Stability, Clouds, and Severe Weather

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Stability, Clouds, and Severe Weather

• AOS 101 Discussions 301/302/303
• Oct. 13-15th, 2008

Discussion Leader Brian Miretzky
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Review

• Discussion of Homework 3
• Turn in hw 4
• Weekly discussions
• Turn in badger forecasts
• This weeks topic

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Test Review

• Draw profile for different types of precip.
• Stability
• Station model for today
• Cloud identification
• Any questions?

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Last week

• Simple thermodyamic chart (TEMP vs. Height)

T of parcel gt T of environment the parcel is
positively buoyant (unstable) (or less stable) T
of parcel lt T of environment the parcel is
negatively buoyant (stable) T of parcel T of
environment the parcel is neutrally buoyant
(neutral)
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Intro to Log-P Diagrams
Instead of height as y-axis a log scale of
pressure (mb) is used due to the exponential
decrease of pressure observed with height in the
atmosphere. This is referred to as Stuve
Diagram
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Intro. To Skew-T Log-P Diagrams
Different from Stuve because temp. is skewed up
and to the right. Allows for proportionality of
area under the curve and work. Remember first
law of thermo. and adiabatic definition. Thus
the area under the curve is proportional to
energy.
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Skew-T Log-P Diagrams

• Used to plot information gained about the
  atmosphere by radiosondes
• Allow us to ID stability of a layer
• Allow us to ID air masses
• Tell us about the moisture in a layer
• Help us ID cloud layers
• Allow us to speculate on processes occurring

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Skew-T Log-P
Green Dry Adiabatic Lapse Rate Blue (curvy)
Moist Adiabatic Lapse Rate Blue (diagonal)
Temperature Purple Mixing Ratio Black
Dewpoint (left), Temperature (right)
Pressure ?
Temperature ?
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Skew-T Log-P
Green Dry Adiabatic Lapse Rate Light Blue
(dotted) Moist Adiabatic Lapse Rate Blue
(diagonal) Temperature Dark Green (dotted)
Mixing Ratio Black Dewpoint (left),
Temperature (right)
Pressure ?
Temperature ?
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Skew-T Log-P
On a Skew-T diagram, like the one from Omaha to
the left Temperature lines (also used for
Dewpoint temperature) slope up and to the
right. In this case, only at the tropopause
inversion does the temperature rise with height.
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Skew-T Log-P
Dry Adiabats represent the dry adiabatic lapse
rate of 9.8 oC/km. This represents the change
in temperature a parcel of air would undergo if
lifted (or lowered) dry (without heat transfer
like latent heating)
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Skew-T Log-P
Moist Adiabats represent the moist adiabatic
lapse rate, which varies based on temperature.
Because warm temperatures can hold lots of
water vapor, the MALR is close to 6 oC/km (more
water vapor condenses and releases latent heat,
warming the air). This represents the change in
temperature a parcel of air would undergo if
lifted (or lowered) saturated.
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Skew-T Log-P
Mixing Ratio lines are drawn so that you can
figure out what the mixing ratio is based on the
dewpoint profile. Match the dewpoint to a mixing
ratio line, and trace the line down to the bottom
of the Skew-T to find out the mixing ratio at
that level.
The mixing ratio that the temperature lies on is
the saturation mixing ratio
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Inversion Layer
A layer where temperature raises with height
Careful Remember that temperature lines slope
up to the right!
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Moist Layer
Often very close to saturation
Moist Layer Dewpoint is close to Temp.
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Dry Layer
Sometimes has been advected from high,
mountainous terrain
Dry layer layer in which the dew point and
actual temperature are far from one another
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Saturated Layer
Dewpoint and Temp. are equal Saturation, and
thus, condensation (CLOUDS) are present
The presence of clouds can be assumed where the
dew point and actual temperature are equal to
each other and reach saturation.
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Quick Detour on clouds and precipitation

• Clouds categorized by height and type
• Height determined at cloud base
• -high level, prefix cirr
• Ex.- cirrostratus
• -mid level, prefix alto
• Ex.- altostratus
• -low level, no prefix
• Ex.- stratus, cumulonimbus

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Clouds and Precipitation cont.

• Why can sometimes thermals form clouds and
  sometimes not?
• Depends on moisture and stability
• Why can sometimes clouds give precipitation and
  sometimes not? not yet discussed but will be in
  the coming week/s in lecture
• Depends on droplet growth big enough to fall out
  of a cloud

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Remember our lifting mechinisms

• Surface convergence of the wind
• Orography (lifting from air moving up the slope
  of a mountain)
• Fronts
• Convection, simply from heating the surface of
  the earth
• - The first three could lift a parcel through a
  stable layer, convection can not
• Numerous other conditions of which only some
  will be discussed
• Upper tropospheric jets
• Cyclones at all levels
• Outflow from prior thunderstorms
• Sea breezes, or lake breezes
• Etc.

