Transcript convection: single cell

Convective Dynamics
Single cell cumulus
Review of Skew-T
Diagram
Desirable Properties of a thermodynamic diagram:
• Area on diagram proportional to energy
• As many lines representing basic processes
should be ~straight
• Vertical coordinate proportional to height
• Dry adiabatsshould be at near right angles to
isotherms
• Saturation adiabats should be at large angle to dry
• adiabats in lower atmosphere
The Skew T-Log P diagramwas selected by the Air Weather
Service as the most convenient thermodynamic diagram for
general use.
•The most commonly used diagram in the United States.
•Current soundings, model soundings, and archived
soundings are available in Skew-T Log-P form at several
websites and in analysis programs such as GARP.
Saturation Adiabats: the path that a saturated air parcel follows as it rises
pseudo- moist-adiabatically through the atmosphere.
Pseudo-moist-adiabatically: All condensed moisture immediately precipitates
from parcel. Moist adiabatically: All condensed moisture remains in parcel.
Winds are plotted in standard staff/barb format on the line to the right
of the diagram
http://weather.uwyo.edu/upperair/sounding.html
Thunderstorms
• Definition: A thunderstorm is a local storm, invariably
produced by a cumulonimbus cloud, that always is
accompanied by lightning and thunder. It usually contains
strong gusts of wind, heavy rain, and sometimes hail.
• We often use the word "convection" to describe such
storms in a general manner, though the term convection
specifically refers to the motion of a fluid resulting in the
transport and mixing of properties of the fluid. To be
more precise, a convective cloud is one which owes its
vertical development, and possibly its origin, to
convection (upward air currents).
Modes of Convection / Storm Classification
• Although a continuous spectrum of storms
exists, meteorologists find it convenient to
classify storms into specific categories
according to their structure, intensity,
environments in which they form, and
weather produced.
The most basic classification includes:
• Single-cell or air-mass storm Typically last 20-30 minutes. Pulse storms
can produce severe weather elements such as downbursts, hail, some
heavy rainfall and occasionally weak tornadoes.
• Multicell cluster storm A group of cells moving as a single unit, with each
cell in a different stage of the thunderstorm life cycle. Multicell storms
can produce moderate size hail, flash floods and weak tornadoes.
• Multicell Line (squall line) Storms - consist of a line of storms with a
continuous, well developed gust front at the leading edge of the line. Also
known as squall lines, these storms can produce small to moderate size
hail, occasional flash floods and weak tornadoes.
• Supercells Defined as a thunderstorm with a rotating updraft, these
storms can produce strong downbursts, large hail, occasional flash floods
and weak to violent tornadoes.
Ordinary or "air-mass“ storm
• Main Characteristics
– Consists of a single cell (updraft/downdraft pair)
– Forms in environment characterized by large conditional instability and
weak vertical shear
– Vertically erect  built-in self-destruction mechanism
– Can produce strong straight-line winds or microburst
– Life cycle is generally < 1 hour, usually 30-45 min
– These storms form in weakly-forced environments, and are driven
primary by convective instability rather than the ambient winds
– They are some times called "air-mass" storm because they form within
air-masses with more-or-less horizontal homogeneity
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.
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
(a) – developing stage
(c) Mature stage
(b) – maturing
(d) – dissipating stage
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)
where L = liquid+ice water content. Note that the water vapor
contributes to the buoyancy of air parcel and the combined effect
can be expressed in terms of virtual potential temperature
Three stages of single-cell storm development
Towering stage
Mature Stage
Dissipating Stage
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
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 q’
(remember earlier vertical momentum equation?). The saturated air
parcel ascends following the moist adiabat, along which the equivalent
potential temperature qe 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)
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
CAPE
dt
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)
ARPS
Simulation
of a Single
Cell Storm
• May 20, 1977 Del City
Supercell Storm
Sounding – used
without
environmental wind
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(q’/q)
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.
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-destruction mechanism mentioned earlier
• The cold downdraft sometimes form a cold pool that propagates away
from the cell above, further removing the lifting underneath 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 (imagine pushing 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
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 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 wider clouds, 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 from becoming too narrow, hence the typical aspect ratio
of clouds is 1.
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, parcel theory tends to overestimate the intensity of updraft.
