Transcript MET 2204 METEOROLOGY

MET 2204
METEOROLOGY
Presentation 6:
Thunderstorms.
Presented by Mohd Amirul for AMC
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Presentation Outline
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Introduction
Formation of Thunderstorms
Size of Thunderstorms
Types of Thunderstorm
Thunderstorm’s Hazards
Do’s and Don’ts Thunderstorm Flying
Weather Radar
Conclusion
Presented by Mohd Amirul for AMC
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Part 2
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Learning Outcomes
• At the end of this session, student should be
able to:
– Characterized the thunderstorms
– Understanding the concept of thunderstorm
developments.
– Be aware of the hazardous and Do’s and Don’ts of
thunderstorm flying.
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Introduction
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What is the thunderstorm??
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• Thunderstorms: a STORM caused by strong
rising air currents and characterized by
thunder and lightning and usually heavy rain
or hail.
• One of the most thrilling and dangerous types
of weather phenomena.
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Where and when THUNDERSTORM
can occur??
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• Tropical regions- Thunderstorms occur year
around
• Midlatitude regions- Thunderstorm occur
frequently in spring, summer, and fall.
• Arctic regions- Thunderstorm occur frequently
during summer.
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How do thunderstorms form??
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Formation of Thunderstorm
• For a thunderstorm to form, the air must
have:
1. Sufficient water vapor
2. Unstable lapse rate (unstable air)
Rapid decrease of temperature with altitude. Caused
by surface warmer than air. Strong heating of surface
produce rising air (air become less dense since it
warmer.)
3. Initial upward boost (lifting)
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• Initial upward boost (lifting)
• Strong upward current (updraft)
• Can be produced by:
– Surface heating
» Warm air will go up
– Converging (meet) wind
– Slopping terrain
» E.g.: mountain and valley wind
– Frontal surface
» Two masses of air of different densities.
– Or any combination of these that can produce lifting air.
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• There are THREE stages of thunderstorm’s
development:
1. The Cumulus Stage (Cloud formation stage)
2. The Mature Stage
3. The Dissipating Stage
• Thunderstorm may be in a cluster (bunch) of
cells in a different stages.
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The Cumulus Stage (Cloud
formation stage)
• Every thunderstorm begins in a CUMULUS cloud.
– Review the formation of the cloud and
precipitation!!!
• The updraft (strong lifting flow/ strong upward
current)
– Is a key feature of cumulus stage.
– Varies in strength and extends from very near
surface to cloud top (increasing in strength when
increasing in altitude.)
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• Water droplets are quite small but grow to
raindrop size as the clouds grow
• The rise air carries the liquid water above
freezing level (increasing altitude, decreasing
temperature). May create icing hazard.
• Will fall as the raindrops when they become
heavier.
• The cold rain drags air with it creating a
downdraft (coexist with updraft).
• Now the cell has reached MATURE STAGE.
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The Cumulus Stage
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The Mature Stage
• Sign that cell has entered matured
stage and downdraft has developed
 Precipitation begins to fall from
the cloud base.
• This cloud has become a
cumulonimbus. The cumulonimbus
is now a thunderstorm cell.
• Downdraft remains cooler than
surrounding air and retards heating.
• Downward speed is accelerated
(colder more dense).
– May exceed 2500 ft per minute
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Cumulonimbus cloud
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• The downrushing air spreads outward at the
surface produce:
– Strong and gusty (rough) wind
– Sharp temperature drop
– Rapid rise in pressure.
*We call this surface wind as “plow wind” and its
leading edge as “first gust”.
• Meanwhile the updraft reach a maximum
speed possibly exceeding 6000 ft per minute.
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• Updraft and downdraft in close
proximity create a strong
vertical shear and a very
turbulent (unstable flow)
environment.
• All thunderstorm hazards reach
their greatest intensity during
the mature stage.
The Mature stage
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The Dissipating Stage
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Storm stop rapidly
Downdraft less intense
Rain has ended
Now only harmless cloud
remains.
