mesocyclone convective complex (MCC)
• A nearly circular cluster of many interacting thunderstorm cells with a
lifetime of at least 6 hrs, and often 12-24 hrs
• Thousands of times larger than a single cell
• Primarily warm season phenomena (March – September)
• Usually develop at night over the eastern 2/3 of the U.S.
• Is not associated with a front
• Usually develops during weak synoptic-scale flow, often develops
near an upper-level ridge of high pressure, and on the cool side of a
stationary front
• A low level jet feeds warm humid air into the system
– Supercell thunderstorms are long-lived single cell storms
• Exceptionally strong updraft, with rotational circulation that may
evolve into a tornado
13
Thunderstorm Classification
Radar image of a squall line
stretching from Texas to Illinois
Infrared satellite image showing
meso-scale convective
complexes over western Kansas
14
and most of Arkansas
The Geography of Thunderstorms
Frequency decreases with distance from equator. None above 60o
Most occur during summer’s warm temperatures.
The Geography of Thunderstorms
Thunderstorm Frequency
• Probably 1500 to 2000 thunderstorms active
around the world at any given time.
Severe Thunderstorms
• A severe thunderstorm is accompanied by locally
damaging surface winds, frequent lightning, or large
hail
– Surface winds stronger than 50 kts (58 mph) and/or hailstones
0.75 in. (1.9 cm) or larger in diameter
– May also produce flash floods or tornadoes
• What causes some thunderstorms to be severe?
– Key is vertical wind shear, the change in horizontal wind
speed and direction with increasing altitude
• Weak vertical wind shear favors short-lived updrafts, low cloud tops,
and weak thunderstorms
• Strong vertical wind shear favors vigorous updrafts, great vertical
cloud development, and severe thunderstorms
• With increasing vertical wind shear, the inflow of warm humid air is
sustained for a longer period because the gust front cannot advance as
far from the cell. Also, most precipitation falls alongside the titled
updraft, sustaining the updraft.
American Meteorological Society Education Program
19
Severe Thunderstorms
A synoptic weather pattern that favors development of severe
thunderstorms. A dryline is the western boundary of the mT air
mass and brings about uplift in a manner similar to a cold front.
American Meteorological Society Education Program
20
Severe Thunderstorms
• The polar front jet stream produces strong vertical wind
shear
– This maintains a vigorous updraft
– This supports great vertical development of thunderstorms
– The jet contributes to stratification of air that increases the
potential instability of the troposphere
• A jet streak induces both horizontal divergence and convergence of air
in the upper troposphere
• Convergence occurs in the right front quadrant of a jet streak, causing
weak subsidence of air
• Sinking air is compressionally warmed and forms an inversion
(capping inversion) over the mT air mass
• The underlying air mass becomes more humid
• Contrast between air layers mounts
• All that is needed is a lifting mechanism for severe weather to occur
American Meteorological Society Education Program
21
Severe Thunderstorms
A temperature sounding that favors the development of severe thunderstorm cells. A
capping inversion separates subsiding dry air aloft from warm, humid air near the
surface.
American Meteorological Society Education Program
22
Severe Thunderstorms
Mammatus clouds occur on the underside of a thunderstorm anvil and
sometimes indicate a severe storm system. Their appearance is caused by blobs
of cold, cloudy air that descend from the anvil into the clear air beneath the
anvil.
23
American Meteorological Society Education Program
Thunderstorm Hazards
• Lightning
– A brilliant flash of light caused
by an electrical discharge
within a cumulonimbus cloud
or between the cloud and
Earth’s surface
– Direct hazard to human life
– Ignites forest and brush fires
– Very costly to electrical utilities
– Lightning detection network
provides real-time information
American Meteorological Society Education Program
24
Thunderstorm Hazards
• Lightning, continued
– What causes lightning?
