Download Chapter 3 Convective Dynamics 3.2 Ordinary or "air

Chapter 3
Convective Dynamics
Photographs © Todd Lindley
3.2 Ordinary or "air-mass“ storm
• 3.2.1. Main Characteristics
§ Consists of a single cell (updraft/downdraft pair)
§ Forms in environment characterized by large conditional instability and
weak vertical shear
§ Vertically erect à built-in self-destruction mechanism
§ Can produce strong straight-line winds or microburst
§ Life cycle is generally < 1 hour, usually 20-45 min
§ These storms form in weakly-forced environments, and are driven
primary by convective instability rather than the ambient winds
§ They are sometimes called "air-mass" storm because they form within the
same air-masses with more-or-less horizontal homogeneity
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Photograph by: NSSL
§ This is a single cell storm, looking east from about 15 miles. The storm
was moving east (into the photo). Some of the anvil cloud has been left
behind the storm, but the greater portion of the anvil is blowing off in
advance of the storm and is not observable from this perspective.
(May storm in the Texas Panhandle near Amarillo.)
• This late May storm in Oklahoma, looking northeast from about
20 miles, occurred with weak to moderate vertical wind shear. It
did not produce any severe weather.
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Life cycle of a non-severe single cell storm in weak wind shear
Radar history of the severe pulse storm – often with larger instability
Example of Single-cell Life Cycle
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3.2.2. Basic Dynamics
(forces acting on an air parcel in the vertical)
Perturbation Vertical Momentum Equation
(base-state hydrostatic equations has been
subtracted off on the RHS)
water
Three stages of single-cell storm
development
Towering stage
Mature Stage
Dissipating Stage
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LCL – Lifting
condensation level
LFC – Level of free
convection – level at
which the parcel is
warmer than the
Environment
EL – Equilibrium
Level – level at which
the parcel’s T becomes
the same as the
environment again
Development of Single Cell Storm
Step 1
§ In the absence of frontal or other forcing, daytime heating of
the PBL causes the convective temperature to be reached.
Thus, there is no ‘negative area’ on the skew-T diagram for an
air parcel rising from the surface – the lid is broken
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Development of Single Cell Storm
Step 2
§ Updraft forms – once the air reaches the LCL, latent heat is released
due to condensation:
- L dqv = Cp dT
§ For every 1g/kg of water condensed, the atmosphere warms about 3
degrees. This feeds into the buoyancy term through an increase in θ’
(remember earlier vertical momentum equation?). The saturated air
parcel ascends following the moist adiabat, along which the equivalent
potential temperature θ e is conserved.
§ Until the ‘Equilibrium level’ is reached, the air parcel is warmer than
the environment, which keeps the buoyancy positive (without the
effect of water loading – see later)
Development of Single Cell Storm
Step 2 – continued …
§ When cloud forms, part of it is carried upward by the draft and the
other part falls off the updraft. The ‘weight’ of this liquid water
makes the air parcel heavier, this ‘water loading’ effect acts to reduce
the positive buoyancy.
−gL
B = g(θ’/θ)
10 x 3/300
~
- 10 x 0.01 kg/kg
Therefore 10 g/kg of cloud or rain water will offset 3 K temperature
surplus.
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Development and decay of Single Cell Storm
• When the cloud grows to a stage that the updraft becomes too ‘heavy’
because of water loading, it will collapse, updraft then turns into
downdraft.
• Another important process that contributes to the collapse is the
evaporative cooling. When cloud grows, cloud droplets turn into
larger rain drops that fall out of the updraft, reaching the lower level
where the air is sub-saturated. The rain drops will partially evaporate
in this sub-saturated air, producing evaporative cooling. This cooling
enhances the downdraft.
• In the absence of vertical wind shear, the cell is upright, this
downdraft would then disrupt the low-level updraft, causing the cell
to dissipate. This is the built-in self-distruction mechanism mentioned
earlier
• The cold downdraft sometimes form a cold pool that propagates away
from the cell above, further removing the lifting undernearth the cell
The effect of pressure gradient force
• In addition to buoyancy force and water loading, another force that is also
acting on the rising parcel is the vertical pressure gradient force (PGF)
• When an air parcel rises (due to buoyancy), it has to push off air above it,
creating higher pressure (positive p’) above (imaging push yourself through
a crowd)
• Below the rising parcel, a void is created (imagine a vacuum cleaner),
leading to lower pressure at the cloud base
H
PGF
L
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Effect of Pressure Gradient Force
• The higher pressure above will push air to the side, making room for
the rising parcel, while the lower pressure below attracts surrounding
air to compensate for the displaced parcel
• Such a positive-negative pattern of p perturbation creates a downward
pressure gradient. The PGF force therefore opposes the buoyancy
force, therefore acts to reduce the net upward forcing.
