multicell storms

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Multicell Storms Multicell Storms METR 4433: Mesoscale Meteorology METR 4433: Mesoscale Meteorology Spring 2006 Semester Spring 2006 Semester Adapted from Materials by Drs. Kelvin Droegemeier, Frank Adapted from Materials by Drs. Kelvin Droegemeier, Frank Gallagher III and Ming Xue Gallagher III and Ming Xue School of Meteorology School of Meteorology University of Oklahoma University of Oklahoma

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Multicell Storms. METR 4433: Mesoscale Meteorology Spring 2006 Semester Adapted from Materials by Drs. Kelvin Droegemeier, Frank Gallagher III and Ming Xue School of Meteorology University of Oklahoma. Multicellular Thunderstorms. - PowerPoint PPT Presentation

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Page 1: Multicell Storms

Multicell StormsMulticell Storms

METR 4433: Mesoscale MeteorologyMETR 4433: Mesoscale MeteorologySpring 2006 SemesterSpring 2006 Semester

Adapted from Materials by Drs. Kelvin Droegemeier, Frank Adapted from Materials by Drs. Kelvin Droegemeier, Frank Gallagher III and Ming XueGallagher III and Ming Xue

School of MeteorologySchool of MeteorologyUniversity of OklahomaUniversity of Oklahoma

Page 2: Multicell Storms

                                            

Page 3: Multicell Storms

Multicellular Multicellular ThunderstormsThunderstorms

So far we have discussed the So far we have discussed the structure of air mass or single cell structure of air mass or single cell thunderstorms.thunderstorms.

We can think of these types of We can think of these types of storms as a single “cell” where each storms as a single “cell” where each cell is:cell is:– Independent– Has a complete life cycle– Has a life cycle of 30 minutes to an hour– Is usually weak

Page 4: Multicell Storms

Multicellular Multicellular ThunderstormsThunderstorms

We know that many thunderstorms can We know that many thunderstorms can persist of longer periods of time.persist of longer periods of time.– These storms are made up of many cells.– Each individual cell goes through a life cycle but

the group persists.– These storms are called multicellular

thunderstorms, or simply multicells– Multicellular storms consist of a series of evolving

cells with each one, in turn, becoming the dominant cell in the group.

Page 5: Multicell Storms

Multicell StormsMulticell Storms Multicell cluster stormMulticell cluster storm - A group of cells moving - A group of cells moving

as a single unit, often with as a single unit, often with each cell in a each cell in a different stage of the thunderstorm life cycledifferent stage of the thunderstorm life cycle. . Multicell storms can produce moderate size Multicell storms can produce moderate size hail, flash floods and weak tornadoes. hail, flash floods and weak tornadoes.

Multicell Line (squall line) StormsMulticell Line (squall line) Storms - consist of - consist of a a line of stormsline of storms with a continuous, with a continuous, well well developed gust front at the leading edgedeveloped gust front at the leading edge of the of the line. Also known as squall lines, these storms line. Also known as squall lines, these storms can produce small to moderate size hail, can produce small to moderate size hail, occasional flash floods and weak tornadoes. occasional flash floods and weak tornadoes.

Page 6: Multicell Storms

Multicell Storm WeatherMulticell Storm Weather

Multicell severe weather can be of any Multicell severe weather can be of any variety, and generally these storms are variety, and generally these storms are more potent than single cell storms, but more potent than single cell storms, but considerably less so than supercells, considerably less so than supercells, because closely spaced updrafts compete because closely spaced updrafts compete for low-level moisturefor low-level moisture. .

Organized multicell storms have higher Organized multicell storms have higher severe weather potential, although severe weather potential, although unorganized multicells can produce pulse unorganized multicells can produce pulse storm-like bursts of severe events.storm-like bursts of severe events.

Page 7: Multicell Storms

Multicellular Multicellular ThunderstormsThunderstorms

With air mass storms, the outflow boundaries are usually tooweak to trigger additional convection. In multicell storms, theoutflow boundary does trigger new convection.