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Size of Thunderstorms
• Individual thunderstorm measure from less
than 5 miles to more than 30 miles in
diameter.
• Cloud base range from few hundred feet in
very moist climates to 10000ft or higher in
drier regions.
• Clouds tops generally range from 25000 ft to
45000 ft but occasionally extend above 65000
ft.
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Type of Thunderstorm
• Duration of mature stage is closely related to
severity of the thunderstorm.
• Type of thunderstorm:
1. Air Mass type (Heat type)
2. Steady State type (Frontal type)
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The Air Mass type (Heat type)
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Last only for 1 hour or 2 hour.
Produce only moderate gust and rainfall
Results from surface heating.
When storm reach mature stage, rain falls
through or immediately beside the updraft.
• Falling precipitation induces frictional drag.
• The frictional drag will retards the updraft and
reverse it to downdraft.
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• The downdraft and cold precipitation cool the
lower portion of the storm and the underlying
surface.
• Therefore cuts of inflow of water vapor.
• Eventually the storm runs out energy and dies.
• Storm is self-destructive cell. (usually have life
cycle of 20 minutes to 1 and half hours)
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• Since air mass thunderstorm result from
surface heating;
– At land: they reach maximum intensity and
frequency over land during middle and late
afternoon.
– At sea: they reach maximum intensity during late
hours of darkness when land temperature is
coolest and cold air flows off the land over the
relatively warm water.
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• Other characteristics:
– Isolated
– Most common over land in summer
– Form day by day and clear by night.
– Occur in cols (gap between mountain peak) and
weak lows.
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The Steady State type (Frontal
type)
• Associated with weather systems:
– Fronts (the atmospheric phenomenon created at
the boundary between two different air masses),
converging winds, rough aloft.
– The storm induce force upward motion
– Often form into squall lines.
– High intensity by afternoon heating.
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• Precipitation falls outside the updraft. This will
allow updraft to continue unabated (continue
at full strength/intensity.
• Therefore the mature stage updraft becomes
stronger and last much longer than in air mass
storms.
• Steady state here means thunderstorm may
persist (continue to exist) for several hours.
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• Other characteristics:
– Most frequent in winter.
– Form over land or sea and day or night.
– Usually form in a line at a cold front or occlusion
(colder air surrounds a mass of warm air and
forces it aloft).
– Occur in active depression
– Can be accompanied by a line of squall.
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Thunderstorm's Hazards
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TORNADOES
SQUALL LINES
TURBULENCES
ICING
HAIL
LOW CEILING AND VISIBILITY
EFFECT ON ALTIMETERS
THUNDERSTORM ELECTRICITY
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The Tornadoes
• The most violent thunderstorms draw air into their
cloud bases with great vigor (energy/strength).
• If the incoming air has any initial rotating motion, it
often forms an extremely concentrated vortex from
the surface well into the cloud.
• Meteorologists have estimated that wind in such a
vortex can exceed 200 knots (370.4 km/hr!!);
pressure inside the vortex is quite low.
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• The strong winds gather dust and
debris, and the low pressure
generates a funnel-shaped (a
conical shape with a wider and a
narrower opening at the two ends)
cloud extending downward from
the cumulonimbus base.
• If the cloud does not reach the
surface, it is a “funnel cloud,”
• if it touches a land surface, it is
a “tornado,”
• if it touches water, it is a
“water spout,”.
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Funnel Cloud
Tornado
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Water Spout
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• Tornadoes occur with isolated thunderstorms at
times, but much more frequently, they form with
steady state thunderstorms associated with cold
fronts or squall lines.
• An aircraft entering a tornado vortex is almost
certain to suffer structural damage. Since the vortex
extends well into the cloud, any pilot inadvertently
caught on instruments in a severe thunderstorm
could encounter a hidden vortex.
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• Families of tornadoes have been observed as
appendages of the main cloud extending several
miles outward from the area of lightning and
precipitation. Thus, any cloud connected to a severe
thunderstorm carries a threat of violence.