• Large differences in electrical charge develop within a cloud, between
clouds, or between a cloud and the ground
– Upper portion and much smaller region of the cumulonimbus cloud
become positively charged, with a disk-shaped zone of negative charge in
between. A positive charge is induced on the ground directly under the
cloud
• Lightning may forge a path between oppositely charged regions
• Charge separation within a cloud may be due to collisions between
descending graupel striking smaller ice crystals in their path. At
temperatures < -15 °C (5 °F) graupel become negatively charged
while ice crystals become positively charged. Vigorous updrafts carry
ice crystals to upper portions of the cloud.
• Positive charge near cloud base also due to graupel-ice crystal
collision, but temps > -15 °C (5 °F) induce positive charge to
graupel and negative charge to ice crystals
American Meteorological Society Education Program
25
Ice plays a vital role in lightning generation
Thunderstorm Hazards
• Lightning, continued
– A cloud-to-ground lightning flash involves a regular sequence of events
• Stepped ladders: streams of electrons surge from the cloud base to the ground
in discrete steps
• Return stroke: forms as an ascending electric current when the positive and
negative charges recombine; often emanates from tall, pointed structures
• Dart leaders, subsequent surges of electrons from the cloud, follow the same
conducting path
• Sequence takes place in < two-tenths of a second
– Lightning causes intense heating of air so rapidly that air density cannot
initially respond
• Shock wave is generated and propagates outward, producing sound waves
heard as thunder
– Flash-to-bang method: Thunder takes about 3 seconds to travel 1 km (or 5
seconds to travel 1 mi)
• If you must wait 9 seconds between lightning flash and thunderclap, the
lightning is about 3 km (1.8 mi) away
American Meteorological Society Education Program
27
Thunderstorm Hazards Lightning
American Meteorological Society Education Program
28
Thunderstorm Hazards
• Downbursts
– Exceptionally strong downdrafts that occur with or without rain
– Starburst pattern causes ground destruction
– Also very dangerous to aircraft because they trigger wind shear
• Aircraft have warning systems that use the same principle as Doppler radar
– A macroburst cuts a swath of destruction > 4 km (2.5 mi) wide with surface winds
that may top 210 km per hr (130 mph)
– A microburst is smaller and shorter lived
– Derecho: a family of straight-line downburst winds that may be hundreds of
kilometers long; sustained winds in excess of 94 km per hr (58 mph)
American Meteorological Society Education Program
29
Thunderstorm Hazards
• Flash Floods
– Short-term, localized, often unexpected rise in stream level
usually in response to torrential rain falling over a relatively
small geographical area
– Caused by excessive rainfall in slow moving or stationary
thunderstorm cells
– Atmospheric conditions that favor flash floods:
• More common at night and form in an atmosphere with weak vertical
wind shear and abundant moisture through great depths
• Precipitation efficient atmosphere has high values of precipitable
water and relative humidity and a thunderstorm cloud base with
temperatures above freezing
American Meteorological Society Education Program
30
Hail
• Formation
• Largest?
Coffeyville, KS, 1970
(1.75 lb, 14 cm diameter)
Thunderstorm Hazards
• Hail
– Frozen precipitation in the form of balls or lumps of ice > 5
mm (0.2 in.) in diameter, called hailstones
– Almost always falls from cumulonimbus clouds that are
characterized by a strong updraft, great vertical development,
and an abundance of supercooled water
– Develops when an ice pellet is transported vertically through
portions of the cloud containing varying concentrations of
supercooled water droplets
• Composed of alternating layers of glaze and rime
– Grows by accretion (addition) of freezing water droplets and
falls out of cloud base when it becomes to large and heavy to
be supported by updrafts
American Meteorological Society Education Program
32
Accretionary Lapilli are
perhaps best explained by a
process which involves
vertically developed clouds
and ice, especially hail and
graupel.
Likely microphysical
processes and
particles in volcanic
cloud.
C Textor et al., 2006,
JVGR 150: 359-373.