Sill, it provides a useful upper-limit for the convection intensity
ARPS Simulation of a Single Cell Storm
T-equivalent buoyancy+qw+ref
Eq. Pot. Temp.+qw+Ref+wind t=0
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
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
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
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
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.
Assignment #4
• Use skew-T diagram to diagnose the convection
How much surface heating (K) would it be to generate
the free convection from near the ground. How
about given more moisture, such as?
Assuming adiabatic rising without water load, all CAPE
would convert into the kinetic energy in the updraft,
calculate the vertical velocity when the air parcel
reach the LNB (level of neutral buoyancy)
Convective Dynamics
Multicell Storms and Density Currents
Photographs © Todd Lindley
Where are single cells and multi cells?
Multicell Storm
• Multicell cluster storm - A group of cells moving as a single
unit, often with each cell in a different stage of the
thunderstorm life cycle. Multicell storms can produce
moderate size hail, flash floods and weak tornadoes.
• Multicell Line (squall line) Storms - consist of a line of storms
with a continuous, well developed gust front at the leading
edge of the line. Also known as squall lines, these storms can
produce small to moderate size hail, occasional flash floods
and weak tornadoes.
Multicell Storm Weather
• Multicell severe weather can be of any variety,
and generally these storms are more potent than
single cell storms, but considerably less so than
supercells, because closely spaced updrafts
compete for low-level moisture.
• Organized multicell storms have higher severe
weather potential, although unorganized
multicells can produce pulse storm-like bursts of
severe events.
Multiple Cells as Seen by Radar
• Radar often reflects the
multicell nature of these
storms, as seen with the
central echo mass and its three
yellowish cores in the lower
portion of this picture.
• Occasionally, a multicell storm
will appear unicellular in a lowlevel radar scan, but will display
several distinct tops when a tilt
sequence is used to view the
storm in its upper portion
4 cells
This one might also
contain multiple cells
Life Cycle of Multicell Storms
n-2
n-1
n
n+1
Life cycle - evolution of cells in a
multicell cluster
•
This illustration portrays a
portion of the life cycle of a
multicell storm.
As cell 1 dissipates at time = 0,
cell 2 matures and becomes
briefly dominant. Cell 2 drops its
heaviest precipitation about 10
minutes later as cell 3
strengthens, and so on.
•
A closer view at T = 20 minutes (from in the earlier slide) shows that cell 3 still has the
highest top, but precipitation is undercutting the updraft in the lower levels. New echo
development is occurring aloft in cells 4 and 5 in the flanking line, with only light rain
falling from the dissipating cells 1 and 2 on the northeast side of the storm cluster.
•
The inset shows what the low-level PPI (plane-position indicator) radar presentation might
look like. This storm appears to be unicellular but the several distinct echo tops tell us
otherwise.
A Real Example of Multicell storm
• Here is a real storm, with radar superimposed. Observe the physical
similarities to the previous slide. This Texas Panhandle storm was nonsevere. Looking north-northeast from about 20 miles. Note that the
updraft numbering is reversed.
Height (3-12km)
Time (0-21min)
The growth of a multicell
storm
Cell Motion versus Storm Motion
•
Cells inside a storm
(system) do not
necessarily move at the
same speed and/or
direction as the overall
storm system
•
The storm system can
move as a result of the
successive growth and
decay of cells
•
It can also move because
the cell motion
•
Environmental winds can
have significant influence
on the cell and/or storm
movement, but the storms
do not necessarily follow
the wind.
Cell Regeneration in Multicell
storms
• Before we discuss the cell regeneration in multicell storms, we
will first look the gust front dynamics, which plays an
important role in long-lasting convective systems
A Schematic Model of a Thunderstorm
and Its Density Current Outflow
Downdraft
Circulation
- Density
Current
in a Broader
Sense
(Simpson 1997)
Role of cold pool in a convective
storms
At the surface, the cold pool
propagates in the form of density
or gravity current
Thunderstorm Outflow /Gust Front as reviewed by the blown
dust
The Gust front propagates along the surface in the form of
density or gravity current
A density current, or gravity current, is a region of dense fluid
propagating into an environment of less dense fluid because of
the horizontal pressure gradient across the frontal surface.