Cell #1Mature

Page 8: Multicell Storms

With air mass storms, the outflow boundaries are usually tooweak to trigger additional convection. In multicell storms, theoutflow boundary does trigger new convection.

Cell #1Mature

Cell #2Cumulus

Multicellular Multicellular ThunderstormsThunderstorms

Page 9: Multicell Storms

After about 20 minutes or so, the second cell becomes thedominant cell. Cell #1 is now dissipating, and a new cell (#3)is starting.

Cell #1Dissipating

Cell #2Mature

Cell #3Cumulus

Multicellular Multicellular ThunderstormsThunderstorms

Page 10: Multicell Storms

Multicellular Multicellular ThunderstormsThunderstorms

After about 20 minutes or so, the third cell becomes thedominant cell. This process may continue as long as

atmospheric conditions are favorable for new convection.

Cell #1AlmostGone

Cell #3Mature

Cell #4Cumulus

Cell #2Dissipating

Page 11: Multicell Storms

Multicellular Multicellular ThunderstormsThunderstorms

A cluster of short lived single cells.A cluster of short lived single cells. Cold outflow from each cell combines to Cold outflow from each cell combines to

form a much larger and stronger gust form a much larger and stronger gust front.front.

Convergence along the gust front tends Convergence along the gust front tends to trigger new updraft development. to trigger new updraft development. This is the strongest in the direction of This is the strongest in the direction of storm motion.storm motion.

New cell growth often appear New cell growth often appear disorganized to the naked eye.disorganized to the naked eye.

Page 12: Multicell Storms

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.

Page 13: Multicell Storms

n+1

n-2 n-1 n-2 n-1 n n

Life Cycle of Multicell Storms

Page 14: Multicell Storms

A closer view at T = 20 minutes (from in the earlier slide) shows that cell 3 still has the highest 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 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 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. 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 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.like. This storm appears to be unicellular but the several distinct echo tops tell us otherwise.

Page 15: Multicell Storms

A Real Example of A Real Example of Multicell stormMulticell storm

Here is a real storm, with radar superimposed. Observe the Here is a real storm, with radar superimposed. Observe the physical similarities to the previous slide. This Texas physical similarities to the previous slide. This Texas Panhandle storm was non-severe. Looking north-northeast Panhandle storm was non-severe. Looking north-northeast from about 20 miles. Note that the updraft numbering is from about 20 miles. Note that the updraft numbering is

reversed.reversed.

Page 16: Multicell Storms
Page 17: Multicell Storms
Page 18: Multicell Storms
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Radar often reflects the Radar often reflects the multicell nature of these multicell nature of these storms, as seen with the storms, as seen with the central echo mass and its central echo mass and its three yellowish cores in the three yellowish cores in the lower portion of this lower portion of this picture.picture.

Occasionally, a multicell Occasionally, a multicell storm will appear storm will appear unicellular in a low-level unicellular in a low-level radar scan, but will display radar scan, but will display several distinct tops when a several distinct tops when a tilt sequence is used to tilt sequence is used to view the storm in its upper view the storm in its upper portionportion

4 cellsThis one might also contain multiple cells

Multicell Storm on RadarMulticell Storm on Radar

Page 20: Multicell Storms

Multicellular Multicellular ThunderstormsThunderstorms

Conditions for developmentConditions for development– Moderate to strong conditional instability

Once clouds form, there is a significant amount of buoyant energy to allow for rapid cloud growth

– Low to moderate vertical wind shear Little clockwise turning

Page 21: Multicell Storms
Page 22: Multicell Storms

Importance of Vertical Wind Importance of Vertical Wind ShearShear

Single cellSingle cell– Weak shear - storm is vertically stacked– Outflow boundary may “outrun” the motion

of the storm cell– New storms that develop may be too far

from the original to be a part of it MulticellMulticell

– Weak to moderate shear keeps gust front near the storm updraft – triggers new cells