• Frequently, cumulonimbus ‘mamma’ clouds occur in
connection with violent thunderstorms and
tornadoes. The cloud displays rounded, irregular
pockets or festoons from its base and is a signpost of
violent turbulence. Surface aviation reports
specifically mention this and other especially
hazardous clouds.
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Cumulonimbus Mamma clouds, associated with cumulonimbus clouds,
indicate extreme instability.
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The Squall Lines
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A squall line is a non-frontal, narrow band of active and
severe thunderstorms.
Often it develops ahead of a cold front in moist, unstable air,
but it may develop in unstable air far removed from any
front.
The line may be too long to easily detour and too wide and
severe to penetrate (difficult to avoid).
It often contains severe steady-state thunderstorms and
presents the single most intense weather hazard to aircraft.
It usually forms rapidly, generally reaching maximum
intensity during the late afternoon and the first few hours of
darkness.
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Squall line thunderstorms
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Squall Line, Atlantic Ocean June 1985 (image from NASA)
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The Turbulence
• Hazardous turbulence is present in all thunderstorms; and in
a severe thunderstorm,
• it can damage an airframe.
• Strongest turbulence within the cloud occurs with shear
between updrafts and downdrafts. Outside the cloud, shear
turbulence has been encountered several thousand feet
above and 20 miles laterally from a severe storm.
• A low level turbulent area is the shear zone between the
plow wind and surrounding air.
• Often, a “roll cloud” on the leading edge of a storm marks the
eddies in this shear. The roll cloud is most frequent with cold
frontal or squall line thunderstorms and signifies an extremely
turbulent zone.
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Schematic cross section of a thunderstorm.
Note areas outside the main cloud where turbulence may be encountered.
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• It is almost impossible to hold a constant
altitude in a thunderstorm, and maneuvering
in an attempt to do so greatly increases
stresses on the aircraft. Stresses will be least if
the aircraft is held in a constant attitude and
allowed to “ride the waves.”
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The Icing
• Updrafts in a thunderstorm support abundant liquid water;
and when carried above the freezing level, the water becomes
supercooled. When temperature in the upward current cools
to about −15° C, much of the remaining water vapor
sublimates as ice crystals; and above this level, the amount of
supercooled water decreases.
• Supercooled water freezes on impact with an aircraft. Clear
icing can occur at any altitude above the freezing level; but at
high levels, icing may be rime (frost) or mixed rime and clear.
The abundance of supercooled water makes clear icing very
rapid between 0° G and −15° C, and encounters can be
frequent in a cluster of cells. Thunderstorm icing can be
extremely hazardous.
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Rime Ice on the wing of a plane. It is supercooled water freezing
very quickly. It is lighter than other ice, but there is a danger in
losing air foil due to bumps on the ice.
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The Hail
• Hail competes with turbulence as the greatest
thunderstorm hazard to aircraft.
• Supercooled drops above the freezing level begin to
freeze. Once a drop has frozen, other drops latch on
and freeze to it, so the hailstone grows—sometimes
into a huge iceball.
• Large hail occurs with severe thunderstorms usually
built to great heights. Eventually the hailstones fall,
possibly some distance from the storm core.
• Hail has been observed in clear air several miles from
the parent thunderstorm.
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• As hailstones fall through the melting level, they
begin to melt, and precipitation may reach the
ground as either hail or rain. Rain at the surface does
not mean the absence of hail aloft. You should
anticipate possible hail with any thunderstorm,
especially beneath the anvil (mostly ice, form in the
upper parts of thunderstorms) of a large
cumulonimbus.
• Hailstones larger than one-half inch in diameter can
significantly damage an aircraft in a few seconds.
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Hail damage to an aircraft.
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Low Ceiling and Visibility
• Visibility generally is near zero within a thunderstorm
cloud.
• Ceiling (maximum altitude at which a plane can fly)
and visibility also can become restricted in
precipitation and dust between the cloud base and
the ground.