Thunderstorm Hazards
• Hail, continued
– May accumulate on the ground in a long, narrow strip known
as a hailstreak; typically 2 km (1.2 mi) wide and 10 km (6.2
mi) long
– The figure below is a model of hailstreak development
American Meteorological Society Education Program
35
Tornadoes
• About 10% of the annual
10,000 U.S. severe
thunderstorms produce
tornadoes
• A tornado is a violently rotating
column of air in contact with
the ground
• Most are small and short-lived
and often strike sparselypopulated regions
• The most prolific tornado
outbreak on record occurred
over the Great Plains and
Midwest on 29-30 May 2004
– More than 180 tornadoes were
reported
36
Lightning
• discharge of electricity that
occurs in mature thunderstorms
• Cause: charge separation in
cloud sets up electrical potential
• Role of lightning is to equalize
these differences in electrical
potential.
• Important fixer of Nitrogen.
Stepped leader
Electrons down
Upward leader
Protons up
Return stroke
Circuit complete
Repeats every few microseconds with new leader.
USA: Real-time Lightning
http://www.weather.com/
Overshooting Top
Overshooting Top
• Overshooting top - characteristic of a strong
updraft
• The updraft goes higher than the rest of the
clouds near it (in the anvil)
• Overshoots the tropopause or equilibrium
level btwn the troposphere & stratosphere
• Updraft penetrates stratosphere and then is
forced back down to equilibrium level
American Meteorological Society Education Program
Supercell Thunderstorms
• A supercell thunderstorm is a t.s. with a deep rotating
updraft (mesocyclone)
• Updraft elements usually merge into the main rotating
updraft and then accelerate rapidly
• Flanking updrafts "feed" the supercell updraft, rather than
compete with it
• Small percentage of all t.s.’s are supercells but they cause
the majority of damage
Diagram of a Supercell
A Look from the SE
Umbrella Cloud
High winds at 10-11 km height made the
volcanic cloud spread like a mushroom
GRL
32L24808
no L24808
GRL 32
2005
Features of Supercells
• Mesocyclone (p.125) organizes updraft and downdraft and
keeps them separate
• Updraft is slanted downwind (aloft) so hail/rain doesn’t fall
through it and kill it
• Supercell can last for hours and travel a hundred plus miles
• Often moves to the right of the mean flow - has to do with
rotation (vorticity) and propagation
• What does propagation mean?
How Supercells Move
• Movement = Advection + Propagation
• This little formula applies to pretty much everything in
weather
• advection = just the horizontal transport of the feature (like
a supercell) along with the winds
• propagation = development of the feature (usually happens
towards inflow or flanking line in the case of a supercell)
American Meteorological Society Education Program
Mammatus
Schultz et al., 2006, J Atmos Sci 63: 2409-2435
Schultz et al., 2006, J Atmos Sci 63: 2409-2435
Schultz et al., 2006, J Atmos Sci 63: 2409-2435
Schultz et al., 2006, J Atmos Sci 63: 2409-2435
Schultz et al., 2006, J Atmos Sci 63: 2409-2435
Cloud polarization lidar of cirrus mammatus 0100-0230 UTC 10 Sept
1994, Salt Lake City
Schultz et al., 2006, J Atmos Sci 63: 2409-2435
Schultz et al., 2006, J Atmos Sci 63: 2409-2435
Schultz et al., 2006, J Atmos Sci 63: 2409-2435
Fully glaciated volcanic cloud with abundant
CCN
Above Ephrata on 18 May
Photo by Douglas Miller
•
•
•
•
•
Density of cloud increases as a whole
Latent heat effects significant in rise and possibly in fall
Bright band effects during descent (thawing)
Overall sublimation/evaporation
Quite different from a thunderstorm
Durant et al., 2008, JGR 113
Few IN
Bergeron
Large Ice HM
Precipitation
Many IN
Small ice HM
Little Precip
Sublimation
Meteorological Cloud
Volcanic Cloud
Virga – NOAA photo
Virga -- Australia Severe Weather
Conceptual Model: Distal Fallout
Durant et al., 2009, JGR