– New development forms adjacent to the older cells and connects with the old cell

Page 23: Multicell Storms

A Schematic Model of a Thunderstorm A Schematic Model of a Thunderstorm and Its Density Current Outflowand Its Density Current Outflow

Downdraft Circulation- Density Current in a Broader Sense

(Simpson 1997)

Page 24: Multicell Storms

Cell and Storm System Cell and Storm System MotionMotion

Page 25: Multicell Storms

Multicellular Multicellular ThunderstormsThunderstorms

On the previous diagram, there are two On the previous diagram, there are two arrows that show the “cell motion” and arrows that show the “cell motion” and the “storm motion.”the “storm motion.”

Notice that they are different. Why?Notice that they are different. Why? New cells tend to form on the side of New cells tend to form on the side of

the storm where the warm, moist air at the storm where the warm, moist air at the surface is located.the surface is located.

In the central Plains, this is often on the In the central Plains, this is often on the south or southeast side.south or southeast side.

1 12

Warm, Moist SurfaceAir (inflow)

Average Wind

Page 26: Multicell Storms

Cell Motion versus Storm Cell Motion versus Storm MotionMotion

Cells inside a storm Cells inside a storm (system) do not (system) do not necessarily move at the necessarily move at the same speed and/or same speed and/or direction as the overall direction as the overall storm systemstorm system

The storm system can The storm system can move as a result of the move as a result of the successive growth and successive growth and decay of cellsdecay of cells

It can also move because It can also move because the cell motionthe cell motion

Environmental winds can Environmental winds can have significant influence have significant influence on the cell and/or storm on the cell and/or storm movement, but the storms movement, but the storms do not necessarily follow do not necessarily follow the wind.the wind.

Page 27: Multicell Storms

Multicellular Multicellular ThunderstormsThunderstorms

Individual cells typically move with Individual cells typically move with the mean (average) wind flowthe mean (average) wind flow

The storm system moves The storm system moves differently – by differently – by discrete discrete propagationpropagation

Multicell storms may last a long Multicell storms may last a long time. They constantly renew time. They constantly renew themselves with new cell growth.themselves with new cell growth.

Page 28: Multicell Storms

The The growth of growth of a multicell a multicell

stormstorm

Time (0-21min)

Height (3-12km)

Page 29: Multicell Storms

Multicellular Thunderstorm Multicellular Thunderstorm HazardsHazards

Heavy rain -- FloodingHeavy rain -- Flooding Wind damageWind damage HailHail LightningLightning Tornadoes -- Usually weakTornadoes -- Usually weak Multicell storms are notorious for heavy Multicell storms are notorious for heavy

rain and hailrain and hail

Page 30: Multicell Storms

Cell Generation in Multicell Cell Generation in Multicell stormsstorms

Before we discuss the cell regeneration in Before we discuss the cell regeneration in multicell storms, we will first look the multicell storms, we will first look the gust gust front dynamicsfront dynamics, which plays an important role , which plays an important role in long-lasting convective systemsin long-lasting convective systems

We’ll now begin to use some of what we We’ll now begin to use some of what we learned in vorticity!learned in vorticity!

Page 31: Multicell Storms

A Schematic Model of a Thunderstorm A Schematic Model of a Thunderstorm and Its Density Current Outflowand Its Density Current Outflow

Downdraft Circulation- Density Current in a Broader Sense

(Simpson 1997)

Page 32: Multicell Storms

At the surface, the cold pool At the surface, the cold pool propagates in the form of density or propagates in the form of density or

gravity currentgravity current

Salt Water

Fresh Water

“Haboob” in the Sudan

Note lobe & cleft structures

Laboratory Current

Page 33: Multicell Storms

Other Geophysical Density Other Geophysical Density CurrentsCurrents

(Lava flow on Surtsey, Iceland 1963)(Lava flow on Surtsey, Iceland 1963)