• The hazards are increased when associated with the
other thunderstorm hazards of turbulence, hail, and
lightning which make precision instrument flying
virtually impossible.
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Effect on Altimeter
• Altimeter: An instrument that measures the height
above ground; (used in navigation)
• Pressure usually falls rapidly with the approach of a
thunderstorm.
• If the altimeter setting is not corrected, the indicated
altitude may be in error by over 100 feet.
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Thunderstorm Electricity
• Electricity generated by thunderstorms is
rarely a great hazard to aircraft, but it may
cause damage and is annoying to flight crews.
Lightning is the most spectacular of the
electrical discharges.
• Example of Thunderstorm electricity:
– Lightning
– Precipitation Static
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Lightning
• A lightning strike can puncture the skin of an aircraft and can
damage communication and electronic navigational
equipment. Lightning has been suspected of igniting fuel
vapors causing explosion;
• however, serious accidents due to lightning strikes are
extremely rare.
• Nearby lightning can blind the pilot rendering him
momentarily unable to navigate either by instrument or by
visual reference.
• Nearby lightning can also induce permanent errors in the
magnetic compass. Lightning discharges, even distant ones,
can disrupt radio communications on low and medium
frequencies.
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• A few pointers on lightning:
– The more frequent the lightning, the more severe
the thunderstorm.
– Increasing frequency of lightning indicates a
growing thunderstorm.
– Decreasing lightning indicates a storm nearing the
dissipating stage.
– At night, frequent distant flashes playing along a
large sector of the horizon suggest a probable
squall line.
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Presented by Mohd Amirul for AMC
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Lightning Formation
• The sky is filled with electric charge. In a calm sky, the + and charges are evenly spaced throughout the atmosphere.
Therefore, a calm sky has a neutral charge.
• Inside a thunderstorm, the electric charge is spread out
differently. A thunderstorm is made up of ice crystals and
hailstones. The ice crystals have a positive charge, and the
hailstones have a negative charge. An updraft pushes the ice
crystals to the top of the thunderstorm cloud. At the same time,
the hailstones are pushed to the bottom of the thunderstorm
by its downdraft. These processes separate the positive and
negative charges of the cloud into two levels: the positive
charge at the top and the negative charge at the bottom.
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Lightning Formation
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Precipitation Static
• Precipitation static, a steady, high level of noise in radio
receivers is caused by intense corona discharges (an electrical
discharge accompanied by ionization of surrounding
atmosphere) from sharp metallic points and edges of flying
aircraft. It is encountered often in the nearby region of
thunderstorms.
• When an aircraft flies through clouds, precipitation, or a
concentration of solid particles (ice, sand, dust, etc.), it
accumulates a charge of static electricity. The electricity
discharges onto a nearby surface or into the air causing a
noisy disturbance at lower frequencies.
Ionization: the formation of ions by separating atoms or
molecules
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Do’s and Don’ts Thunderstorm
Flying
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• Above all, remember this: never regard any
thunderstorm lightly, even when radar
observers report the echoes are of light
intensity. Avoiding thunderstorms is the best
policy.
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• Following are some do's and don'ts of thunderstorm
avoidance:
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Don't land or take off in the face of an approaching thunderstorm. A
sudden gust front of low level turbulence could cause loss of control.
Don't attempt to fly under a thunderstorm even if you can see
through to the other side. Turbulence and windshear under the
storm could be disastrous.
Don't fly without airborne radar into a cloud mass containing
scattered embedded thunderstorms. Scattered thunderstorms not
embedded usually can be visually circumnavigated.
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4. Don't trust the visual appearance to be a reliable indicator of the
turbulence inside a thunderstorm.
5. Do avoid by at least 20 miles any thunderstorm identified as severe or
giving an intense radar echo. This is especially true under the anvil of
a large cumulonimbus.
6. Do remember that vivid and frequent lightning indicates the
probability of a severe thunderstorm.
7. Do regard as extremely hazardous any thunderstorm with tops
35,000 feet or higher whether the top is visually sighted or
determined by radar.
