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Thunderstorms
AOSC 200
Tim Canty
Class Web Site: http://www.atmos.umd.edu/~tcanty/aosc200
Topics for today:
Thunderstorms
Lecture 22
November 15, 2016
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1
Today’s Weather Map
http://www.hpc.ncep.noaa.gov/html/sfc2.shtml
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2
Today’s Forecast
http://www.hpc.ncep.noaa.gov/national_forecast/natfcst.php
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What affects thunderstorm growth?
• Lifting mechanism: thunderstorms can’t grow unless
something causes the air to rise in the first place
• Environmental temperature: determines stability of
atmosphere
• Moisture
• Wind speed and direction at all levels: wind shear helps
increase duration of thunderstorm
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4
Types of Thunderstorms
• Single cell
• Multicell
• Squall line
• Mesoscale Convective Complex
• Supercell
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5
Single Cell (Air-mass thunderstorm)
Cumulus stage: needs source of warm, moist air
Water condenses and cloud spreads out in all directions
Dry air entrained in from all sides – promotes growth of droplets
Precipitation begins to form
Fig
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10-2 Essentials of Meteorology
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Single Cell (Air-mass thunderstorm)
Mature stage: lightning, rain, and small hail
Updrafts increase – promotes further droplet growth (see Ch 5)
Precipitation falls and evaporates, cooling air beneath cloud. Downdraft
begins and increases in strength
Downdraft hits ground and interacts with updraft
Fig
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10-2 Essentials of Meteorology
7
Single Cell (Air-mass thunderstorm)
Dissipating stage: updraft weakens, downdraft dominates
Source of moisture and energy shuts down, storm weakens
Cloud begins to disappear as dry air entrained into cloud, evaporates
droplets
Often develop in mT air masses, last ~ 1 hour
Fig
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10-2 Essentials of Meteorology
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Multicell Thunderstorms
Wind shear keeps the downdraft from interfering with updraft
Strong down draft aids the updraft (gust front)
Increased updraft aids in formation of new cells
Fig
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10-5 Essentials of Meteorology
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Gust Front
Cold air (downdraft) hits the
grounds and spread out.
Acts like a cold front and
forces up warmer air
Warm air rises, condenses
and can form a shelf cloud
Fig
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10-6 Essentials of Meteorology
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Multicell Thunderstorms: Squall Lines
Line of intense, individual storms
May last 6-12 hours
Occur along boundary of unstable air
(e.g. cold front)
Strong wind shear tilts updraft and
separates it from downdraft
Fig
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10-12 Essentials of Meteorology
11
Multicell Thunderstorms: Squall Lines
Rising air from wave
aids lifting from surface
Pre-frontal squall lines may form ahead of an advancing cold front as
the air aloft forms waves downwind from the cold front
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12
Derechos
Line of storms (squall line) forms along cold front
Updraft brings in warm, humid air
Downdraft spreads cool air along the surface
http://earthsky.org/earth/what-you-need-to-know-about-derechos
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13
Derechos
Cold air spreads out along the front, creating bow shape
This causes more storms to develop and spread out along the front
These storms strengthen updraft, lead to “upshear tilting”
Draws in upper level winds from behind the cloud: “rear inflow jet”
http://earthsky.org/earth/what-you-need-to-know-about-derechos
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14
Derechos
Fast, upper level winds drawn down and reach surface
Increase speed of gust front moves storms forward , spread out along front
http://earthsky.org/earth/what-you-need-to-know-about-derechos
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15
Multicell Thunderstorms: Mesoscale Convective Complex
Composed of multiple single-cell
storms in different stages of
development
Individual thunderstorms supports
formation of other convective
cells
To last a long time, good supply of
moist air near surface is needed
Fig 11-14 Meteorology: Understanding the Atmosphere
16
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Supercells Thunderstorms
http://addins.waow.com/blogs/weather/2012/06/amazing-ufo-likeclouds/supercell-vega-tx
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17
Supercells Thunderstorms
Elevated mixed layer
prevents convection
until enough energy is
built up to break
through the inversion
Fig
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10-u3 Essentials of Meteorology
18
Supercells Thunderstorms
Large single cell storm
Needs very unstable atmosphere
Strong wind sheer that changes
direction with height leads to
rotation of entire storm
Fig
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10-19 Essentials of Meteorology
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Supercells Thunderstorms
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Supercells Thunderstorms
Fig
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This material may not be reproduced or redistributed, in whole or in part, without written permission from Tim Canty
10-17 Essentials of Meteorology
21
Supercells Thunderstorms
If overshooting top lasts for a while it indicates possibility of:
• Strong winds, tornado
• Heavy rain
• Hail
• Lightning
Strong convection, air rises rapidly and retains some momentum when it
reaches the tropopause. Air may push into tropopause (and rebound).
Fig
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10-17 Essentials of Meteorology
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Overshooting Top
http://stormvisuals.com/storage/seabreeze_bomb_overshoot001.
jpg?__SQUARESPACE_CACHEVERSION=1303878698655
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Supercells Thunderstorms
Updraft
U
pdraft and
and downdraft
downdraft wrap
wrap around
around each
each o
other
ther
Strong winds aloft brought down which may lead to tornado
Cell may last several hours
Fig 10-47 Essentials of Meteorology
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Supercells Thunderstorms
Cold downdraft supports strong
updraft
Warm, moist air brought in to storm
where up and down drafts meet
Tornado typically forms on southern
edge of storm
Fig 11-17 Meteorology: Understanding the Atmosphere
25
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Tornadoes
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26
Tornadoes
Many theories about tornado formation
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27
Tornadoes: Life Cycle
Wall
Cloud
Wall cloud: rising warm air meets cold downdraft
Warm air cools and condenses, cloud appears to lower
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28
Tornadoes: Life Cycle
Funnel cloud forms and picks up debris
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Tornadoes: Life Cycle
Funnel cloud forms and picks up debris
https://www.wunderground.com/wximage/WBNC/2
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30
Tornadoes: Life Cycle
Mature tornado at it’s peak intensity and width
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31
Lecture Recap
• Thunderstorms
• conditions
• stability
• single cell
• heat burst
• multicell
• gust front
• shelf cloud
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