Page 34: Multicell Storms

Schematic of a Thunderstorm Schematic of a Thunderstorm OutflowOutflow

(Goff 1976, based on tower measurements)(Goff 1976, based on tower measurements)

Rotor

Page 35: Multicell Storms

Vorticity DynamicsVorticity Dynamics

The equation for relative vertical The equation for relative vertical vorticityvorticity

LocalDerivative

OfRelativeVorticity

Str

etch

ing/

Con

verg

ence TiltingSolenoidal

( )p p w v w u

u v w v ft x y z x y y x x z y z

Advection

f Recall Absolute Vertical Vorticity

Page 36: Multicell Storms

Vorticity DynamicsVorticity Dynamics

The vector vorticity equation for The vector vorticity equation for absolute absolute verticalvertical vorticity vorticity

MaterialDerivative

OfAbsoluteVorticity S

tret

chin

g/C

onve

rgen

ce Tilting Solenoidal Friction

Page 37: Multicell Storms

Vorticity DynamicsVorticity Dynamics

Can derive the same equation for Can derive the same equation for horizontal vorticityhorizontal vorticity – to apply to – to apply to the horizontal rotor at the head of the horizontal rotor at the head of the gust frontthe gust front

In it, the solenoidal term is exactly In it, the solenoidal term is exactly the same form but acts in the the same form but acts in the horizontalhorizontal

Now, recall circulation…Now, recall circulation…

Page 38: Multicell Storms

Circulation DynamicsCirculation Dynamics

Definition of Circulation C V dl

Via Stokes’ Theorem ˆ ˆC V dl V ndA ndA

ˆ ˆdC d d d

V dl ndA ndAdt dt dt dt

Page 39: Multicell Storms

Circulation DynamicsCirculation Dynamics

Mitchell and Hovermale (1977)

Page 40: Multicell Storms

Gust Front PropagationGust Front Propagation The low-level-inflow-relative speed of The low-level-inflow-relative speed of

gust front often to a large extend gust front often to a large extend determines the propagation of the determines the propagation of the storm system. This is almost certainly storm system. This is almost certainly true for 2-D squall lines. Therefore the true for 2-D squall lines. Therefore the determination of gust front speed is determination of gust front speed is important.important.

Gust front/density currents propagate Gust front/density currents propagate due to horizontal pressure gradient due to horizontal pressure gradient across the front – created mainly by the across the front – created mainly by the density difference across the front. density difference across the front.

Page 41: Multicell Storms

Gust Front Gust Front Propagation Speed Propagation Speed – How to Determine – How to Determine

It?It? For an idealized density current shown above, we apply simple For an idealized density current shown above, we apply simple

equation equation

What have we neglected? Friction, Coriolis effect, effect of What have we neglected? Friction, Coriolis effect, effect of vertical motionvertical motion

Now, to simplify the problem, let’s look at the problem Now, to simplify the problem, let’s look at the problem in a in a coordinate system moving with the gust front. coordinate system moving with the gust front. In this coordinate In this coordinate system, the density current/gust front is stationary, and the system, the density current/gust front is stationary, and the front-relative inflow speed front-relative inflow speed is equal to the speed of the gust front is equal to the speed of the gust front propagating into a calm environment. propagating into a calm environment.

We further assume that the We further assume that the flow is steadyflow is steady in this coordinate in this coordinate system, a reasonably valid assumption when turbulent eddies system, a reasonably valid assumption when turbulent eddies are not considered. Thereforeare not considered. Therefore

0

1 'du p

dt x

2

0 0

1 ' / 2 1 '/ 0

u p u pt u

x x x x

Page 42: Multicell Storms

Gust Front Propagation Gust Front Propagation SpeedSpeed

Integrate the steady momentum equation along a streamline Integrate the steady momentum equation along a streamline along the lower boundary from far upstream where u = U and along the lower boundary from far upstream where u = U and p' = 0 to a point right behind the gust front where u=0 and p' = 0 to a point right behind the gust front where u=0 and p'=p'=p:p:

The above is the propagation speed of the gust front as The above is the propagation speed of the gust front as related to the surface pressure perturbation (related to the surface pressure perturbation (p) associated p) associated with the cold pool/density current. with the cold pool/density current.

