Download ATM OCN 100 Summer 2002

ATM OCN 100 - Summer 2002
LECTURES 32 & 33
THUNDERSTORMS:
MESOSCALE CONVECTIVE SYSTEMS
A. INTRODUCTION
– Practical Questions


What is a thunderstorm?
Why do thunderstorms occur?
– Definitions
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MADISON’S CURRENT WEATHER
Madison Weather at 1000 AM CDT
WED AUG 8 2001
Updated twice an hour at :05 and :25
Sky/Weather: SUNNY
Temperature: 83 F (28 C)
Dew Point: 71 F (21 C)
Relative Humidity: 67%
Wind: W7 MPH
Barometer: 30.04F
Surface Weather Map from Today
with Isobars & Fronts
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Current Temperatures (oF) & Isotherms
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Current Dewpoints (oF)
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Current Winds Aloft at 34,000 Ft.
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Tomorrow’s 7AM Forecast
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A. INTRODUCTION (con’t.)
 Definitions
– A thunderstorm - contains thunder;
– A thunderstorm day a day with thunder.
A
Mesoscale Phenomenon
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Cumulonimbus
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Thunderstorm Hazards-
Lightning
Lightning is a killer
(with more than 80 deaths in US per year)!
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Thunderstorm Hazards-
Lightning
Lightning is a killer
(with more than 80 deaths in US per year)!
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Historical Thunderstorm Charge
Model
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New Conceptual Model of Charge
Structure in Deep Convection from
Balloon Data
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Thunderstorm Hazards-
Flash Floods
Flash Flooding is a killer
(with more than 120 deaths in US
per year)!
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Thunderstorm Hazards-
Tornadoes
Tornadoes are killers
(with more than 60 deaths in US
per year)!
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Funnel Cloud
When funnel clouds touch the ground they become…….
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Tornadoes
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Thunderstorm Hazards-
High Winds (Straight line)
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Thunderstorm Hazards-
Large Hail
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B. THUNDERSTORM GENESIS
Essential Ingredients
– Low Level Moisture
– Unstable Atmospheric Lapse Rate
– Trigger (or Kicker)
 Initial Setting & Initiation Processes
– Free Convection
– Forced Convection

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When Convection?
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STABLE CONDITIONS
Compare Environment with DALR
Colder parcel sinks & returns to start
ALTITUDE (m)
2500
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TEMPERATURE (C)
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ABSOLUTELY STABLE CONDITIONS
Compare Environment with SALR & DALR
Colder parcel sinks & returns to start
ALTITUDE (m)
2500
2000
1500
1000
500
0
0
5
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15
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TEMPERATURE (C)
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30
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UNSTABLE CONDITIONS
Compare Environment with DALR
Warmer parcel continues upward
ALTITUDE (m)
2500
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1500
1000
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0
5
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15
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TEMPERATURE (C)
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30
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ABSOLUTELY UNSTABLE CONDITIONS
Compare Environment with SALR & DALR
Warmer parcel continues upward
ALTITUDE (m)
2500
2000
1500
1000
500
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5
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15
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TEMPERATURE (C)
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If STABLE CONDITIONS for Dry
Compare Environment with DALR
Colder parcel sinks & returns to start
ALTITUDE (m)
2500
2000
1500
1000
500
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0
5
10
15
20
25
30
TEMPERATURE (C)
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But UNSTABLE CONDITIONS for Humid
Compare Environment with SALR
Warmer parcel continues upward
ALTITUDE (m)
2500
2000
1500
1000
500
0
0
5
10
15
20
25
30
TEMPERATURE (C)
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CONDITIONALLY STABLE CONDITIONS
ALTITUDE (m)
Compare Environment with SALR & DALR
2500
2000
1500
1000
500
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0
5
10
15
20
25
30
TEMPERATURE (C)
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Stability Criteria
See Fig. 6.14 Moran & Morgan (1997)
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STABILITY CRITERIA



Absolutely Stable
– Layer lapse rate less than saturation
adiabatic lapse rate.
Absolutely Unstable
– Layer lapse rate greater than dry adiabatic
lapse rate.
Conditionally Unstable
– Layer lapse rate lies between dry &
saturation adiabatic lapse rates.
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STATIC STABILITY (con’t.)
•
Processes which change static stability
(instability)
• Cool below, but warm above (stabilize)
• Warm below, but cool above (destabilize)
•
Visual stability indicators
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Mammatus
These pocket clouds form from sinking air ahead of the most severe
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thunderstorms.
If you see these….run for cover!!!!
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Hail Stones
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Hail Storm
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THE “TRIGGER”




Free Convection
– Air ascends due to destabilization;
– Heat surface and/or cool aloft.
Orographic Lifting
– Air ascends mountain slopes.
Frontal Lifting
– Air ascends along cold front.
Dynamic Lifting
– Upper level motions induce lifting.
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C. THUNDERSTORM TYPES
 Single-Cell Thunderstorms
– “Air Mass” Thunderstorms
– Orographic Thunderstorms
 Frontal Thunderstorms
 Squall-Line Thunderstorms
 Mesoscale Convective Complexes (MCC)
 Multiple-Cell Thunderstorms
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D. THUNDERSTORM CLIMATOLOGY
Geographical & Seasonal Distribution
 Time of Day
 Duration of Thunderstorms
 Reported Thunderstorm Intensities

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Thunderstorm Climatology
See Fig. 13.9, Moran & Morgan (1997)
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E. CHARACTERISTICS OF
THUNDERSTORMS
The Thunderstorm Cell Model
 Life Cycle of a Non-Severe
Thunderstorm Cell

– Where…
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Cumulus Stage in thunderstorm life cycle
See Fig. 13.2a, Moran & Morgan (1997)
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Mature Stage in thunderstorm life cycle
See Fig. 13.2b, Moran & Morgan (1997)
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Dissipating Stage in thunderstorm life cycle
See Fig. 13.2c, Moran & Morgan (1997)
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F. SEVERE THUNDERSTORMS

Official Criteria for Severe Weather
– Large hail
(3/4 inch or large diameter);
– and/or

– Damaging Winds
(surface wind gusts of 58 mph or greater).
Unofficial Criteria for Severe Weather
– Tornadoes, Heavy Precipitation, Flash
Flooding, Deadly Lightning
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Vertical Cross section of Severe
Thunderstorm
See Fig. 13.11, Moran & Morgan (1997)
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Ingredients for severe thunderstorm episode
See Fig. 13.13, Moran & Morgan (1997)
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F. SEVERE THUNDERSTORMS (con’t.)
Types of Severe Thunderstorms
 Structure of Severe Thunderstorms
 Synoptic Situations Favoring Severe
Thunderstorm Weather
 Movement of Severe Thunderstorm
Cells

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NWS Watches & Warnings


Severe thunderstorm watch
– Issued by National Weather Service to
alert public that conditions are favorable
for development of severe thunderstorms
(large hail &/or high winds).
Severe thunderstorm warning
– Issued by National Weather Service to
warn public that a severe thunderstorm
has been sighted by storm spotters or has
been indicated by radar.
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Hail Climatology
See Fig. 13.24, Moran & Morgan (1997)
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Tornado Climatology
See Fig. 14.4, Moran & Morgan (1997)
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Cross-section of a
thunderstorm cell
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