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Introduction to Parcel Paths
At what level are the base of these cumulus
clouds??
Remember A rising thermal cools until it
condenses and creates cumulus clouds. The level
at which these cumulus clouds develop is called
the Lifted Condensation Level, or the LCL.
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Skew-T Log-P
Example Parcel Path
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Thunderstorms and Storm Forcing
If it is very stable at this part of the
atmosphere (usually only 1-2 km above the
ground), the clouds that form wont rise any
more, and shallow cumulus clouds will result.
http//www.met.utah.edu/galleries/home_page_images
/cu_fair_wx_nb.JPG/variant/medium
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Thunderstorms and Storm Forcing
If the parcel continues to find itself warmer
than the environment, it will continue to rise,
because it is less dense than the
environment! The parcel continues to rise at the
moist adiabatic lapse rate and condense out its
water vapor. The result is cumulonimbus A
thunderstorm with a very large vertical extent.
Here, the thunderstorm has reached the
tropopause.
http//www.alanbauer.com/images/Weather/Big20cumu
lus202.jpg
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Ingredients for Severe Weather CAPE
CAPE (Convective Available Potential Energy)
The maximum energy available for an ascending
parcel of air. CAPE is proportional to the area
between a parcel path and the environmental lapse
rate! If the environment is unstable with
respect to a rising parcel, the CAPE is positive!
http//www.cimms.ou.edu/doswell/Monograph/Synfig2
4.GIF
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Ingredients for Severe Weather CAPE
The sounding to the right shows a profile with a
very high CAPE. If a parcel were forced above
700 mb, it would continue to rise rapidly due to
the relatively unstable environmental lapse rates
throughout the middle and upper
troposphere. Eventually, the parcel hits the
tropopause, which is very stable. So, the rapid
vertical motions have to stop!
http//www.cimms.ou.edu/doswell/Monograph/Synfig2
4.GIF
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Example of cumulonimbus hitting the tropopause
http//www.youtube.com/watch?vYXMVEpYRqyo
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Convective Instability
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Four Main Types of T-Storms

• Single Cell
• Multi-cell cluster
• Multi-cell line
• Supercell
• Note Supercells are always severe, while the
  other three may be severe or non-severe

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Ingredients for Severe Weather Shear
Wind shear A change in direction or speed of
the wind with height Directional shear Tends
to favor the development of a supercell
thunderstorm Speed shear Tends to favor
multiple smaller thunderstorm cells
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Ingredients for Severe Weather Shear
Shear is important in maintaining a storm
thunderstorm. Without it, the storm ends up
destroying itself with its own downdraft! An
example of this below shows the downdraft
completely cutting off the updraft by the 3rd
frame. With shear, the updraft and downdraft can
work together.
http//physics.uwstout.edu/wx/u10/U10_03.gif
http//www.srh.weather.gov/jetstream/tstorms/life.
htm
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Multi-Cell Cluster

• Group of cells moving as a single unit
• Each cell lasts about 20-30 minutes, but cluster,
  itself can last several hours
• Each cell in a different stage of the life cycle
• Cells take turns being the most dominant
• Organized multi-cell storms have higher severe
  weather potential
• Downbursts, moderate sized hail, flash floods,
  weak tornadoes
• Flash flood threat can be especially significant

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Life Cycle of Multi-Cell Cluster

• At t0, cell 1 is dissipating, cell 2
  maturing/briefly dominant
• At t 10, cell 3 strengthening, cell 2 dropping
  heaviest precipitation
• Train-echo pattern
• Easy to create flash flooding

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Schematic of Classic Supercell

• Hook Echo review
• In this image, hook echo is seen as the curling
  of the precipitation field at the south end of
  the supercell
• Most likely place for tornadoes to form
• Seen in radar reflectivity images

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Cross-section of a Supercell

• Supercells have slanted updrafts/downdrafts
• Downdraft cannot interfere with updraft (source
  of storms energy)
• Allows supercell to sustain itself for hours

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Supercell Structure cont.
The severe weather parameters have to come
together just right to form a supercell. As an
example, the shear cant outweigh the
instability, or vice versa.
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Ingredients for Severe Weather Moisture
Abundance of water vapor in the air is very
important for the development of severe
weather. The higher the relative humidity, the
lower the LCL. A lower LCL makes the cloud bases
lower. This allows a storm to tap into surface
shear, which (as Ill explain soon) favors
tornado development.
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Tornadoes
Good directional shear must be prevalent to form
a tornado! This is because with directional
shear, the updraft in a supercell begins to
rotate as it rises into the storm. Eventually, a
whole portion of the supercell, the mesocyclone
rotates. The downdraft, then, interacts with
the rotating mesocyclone, and on occasion forms a
tornado.