2

0 0

2

2

U p pU

2

0 0

1 ' / 2 1 '/ 0

u p u pt u

x x x x

Page 43: Multicell Storms

Gust Front Propagation Gust Front Propagation SpeedSpeed

This equation is very. The contributions to the This equation is very. The contributions to the surface pressure perturbation from cold pool, surface pressure perturbation from cold pool, upper-level heating, nonhydrostatic effect upper-level heating, nonhydrostatic effect (vertical acceleration) and dynamic pressure (vertical acceleration) and dynamic pressure perturbation can all be included.perturbation can all be included.

2

0 0

2

2

U p pU

Page 44: Multicell Storms

Gust Front Propagation SpeedGust Front Propagation Speed If we assume that the If we assume that the p is purely due the hydrostatic effect p is purely due the hydrostatic effect

of heavier air/fluid (of heavier air/fluid (inside the cold pool of inside the cold pool of depth h (other effects as listed in the previous slide are depth h (other effects as listed in the previous slide are neglected), the above formula can be rewritten as (assuming neglected), the above formula can be rewritten as (assuming pressure perturbation above the cold pool is zero):pressure perturbation above the cold pool is zero):

In this case, the In this case, the speed of density current is mainly speed of density current is mainly dependent on the depth of density current and the density dependent on the depth of density current and the density difference across the frontdifference across the front, not a surprising result. , not a surprising result.

When other effects are included, the speed can be When other effects are included, the speed can be somewhat different. But it is generally correct to say somewhat different. But it is generally correct to say that that deeper and/or heavier (colder) density current/cold pool deeper and/or heavier (colder) density current/cold pool propagates faster.propagates faster.

0

0 00 0

2

'0

h h

gU h

dpbecause dz g dz p g h

dz

Page 45: Multicell Storms

Laboratory CurrentLaboratory Current

Laminar

Turbulent

Page 46: Multicell Storms

Laboratory CurrentLaboratory Current

Page 47: Multicell Storms

Numerical SimulationsNumerical Simulations

Temperature’

Pressure’

Horizontal Wind

Droegemeier and Wilhelmson (1987)

Page 48: Multicell Storms

Numerical SimulationsNumerical Simulations

Droegemeier and Wilhelmson (1987)

Page 49: Multicell Storms

Pressure perturbations Pressure perturbations ahead of the gust frontahead of the gust front

In the previous idealized model in the front-following In the previous idealized model in the front-following coordinate, the inflow speed decreases to zero as the air coordinate, the inflow speed decreases to zero as the air parcel approaches the front from far upstream. There must parcel approaches the front from far upstream. There must be horizontal pressure gradient ahead of the gust front to be horizontal pressure gradient ahead of the gust front to ahead of the gust front and this positive pressure ahead of the gust front and this positive pressure perturbation has to be equal to that produced by cold pool.perturbation has to be equal to that produced by cold pool.

We can rewrite the earlier equation asWe can rewrite the earlier equation as

is constant along the streamline following the is constant along the streamline following the lower boundary which is a special form of the Bernoulli lower boundary which is a special form of the Bernoulli function (with the effect of vertical displacement excluded).function (with the effect of vertical displacement excluded).