c
u=0
c  k (p /  )1/2
In the Laboratory
Fresh Water
Salt Water
Schematic of a Thunderstorm Outflow
(Goff 1976, based on tower measurements)
Rotor
Numerical simulation of density currents showing the
pressure perturbations associated with density current
Pressure perturbations
associated with rotors / rotating
(Kelvin-Halmholtz) eddies
Pressure
perturbations in the
head region and
associated (rotor)
circulation
Cell Regeneration in 2D multicell
storms
• Representative modeling studies
– Fovell et al (1995 JAS), Fovell and Tan (1998,
MWR)
– Lin et al (1998, 2001, JAS)
Fovell, R. G., and P. S. Dailey, 1995: The temporal behavior of numerically simulated multicell-type
storms, Part I: Modes of behavior. J. Atmos. Sci., 52, 2073-2095.
Fovell, R. G., and P.-H. Tan, 1998: The Temporal Behavior of Numerically Simulated MulticellType Storms. Part II: The Convective Cell Life Cycle and Cell Regeneration. Mon. Wea. Rev., 126,
551-577.
Lin, Y.-L., R. L. Deal, and M. S. Kulie, 1998: Mechanisms of cell regeneration,
development, and propagation within a two-dimensional multicell storm. J. Atmos. Sci., 55, 1867-1886.
Lin, Y.-L., and L. Joyce, 2001: A further study of mechanisms of cell regeneration, propagation
and development within two-dimensional multicell storms. J. Atmos. Sci., 58, 2957–2988.
Summary of life cycle
Rearward advection of the growing GFU
Formation and maintenance
of the gust front updraft (GFU)
Cutting off of the growing cell (c1) from
the GFU by the upstream compensating
downdraft
Cell generation and coexistence of
the growing (c2 and c3) and
propagating (c1) cells
Based on Lin et al 1998.
Conceptual Model of Lin et al (1998) for Cell Regeneration
In Lin et al (1998), the following processes are believed to repeat for cell
regeneration (see previous illustration).
• (i) Near the edge of the gust front, the gust front updraft is formed by
the low-level convergence ahead of the gust front near the surface.
• (ii) The upper portion of the gust front updraft grows by feeding on the
midlevel inflow since the gust front propagates faster than the basic
wind, creating mid-level as well as low-level convergence.
• (iii) The growing cell (C1) produces strong compensating downdrafts
on both sides. The downdraft on the upstream (right) side cuts off this
growing cell from the gust front updraft.
• (iv) The period of cell regeneration is inversely proportional to the
midlevel, storm-relative wind speed.
(a) Skew-T plot of the temperature and dewpoint profiles used in the simulations.
This is a smoothed version of the 1430 HNT 22 May 1976 sounding presented in
Ogura and Liou (1980) . (b) Wind profiles used to initialize the simulations (from Lin
et al 1998).
Life Cycle of simulation
2D-multicell storm
Vertical cross sections of vertical
velocity (thin contours in intervals of 1
m s-1) for the U = 10 m s-1 case.
The cold pool / density current may be
roughly represented by the 1 K
potential temperature perturbation
contour (bold dashed) near the
surface.
The rainwater is shaded (>0.0005 g kg
1) and the cloud boundary is bold
contoured (>0 g kg 1). The
corresponding integration time is
shown at the top of each panel (from
Lin 1998).
Cell Regeneration in
2D multicell storms
•
The three stages of a
convective cell.
•
Equivalent potential
temperature (shaded) and
vertical velocity
(contoured) fields were
taken from Fig. 3 . Note
the reference, frame
shown is not fixed in
space, but rather tracks
the cell’s principal updraft.
•
For more details, read
Fovell and Tan (1998). Link
to the PDF format paper is
found at the course web
site.
Summary on Cell Regeneration
Theories
•
Looked harder, the two theories are more complementary than contradictory.
Both examine the rearward movement of older cells and the separation of the cell
from the new cells
•
Lin et al focus on the environmental condition that affects the rearward cell
movement then on the associated cell regeneration.
•
Fovell’s work emphasizes cell and cold pool interaction and the associated gustfront forcing/lifting. The change in the gust-front lifting is considered to play an
important role in modulating the intensity and generation of new cells at the gust
front.
•
Hence, Lin et al’s work looks to the external factor while Fovell et al’s work looks to
the internal dynamics for an explanation of the multi-cellular behavior –in my
opinion - each is looking at a different but complementary aspect of the problem.