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• If you cannot avoid penetrating a thunderstorm, following are
some do's BEFORE entering the storm:
– Tighten your safety belt, put on your shoulder harness if you have one, and
secure all loose objects.
– Plan and hold your course to take you through the storm in a minimum time.
– To avoid the most critical icing, establish a penetration altitude below the
freezing level or above the level of -15 °C.
– Verify that pitot heat is on and turn on carburettor heat or jet engine anti-ice.
Icing can be rapid at any altitude and cause almost instantaneous power
failure and/or loss of airspeed indication.
– Establish power settings for turbulence penetration airspeed recommended in
your aircraft manual.
– Turn up cockpit lights to highest intensity to lessen temporary blindness from
lightning.
– If using automatic pilot, disengage altitude hold mode and speed hold mode.
The automatic altitude and speed controls will increase maneuvers of the
aircraft thus increasing structural stress.
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• Following are some do's and don'ts DURING the thunderstorm
penetration:
– Do keep your eyes on your instruments. Looking outside the cockpit
can increase danger of temporary blindness from lightning.
– Don't change power settings; maintain settings for the recommended
turbulence penetration airspeed.
– Do maintain constant attitude; let the aircraft "ride the waves."
Maneuvers in trying to maintain constant altitude increase stress on
the aircraft.
– Don't turn back once you are in the thunderstorm. A straight course
through the storm most likely will get you out of the hazards most
quickly. In addition, turning maneuvers increase stress on the aircraft.
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Weather Radar
• Weather radar detects droplets of precipitation size.
• Strength of the radar return (echo) depends on drop size and
number.
– The greater the number of drops, the stronger is the echo; and the
larger the drops, the stronger is the echo. Drop size determines echo
intensity to a much greater extent than does drop number.
• Drop size is almost directly proportional to rainfall rate; and
the greatest rainfall rate is in thunderstorms.
– Therefore, the strongest echoes are thunderstorms.
• Hailstones usually are covered with a film of water and,
therefore, act as huge water droplets giving the strongest of
all echoes. Showers show less intense echoes; and gentle rain
and snow return the weakest of all echoes.
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Radar photograph of a line of
thunderstorms.
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• Since the strongest echoes identify thunderstorms, they also
mark the areas of greatest hazards.
• Radar information can be valuable both from ground based
radar for preflight planning and from airborne radar for severe
weather avoidance.
• Thunderstorms build and dissipate rapidly, and they also may
move rapidly. Therefore, do not attempt to preflight plan a
course between echoes. The best use of ground radar
information is to isolate general areas and coverage of
echoes. You must avoid individual storms from inflight
observations either by visual sighting or by airborne radar.
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• The most intense echoes are severe thunderstorms.
Remember that hail may fall several miles from the cloud, and
hazardous turbulence may extend as much as 20 miles from
the cloud. Avoid the most intense echoes by at least 20 miles;
that is, echoes should be separated by at least 40 miles before
you fly between them. As echoes diminish in intensity, you
can reduce the distance by which you avoid them.
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Use of airborne radar to avoid heavy precipitation and turbulence. When
echoes are extremely intense, avoid the most intense echoes by at least 20
miles. You should avoid flying between these very intense echoes unless
they are separated by at least 40 miles. Hazardous turbulence and hail often
extend several miles from the storm centers.
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Conclusion
• Thunderstorm is one of the most thrilling and
dangerous types of weather phenomena.
• Above all, remember this: never regard any
thunderstorm lightly, even when radar
observers report the echoes are of light
intensity. Avoiding thunderstorms is the best
policy.
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Key Points
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Definition of Thunderstorm.
Time and location of Thunderstorm can occur.
Three stages of Thunderstorm’s formation.
Size of Thunderstorm.
Types of Thunderstorm.
Hazards of Thunderstorm.
Do’s and Don’ts flying in Thunderstorm.
Weather Radar.
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End of Presentation #6
5 Minutes for Q/A session
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