Cloud Dynamics, Structure and

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Cloud Dynamics, Structure and..

• Prof. Steven Rutledge
• Department of Atmospheric Science
• Colorado State University
• Presented to the ASP Remote Sensing Symposium
• 3 June 2009

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Outline

• Introduction
• Instability, shear and storm organization
• Air mass and multicell thunderstorms
• Supercellular convection
• Mesoscale Convective Systems

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The mystery of clouds

• Precipitating clouds are of course needed for
  life on our planet
• Fundamental components of the hydrologic cycle
• Clouds play a key role in climate change
• Precipitating clouds present challenges for both
  remote and in-situ sensing
• We will focus mainly on clouds that are
  convective in nature, forming in the troposphere
  where temperature generally decreases with height
• Clouds have been the subject of studies for
  centuries

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The big controls on clouds

• Atmospheric instability
• CAPE is integrated thermal buoyancy assuming
  parcel reaches LFC, level of free convection
• CAPE represent maximum kinetic energy achievable
  by an ascending parcel that does not exchange
  momentum, heat and moisture with its environment
• Vertical shear (drives degree of organization)
• Aerosols, receiving increased attention as of
  late
• African aerosols for example

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Convection as a function of shear and CAPE
Individual convective storms and mesoscale
convective systems
DIVIDING LINE BETWEEN SEVERE AND NON-SEVERE
(Jorgensen and Weckwerth 2003)
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Convection as a function of shear and CAPE
Bulk Richardson No.
Lowest 0-4-km shear
Weisman and Klemp Multicell Rgt30 Supercell 10ltRlt40
R. Johnson (CSU), class notes, Mesoscale Dynamics
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Mature Stage Conceptual Model
AIRMASS THUNDERSTORM
The classical conceptual model of an airmass
thunderstorm from The Thunderstorm Project.
Byers and Braham, 1949. Thunderstorm Project was
the first field project dedicated to
weather research. More on this in a
bit.. Doswell, C. A. III, 2007 Historical
overview of severe convective storms research.
Electronic J. Severe Storms Meteor., 2(1),
125. In the low shear environment that airmass
storms form in, precipitation produced downdraft
chokes off the updraft, leading to rapid storm
dissipation. Lifecycle around 30 min.
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Cloud dynamics and microphysical processes can be
described by a simple 1-D entraining cumulus
model. Concept of Buoyancy, allows parcels to
accelerate in the vertical Vertical momentum
equation can be formulated by considering
a slight perturbation from the base hydrostatic
state First term on RHS is the buoyancy
term Second term on RHS is the pressure
perturbation term Neglecting the pressure
perturbation term, Since the cloud area is
small compared to the area of the clouds
environment,
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A more general form of the Buoyancy term can be
arrived at my combining the Ideal Gas Law and the
virtual temperature expression in perturbation
form
The various contributions to buoyancy are
apparent. Temperature, pressure, vapor and
condensate. Here the primed terms are short hand
notation for the difference in the quantity
between the cloud and the environment. Level
of neutral buoyancy, neglecting pressure and
vapor effects, occurs when
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A host of cloud dynamical problems can be studied
via the use of a simple 1-D entraining plume
model. Lets look at a quick formulation that
includes microphysical processes. Average
properties of a rising cumulus element must
include the effects of turbulent exchange with
the clouds environment. This is of course the
entrainment process. Entrained air is brought in
from sides and mixed instantaneously across the
cloud Process occurs continuously as the cloud
parcel rises
is some arbitrary variable such a rain water
content, heat, etc. m is mass.
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From conservation principles..
It can be shown after some algebra that
(1)
where
is the entrainment rate. The entrainment rate is
parameterized based on conceptual models for
shape of thermal. Jet, thermal and starting
plume models are invoked.
Based on Eqn. (1), a set of equations describing
parcel properties in rising cumulus elements can
easily be arrived at. Vertical momentum
equation
Source term for vertical momentum is of course
buoyancy.
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Water continuity equations can be introduced to
describe vapor and each condensate field
Microphysical processes (S) can be formulated via
bulk parameterization or explicit methods.
Finally, the thermodynamic energy equation can be
derived from principles of Moist Static Energy
These equations can easily be solved to describe
the vertical motion, temperature, water vapor and
rain/precipitation ice fields. This simple model
can be used to investigate a host of problems in
the area of cumulus dynamics.
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From Houze (1993) Cloud Dynamics
The common airmass thunderstorm provides a nice
illustration of the couplings between storm
dynamics, microphysics and electrification. Mixed
phase processes aloft promote intracloud
lightning in developing/mature phase. Descent of
ice and charge then lead to cloud-to-ground
lightning in the mature/dissipating phases.
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Storm charge structureNormal polarity

• Ice based precipitation provides for
  electrification
• Dipole/tripole
• Vertically separated, oppositely charged regions
• Charge regions tied to specific temperature
  regions