2 2 2

0 0 0

/ 2 1 ' ' '0

2 2

u p u p u pC

x x x

2

0

'

2

u p

Page 50: Multicell Storms

Numerical simulation of density currents Numerical simulation of density currents showing the pressure perturbations showing the pressure perturbations

associated with density currentassociated with density current

Page 51: Multicell Storms

Pressure perturbations associated Pressure perturbations associated with rotors / rotating (Kelvin-with rotors / rotating (Kelvin-

Helmholtz) eddiesHelmholtz) eddies

Page 52: Multicell Storms

Pressure perturbations Pressure perturbations associated with rotors / associated with rotors /

eddieseddies Above the density current head there usually exist Above the density current head there usually exist

vorticity-containing rotating eddies. Most of the vorticity is vorticity-containing rotating eddies. Most of the vorticity is generated by the horizontal density/buoyancy gradient generated by the horizontal density/buoyancy gradient across the frontal interface.across the frontal interface.

Associated with these eddies are pressure perturbations Associated with these eddies are pressure perturbations due to another dynamic effect – pressure gradient is need due to another dynamic effect – pressure gradient is need to balance the centrifugal force. The equation, called to balance the centrifugal force. The equation, called cyclostrophic balance and applied to tornadoes, iscyclostrophic balance and applied to tornadoes, is

where n is the coordinate directed inward toward the where n is the coordinate directed inward toward the center of the vortex and Rs is the radius of curvature of center of the vortex and Rs is the radius of curvature of the flow. To overcome centrifugal force, pressure at the the flow. To overcome centrifugal force, pressure at the center of a circulation is always lower. The faster the eddy center of a circulation is always lower. The faster the eddy rotates and the smaller the eddy is, the lower is the rotates and the smaller the eddy is, the lower is the central pressure.central pressure.

21 s

s

Vp

n R

Page 53: Multicell Storms

Pressure Pressure perturbationperturbation

s in the s in the head region head region

and and associated associated

(rotor) (rotor) circulationcirculation

Page 54: Multicell Storms

Effects of Surface FrictionEffects of Surface Friction

Free Slip Surface Drag

Max Wind at Ground Max Wind Elevated (Nose)

Page 55: Multicell Storms

Schematic of a Thunderstorm Schematic of a Thunderstorm OutflowOutflow

(Goff 1976, based on tower measurements)(Goff 1976, based on tower measurements)

Rotor

Page 56: Multicell Storms

NoseNose

Page 57: Multicell Storms

Reversal of Near-Surface Reversal of Near-Surface VorticityVorticity

Solid

Dashed

Page 58: Multicell Storms

n+1

n-2 n-1 n-2 n-1 n n

Recall Life Cycle – New Storm on RightRecall Life Cycle – New Storm on Right

Page 59: Multicell Storms

Model-Simulated 2D-Multicell Model-Simulated 2D-Multicell StormStorm

(Lin and Joyce 2001)(Lin and Joyce 2001)

Updraft solid, downdraft dashed

Newest Cell

Page 60: Multicell Storms

Summary of life cycleSummary of life cycle

Based on Lin et al 1998.

Formation and maintenance of the gust front updraft (GFU)

Rearward advection of the growing 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

Page 61: Multicell Storms

Conceputal Model of Lin et al Conceputal Model of Lin et al (1998) for Cell Regeneration (1998) for Cell Regeneration In Lin et al (1998), the following processes are believed to repeat forIn Lin et al (1998), the following processes are believed to repeat for

cell regeneration (see previous illustration). cell regeneration (see previous illustration).

(i) Near the edge of the gust front, the gust front updraft is formed (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 by the low-level convergence ahead of the gust front near the surface.surface.

(ii) The upper portion of the gust front updraft grows by feeding on (ii) The upper portion of the gust front updraft grows by feeding on the midlevel inflow since the gust front propagates faster than the the midlevel inflow since the gust front propagates faster than the basic wind, creating mid-level as well as low-level convergence. basic wind, creating mid-level as well as low-level convergence.

(iii) The growing cell ((iii) The growing cell (CC11) produces strong compensating downdrafts ) produces strong compensating downdrafts on both sides. The downdraft on the upstream (right) side cuts off on both sides. The downdraft on the upstream (right) side cuts off this growing cell from the gust front updraft. this growing cell from the gust front updraft.