Effect of Pressure Gradient Force
•
The degree of opposition to the buoyancy force depends on the aspect ratio of
the cloud (L/H), or more accurately that of the updraft
•
The effect is larger for wider/large aspect-ratio cloud, and weaker for
narrower/small aspect ratio cloud, because
§ For narrow cloud, a small amount of air has to be displaced/attracted by the rising
parcel, therefore the p perturbation needed to achieve this is smaller, so that the
opposing pressure gradient is smaller (often <<buoyancy) so a narrow cloud can
grow faster
§ PGF is stronger for wide cloud, as a results, the net upward force (buoyancy –
PGF) is significantly reduced, the cloud can only grow slowly. When B and PGF
has similar magnitude, the vertical motion becomes quasi-hydrostatic – this is
typical of large scale broad ascent.
•
Dynamic stability analysis of inviscid flow shows that the infinitely narrow
clouds grow the fastest, but in reality, the presence of turbulent mixing
prevents the cloud to become too narrow, hence the typical aspect ratio of
clouds is 1.
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Skew-T analysis and Parcel Theory
• Skew-T analysis and Parcel Theory typically neglect the effect of PGF
induced by vertical motion, essentially they assume that the
environment is unchanged by the parcel motion. They also neglect the
effect of mixing/friction
• Therefore, they only provide a, though still very useful, upper-limit for
the convection intensity
Convective Available Potential Energy
(CAPE)
• CAPE measures the amount of convective instability, or
more accurately the potential energy of an environmental
sounding – the energy that can be converted into kinetic
energy when an air parcel rises from LFC to EL
• It is based on the simple parcel theory which neglects the
effect of mixing/friction, PGF and sometimes water
loading.
• From CAPE, we can estimate the maximum vertical
velocity that can be reached by a parcel
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CAPE
Skew-T
• The ‘negative’ area is equal
to CIN
(Convective Inhibition)
• The positive area (where air
parcel is warmer than
environment) is equal to
CAPE
• Lifted Index – temperature
excess in 500mb
environment over that of a
parcel lifted from the low
‘moist’ layer (negative
value indicates instability)
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ARPS
Simulation
of a Single
Cell Storm
• May 20, 1977 Del
City Supercell Storm
Sounding – used
without
environmental wind
ARPS Simulation of a Single Cell Storm
T-equivalent buoyancy+qw+ref
Eq. Pot. Temp.+qw+Ref+wind t=0
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ARPS Simulation of a Single Cell Storm
T-equivalent buoyancy+qw+ref
Eq. Pot. Temp.+qw+Ref+wind t=15min
ARPS Simulation of a Single Cell Storm
T-equivalent buoyancy+qw+ref
Eq. Pot. Temp.+qw+Ref+wind t=20min
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ARPS Simulation of a Single Cell Storm
T-equivalent buoyancy+qw+ref
Eq. Pot. Temp.+qw+Ref+wind t=25min
ARPS Simulation of a Single Cell Storm
T-equivalent buoyancy+qw+ref Eq. Pot. Temp.+qw+Ref+wind t=30min
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ARPS Simulation of a Single Cell Storm
T-equivalent buoyancy+qw+ref
Eq. Pot. Temp.+qw+Ref+wind t=45min
ARPS Simulation of a Single Cell Storm
Perturbation Pressure +qw+ref +wind
t=0, 15min
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ARPS Simulation of a Single Cell Storm
Perturbation Pressure +qw+ref +wind
t=20, 25min
ARPS Simulation of a Single Cell Storm
Perturbation Pressure +qw+ref +wind
t=30, 45min
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ARPS Simulation of a Single Cell Storm
Perturbation Pressure +qw+ref +wind
t=45min
High pressure is seen undernearth the cold pool.
Gust front circulation produces strong lifting.
ARPS Simulation of a Single Cell Storm
Animations
(at http://twister.ou.edu/MM2002)
•Buoyancy + Wind + qw + Reflectivity
•Eqivalent Potential Temperature + Wind + qw + Reflectivity
•Perturbation Pressure + Wind + qw + Reflectivity
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