Transcript Lecture8_Stability_Thunderstorms

Stability and Thunderstorms
ESS 111 – Climate and Global Change
Atmospheric Stability & Clouds
• Stability
– Parcel Theory
• Parcel of air
expands and
contracts freely
• Remains as a
single unit
• No heat
exchange with
the outside air
(Adiabatic
Process)
Atmospheric Stability & Clouds
• Stability
– Adiabatic Process
• Parcel expands
when lifted due
to lessening
pressure
• Molecular action
slows = cools at
a steady rate
• Compresses
when forced
down = Warms
at a steady rate
Atmospheric Stability & Clouds
• Lapse Rate: Rate of cooling (or warming)
of the atmosphere:
• Environmental Laps Rate = actual temperature as
measured by a radiosonde (weather balloon)
• Dry Adiabatic Lapse Rate (DALR) – Unsaturated
parcel of air = 10°C per km
• Moist Adiabatic Lapse Rate (MALR) – Saturated
parcel of air = 6°C per km.
Cloud & Storm Updraft Development
– Mechanisms
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Surface Heating – Free Convection
Topography
Convergence of Surface Air
Fronts
– Free Convection: Ground heated by radiation
• Thermal Forms (Convective Updraft)
– Air in contact with warmest ground is warmer and therefore less
dense than surrounding air
– Air rises…..it cools as it rises….it expands due to lessening
pressure acting on it (Adiabatic Processes)
– If the rising (and cooling) parcel that started at the surface
continues to be warmer than the surrounding air, then the parcel is
unstable and will continue to rise.
– Numerous unstable parcels over time (tens of minutes) can
eventually form a thunderstorm updraft.
Cloud Development Triggers
Convection & Clouds
Pockets of warm
air rise as
thermals with
invisible water
vapor, and at the
dew point
temperature
condensation
creates the cloud
base.
Rising air from
below is replaced
by sinking air
from above,
creating areas of
blue sky.
Thunderstorms
A cumulonimbus cloud that produces lightning (and hence thunder)
Very common (globally)
Transport heat and moisture vertically in the atmosphere
Most are not severe
Severe thunderstorm:
a) Hail > 1 inch diameter (¾ inch prior)
b) Wind gust >50 kt (58 mph)
c) Tornado
Ingredients for thunderstorm
initiation:
a) Water vapor
b) Instability
c) Lifting mechanism
For a severe thunderstorm, a 4th
ingredient is:
d) Strong vertical wind shear
(change in wind speed / direction)
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Thunderstorm Facts &
Climatology
• At any given time there are 2000 thunderstorms
in progress
• About 45,000 thunderstorms take place each
day
• Annually, The U.S. experiences about 100,000
thunderstorms.
• About 16 million thunderstorms occur annually
around the world!
• The lightning from these storms strikes Earth
about 100 times each second
Thunderstorm Frequency Map
Global Lightning Frequency
Upscale development of thunderstorms
Individual thunderstorms
• Single cell thunderstorm
• Multicell thunderstorm
• Supercell thunderstorm
– will examine this first
Larger-scale systems with embedded showers
and/or thunderstorms:
• Mesoscale Convective System (MCS)
• Squall line (quasi-linear convective system, QLCS)
• Hurricane
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Single-Cell Thunderstorm
Single-Cell Thunderstorm
• Life span of about 20 to 30 minutes
• Usually not strong enough to produce
severe weather
• Single Cell Pulse Storms often produce
severe weather
– Usually strong winds as it collapses
– Sometimes small hail
Life cycle of a single cell (air mass) thunderstorm
3 stages of development
1
2
3
Fig. 18.3
Vertical shear is weak (as in the SE
during summer)
Form along weak boundaries or
convergence zones.
Microbursts (strong downdrafts and
their outflows) may form during first
part of the mature stage.
Small size: <10 km diameter
Short lifetime: <1 h
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Towering Cumulus (Cumulus Congestus)
Is the Cumulus Cloud still growing?
Cauliflower-type hard top =
Strong updraft & growing storm
Wispy soft top =
Weaker updraft & dying storm
Life Cycle – Mature Stage
Life Cycle – Dissipating (Orphan Anvil)
Air Mass Thunderstorms Summary
• Usually weak (but can produce heavy rain in a short
period of time).
• Usually not severe
• Usually move slowly (weak winds aloft)
• Often develop and dissipate in less than one hour
• Form in a weakly sheared environment and thus have
a BUILT-IN SELF-DESTRUCT MECHANISM that
guarantees a short lifetime (downdraft collapsing down
on the updraft).