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AOS 101
Severe Weather
April 1/3
Lifting Condensation Level (LCL)
• Level at which a parcel lifted from the surface
would reach saturation (i.e. level where
temperature = dewpoint)
• Last week: DALR → T decreases at 10oC/km
• In addition, dewpoint (Td) decreases at 2oC/km
• LCL will coincide with how high above the
ground the cloud bottom is.
Level of Free Convection (LFC)
• Level at which a parcel raised from the surface
would become warmer than the environment.
• Above this level air is able to freely convect, or
ascend unabated to the tropopause.
Equilibrium Level (EL)
• Level at which parcel is no longer warmer than
the environment
• Usually near the tropopause where
environmental lapse rate is near 0oC/km
• Coincides with height of cloud top.
12
EL
HEIGHT (km)
10
Tparcel
8
Tenv
6
SATURATED
LFC
4
2
LCL
Td
-40
-30
-20
-10
0
TEMPERATURE (oC)
10
T
20
30
Ingredients for Thunderstorms
•
•
•
•
Instability
Lifting mechanism
Shear
Moisture
1. INSTABILITY
• In an unstable atmosphere, lifted parcels
of air will be warmer than its
surroundings.
• In this situation, a buoyancy force acts on
the object, accelerating it upward.
Buoyancy Force
Fbuoyancy  g
Tparcel  Tenv
Tenv
• The larger the difference in Tparcel and Tenv , the
larger the force and acceleration (F = ma)
• More buoyancy leads to stronger updrafts, up
to 50 m s-1
CAPE
(Convective Available Potential Energy)
• A measure of how buoyant parcels will be as
they ascend in a thunderstorm cloud.
• Higher CAPE means stronger updrafts and
more intense thunderstorms.
• Is equal to the area between parcel path and
environmental temperature curve when the
parcel is warmer (between the LFC and EL).
12
EL
HEIGHT (km)
10
8
CAPE
6
SATURATED
LFC
4
2
-40
-30
-20
-10
0
TEMPERATURE (oC)
10
20
30
2. LIFTING MECHANISM
• On the typical summer day, the atmosphere will be
conditionally unstable.
– Stable for unsaturated parcels
– Unstable for saturated parcels
• Surface (unsaturated) parcels will not be able to rise on
their own.
• Some mechanism must raise the parcel until it reaches
saturation (LCL) and then past a level at which it is
warmer than its surroundings (LFC).
FREELY
CONVECT
2 km: Te=14oC
LIFTING
MECHANISM
1 km: Te=22oC
Tp=14oC
LFC
SATURATED
Tp=20oC
LCL
UNSATURATED
0 km: Te=30oC
Tp=30oC, Td=22oC
Examples
•
•
•
•
Convergence of winds
Orography (upslope)
Intense surface heating
Outflow boundary from storm in the
vicinity
3. SHEAR
• Difference in winds with
height
1 km
2 km
3 km
sfc
• 2 types:
– Directional shear (wind
changes direction with
height).
– Speed shear (same
direction of winds, but
speed increases with
height).
DIRECTIONAL
3 km
sfc
2 km
1 km
SPEED
Air mass thunderstorm
CUMULUS
MATURE
DISSIPATING
COOL
DOWNDRAFT
WARM
MOIST
UPDRAFT
HEAVY RAIN
LIGHT RAIN
Why shear is needed…
• With no shear,
downdrafts cut off
source of moisture
(updraft)
• Vertical shear
displaces downdraft
from updraft,
allowing t’storm to
continually replenish
moisture
• Storms last longer
and become more
intense.
4. MOISTURE
• Thunderstorms need plentiful source of
moisture to drive circulation
• High moisture → High dewpoint → Low
dewpoint depression → Low LCL → Low LFC →
Less lifting needed for free convection (also
more CAPE).
12
EL
TEMPERATURE 30oC
DEWPOINT 14oC
HEIGHT (km)
10
8
CAPE
6
LFC
4
2
LCL
Td
-40
-30
-20
-10
0
TEMPERATURE (oC)
10
T
20
30
EL
12
TEMPERATURE 30oC
DEWPOINT 22oC
HEIGHT (km)
10
8
CAPE
6
4
LFC
2
LCL
-40
-30
-20
-10
0
TEMPERATURE (oC)
10
20 Td 30
Severe Weather Criteria
• Wind in excess of 58 mph
• Hail larger than .75 inches
• Tornado
WIND
• 58 MPH wind gust or
higher
– Strong winds are
created by downdrafts
which strike the
ground and spread out
– Downburst, Microburst
– Can exceed 120 mph
(weak tornado
strength)
HAIL
• .75 inch hail (nickel
size) or greater
– Strong updrafts keep
ice chucks aloft so
that more water
freezes to them
– Record hailstone: 7.0
inches (volleyball size)
• Aurora, Neb.
• June 22, 2003
TORNADO
• Can have winds up to
318 mph
– Moore, OK – May 3,
1999
• Stay on the ground for
an hour or more
– Record: 1924 TriState Tornado (3.5
hours)
• Track for tens of miles
– Record: Tri-state,
219 miles
• Up to a mile wide
– Record: 2.5 miles –
Hallam, NE – May
22, 2004
• Ranked using the Fajita Scale
– Assessors look at damage, than match to
wind speeds
– F0 = weakest, F5 = strongest
– Only 1% are F4 or F5
– Last F5: Greenville, KS 2007 (first since 1999)
Types of Thuderstorms
•
•
•
•
Airmass (usually not severe)
Multi-cell
Squall line (bow echo, derecho)
Supercell
Multicell
• Groups of cells moving in a line
• Outflow of one storm provides
lifting mechanism for the next
cell in the line
• Can “train” over one area for
hours
• Flash Flooding
Squall Line (Bow Echo, Derecho)
• Continuous line of
storms moving quickly
• Typical if speed shear
but no directional
• Outflow in front of
storm lifts air upwards
• Can persist for over a
day and travel 1000 km
or more
• Very strong winds
(120+ mph)
Supercell
• Most severe of all
storms.
• Produce most strong
tornadoes (F2+) and
large hail (2”+)
• Need directional shear,
winds turning clockwise
with height.
• Hook echo (TVS)
• Mesocyclone
• Overshooting top
Mesocyclone
Overshooting
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