(iv) The period of cell regeneration is inversely proportional to the (iv) The period of cell regeneration is inversely proportional to the midlevel, storm-relative wind speed.midlevel, storm-relative wind speed.

Page 62: Multicell Storms
Page 63: Multicell Storms

Numerical Simulations (M. Numerical Simulations (M. Xue)Xue)

Multicell Storm SimulationMulticell Storm Simulation– http://cirrus.ou.edu/RKW/RKW54c2.ptew/RKhttp://cirrus.ou.edu/RKW/RKW54c2.ptew/RK

W54c2.4.anim.gifW54c2.4.anim.gif Density Current simulationDensity Current simulation

– http://twister.http://twister.ouou..eduedu//DensityCurrentDensityCurrent/LS.html/LS.html

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Cell Regeneration theory Cell Regeneration theory of Fovell et alof Fovell et al

Fovell and Tan (1998, MWR) also examined the cell Fovell and Tan (1998, MWR) also examined the cell regeneration problem using a numerical modelregeneration problem using a numerical model

They noted that theThey noted that the unsteadiness unsteadiness of the forcing at the gust of the forcing at the gust front is one reason why the storm is “multicellular”front is one reason why the storm is “multicellular”. The cells . The cells themselves “feed back” to the overall circulation.themselves “feed back” to the overall circulation.

The multicellular storm establishes new cells on its forward The multicellular storm establishes new cells on its forward (upstream) side, in the vicinity of the forced updraft formed (upstream) side, in the vicinity of the forced updraft formed at the cold pool boundary, that first intensify and then decay at the cold pool boundary, that first intensify and then decay as they travel rearward within the storm’s upward sloping as they travel rearward within the storm’s upward sloping front-to-rear airflow. front-to-rear airflow.

The cells were shown to be convectively active entities that The cells were shown to be convectively active entities that induce local circulations that alternately enhance and induce local circulations that alternately enhance and suppress the forced updraft, modulating the influx of the suppress the forced updraft, modulating the influx of the potentially warm inflow. potentially warm inflow.

An explanation of the timing of cell regeneration was given An explanation of the timing of cell regeneration was given that involves two separate and successive phases, each with that involves two separate and successive phases, each with their own timescales. their own timescales.

Page 65: Multicell Storms

Pressure field induced by perturbation buoyancy Pressure field induced by perturbation buoyancy (derived from u and w momentum equations:(derived from u and w momentum equations:

Equation of the horizontal component of Equation of the horizontal component of vorticity (in the x-z plane), neglecting friction,vorticity (in the x-z plane), neglecting friction,

We call generation of horizontal vorticity by We call generation of horizontal vorticity by horizontal gradient of buoyancy the baroclinic horizontal gradient of buoyancy the baroclinic generation of vorticitygeneration of vorticity

2 ( ')'b

Bp

z

horizontalvorticityin ydirection

'

u w

z xd B

dt x

Cell Regeneration theory Cell Regeneration theory of Fovell et alof Fovell et al

Page 66: Multicell Storms

Schematic Schematic illustrating the effect illustrating the effect

of an individual of an individual convective cell on convective cell on

the storm’s low-level the storm’s low-level circulationcirculation

Panel (a) shows the BPGA (buoyancy Panel (a) shows the BPGA (buoyancy pressure gradient acceleration) pressure gradient acceleration) vector field associated with a finite, vector field associated with a finite, positively buoyant parcel. positively buoyant parcel.

Panel (b) shows the full FPanel (b) shows the full Fbb field and field and

the circulatory tendency associated the circulatory tendency associated with baroclinic vorticity generationwith baroclinic vorticity generation..

Panel (c) presents an analysis of the Panel (c) presents an analysis of the circulation tendency at the subcloud circulation tendency at the subcloud cold pool (stippled region) cold pool (stippled region) boundary. boundary.

Panel (d) adds a positively buoyant Panel (d) adds a positively buoyant region with its attendant circulatory region with its attendant circulatory tendency, illustrating the initial tendency, illustrating the initial formation of a convective cell. formation of a convective cell.