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Basic premise is that large and small ice
particles along with supercooled droplets,
collide and rebound in a cloud, with charge of
opposite sign being retained on the graupel and
small ice particles, respectively. Graupel
charges negatively under certain conditions and
positively under other conditions. Depositional
surface state and water coated surface state
associated with positive charge on that particle.
Adapted from Williams, Scientific American
Takahashi (1978) J. Atmos Sci. 10-4 esu 33
fC Femto 10e-15
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The Thunderstorm
Project Directed by Professors H. Byers and R.
Braham at the University of Chicago. It was
clear at the end of World War II that both
civilian and military aircraft could avoid flying
in and around thunderstorms. To promote aviation
safety, information was needed concerning the
internal structure and behavior of
thunderstorms. The Thunderstorm Project took
advantage of equipment and people that were at
the end of World War II. Twenty two railroad
freight cars and ten P-61C Black Widow aircraft
were made available. Radars, sounding equipment
and surface instrumentation were deployed.
Instrumented sailplane observations of
thunderstorms were also carried out. Results
published in The Thunderstorm (1949). Revealed
detailed information on airmass and multicell
thunderstorms.
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Conceptual model for the multicell storm from The
Thunderstorm Project..
From Houze (1993) Cloud Dynamics
A collection of air mass storms undergoing
individual lifecycles Young cells contain a
single updraft with developing precipitation Matu
re cells have both an updraft and a downdraft and
produce heavy rain Dissipating cells contain a
downdraft and produce light rain
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An example of a multicell as viewed by the
CSU-CHILL radar
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Corresponding image of differential reflectivity
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The more organized multicell thunderstorm
Cells undergo the three stage lifecycle as they
move through the storm
Browning et al. (1976), Quart. J. Roy. Met. Soc.
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Adapted from Houze (1993) Cloud Dynamics
Supercell storm. Often accompanied by
severe weather, wind, hail, tornadoes. Conceptual
model identifies a single updraft. Ambient
shear allows for storm-scale rotation, the
mesocyclone. Characterizes the pre-tornadic
phase. Mesocyclone is mainly at storm
mid-levels at this stage.
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Supercell conceptual model
CHARACTERISTICS Weak echo vault Leading shelf
cloud denoting updraft Hail formation process,
rapid growth by accretion as graupel pass through
areas of large supercooled liquid water
content
-40 C
Hail fallout
Browning and Foote (1976)
Fallout of heavy rain
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29 June 2000 supercell storm observed during
STEPS 2000 Dual-Doppler derived flow structure
One large updraft, downdraft structure.
Characteristic of supercell storm.
Tessendorf et al. (2005), J. Atmos. Sci.
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Key dynamics of the supercell Forward and rear
flank downdraftscritical for generation of low
level horizontal vorticity and then vertical
vorticity through tilting. The tilting of the low
level vorticity associated with the
RFD/FFD converging with the warm inflow is a
major source of vorticity for lower portion of
the mesocyclone. Tornado is thought to form when
lower portion of the mesocyclone intensifies via
this mechanism. Lemon and Doswell (1979)
Tornadic phase
Cold outflow
Warm, moist inflow
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Example of supercell as viewed by the CSU-CHILL
radar, 21 May 2004
Mesocyclone and TVS signatures
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What physical interpretations can you make
here..
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But here in Colorado Nonsupercell
tornadoes prevail. Some times referred to as
gustnadoes. Stretching of low-level horizontal
vorticity by storm updraft
Wakimoto and Wilson (1989)
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Background Positive CG hypotheses
Williams (2001)
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29 June 2000 overviewPositive CGs

• Max reflectivity 70 dBZ, Max updraft 50 m/s

Classic supercell, CG lightning predominately
positive
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Graupel, updraft, flash rate (FR) evolution
29 June 2000 (CG supercell)
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29 June charge structure
(CG supercell)

• Inverted tripole in precipitation dipole in
  updraft
• Lower negative charge present in region of CGs

Radar data time 2325 UTC NLDN data time
2320-2330 UTC LMA data time 232442-232457 UTC
Wiens et al. 2005
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The Mesoscale Convective System
Houze, Rutledge, Biggerstaff and Smull (1989),
BAMS
Driven by organized, linear convection forming on
a feature like a surface front or outflow
boundary. Convective line contains individual,
intense convective elements that slope slightly
rearward with height owing to vorticity
considerations between ambient low level shear
and cold pool. Rottuno, Klemp and Wiesman
(1988).
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From Newton and Newton, J. of Meteorology, 1959.
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Johnson and Hamilton (1988)
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Trailing stratiform region contains a number of
interesting pressure features. Mesohigh Wake
low
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In a more weakly-sheared environment, PV dynamics
produces mid-level vortex in trailing stratiform
region. Vortex produces asymmetric pattern to
stratiform precipitation.
8 May 2009
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Courtesy W. Lyons
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20 June 2007 MCS
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Convection initiated along colliding gust fronts.
Gust front clearly depicted in Zdr field from
CSU-CHILL radar.
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