Panel (e) shows the cell’s effect at a Panel (e) shows the cell’s effect at a subsequent time (Fig.10 of Fovell subsequent time (Fig.10 of Fovell and Tan 1998).and Tan 1998).

Page 67: Multicell Storms

The influence of transient The influence of transient cell’s circulation on new cell cell’s circulation on new cell

generationgeneration At first, the positively buoyant air created by latent heating within At first, the positively buoyant air created by latent heating within

the incipient cell is located the incipient cell is located above the forced updraft, above the forced updraft, as depicted as depicted in Fig. 10d. in Fig. 10d.

The new cell’s circulation enhances the upward acceleration of The new cell’s circulation enhances the upward acceleration of parcels rising within the forced updraft while partially parcels rising within the forced updraft while partially counteracting the rearward push due to the cold pool’s circulation. counteracting the rearward push due to the cold pool’s circulation.

As a result, the forced lifting is stronger and parcels follow a more As a result, the forced lifting is stronger and parcels follow a more vertically oriented path than they would have been able to without vertically oriented path than they would have been able to without the condensationally generated heating. the condensationally generated heating.

  

Page 68: Multicell Storms

The influence of the transient cell’s circulation depends on its The influence of the transient cell’s circulation depends on its phasing relative to the forced updraft.phasing relative to the forced updraft.

When the cold pool circulation dominates, the new cell and its When the cold pool circulation dominates, the new cell and its positive buoyancy will be advected rearward. positive buoyancy will be advected rearward.

As it moves away from the forced updraft, the intensifying cell As it moves away from the forced updraft, the intensifying cell soon begins to exert a deleterious effect on the low-level lifting, soon begins to exert a deleterious effect on the low-level lifting, as depicted in Fig. 10e. as depicted in Fig. 10e.

Instead of reinforcing upward accelerations in the forced lifting, Instead of reinforcing upward accelerations in the forced lifting, the new cell is assisting the cold pool circulation in driving the the new cell is assisting the cold pool circulation in driving the rising parcels rearward. Thus, at this time, the forced lifting is rising parcels rearward. Thus, at this time, the forced lifting is weaker than it would have been in the absence of convection. weaker than it would have been in the absence of convection.

As the cell continuing moving rearward, its influence wanes, As the cell continuing moving rearward, its influence wanes, permitting the forced updraft to reintensify as the suppression permitting the forced updraft to reintensify as the suppression disappears.disappears.

The influence of transient The influence of transient cell’s circulation on new cell cell’s circulation on new cell

generationgeneration

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Summary of Cell Summary of Cell Regeneration TheoriesRegeneration Theories

Examined closely, the two theories are more complementary Examined closely, the two theories are more complementary than contradictory. Both examine the rearward movement of than contradictory. Both examine the rearward movement of older cells and the separation of the cell from the new cellsolder cells and the separation of the cell from the new cells

Lin et al focuses on the environmental conditions that affect Lin et al focuses on the environmental conditions that affect the rearward cell movement and the associated cell the rearward cell movement and the associated cell regeneration.regeneration.

Fovell’s work emphasizes cell and cold pool interaction and Fovell’s work emphasizes cell and cold pool interaction and the associated gust-front forcing/lifting. The change in the the associated gust-front forcing/lifting. The change in the gust-front lifting is considered to play an important role in gust-front lifting is considered to play an important role in modulating the intensity and generation of new cells at the modulating the intensity and generation of new cells at the gust front.gust front.

Hence, Lin et al’s work looks to external factors while Fovell et Hence, Lin et al’s work looks to external factors while Fovell et al’s work looks to internal dynamics for an explanation of the al’s work looks to internal dynamics for an explanation of the multi-cellular behavior, so each could be looking at a different multi-cellular behavior, so each could be looking at a different but complementary aspect of the problem.but complementary aspect of the problem.