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Some heavy precipitation issues. Heavy precipitation at a location = intensity x longevity. Common sources of heavy precipitation in U.S. Mesoscale convective systems and vortices Orographically induced, trapped or influenced storms Landfalling tropical cyclones.

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Common sources of heavy precipitation in u s
Common sources of heavy precipitation in U.S.

  • Mesoscale convective systems and vortices

  • Orographically induced, trapped or influenced storms

  • Landfalling tropical cyclones



Mcss precipitation facts
MCSs & precipitation facts

  • Common types: squall-lines and supercells

  • Large % of warm season rainfall in U.S. and flash floods (Maddox et al. 1979; Doswell et al. 1996)

  • Initiation & motion often not well forecasted by operational models (Davis et al. 2003; Bukovsky et al. 2006)

    • Boundary layer, surface and convective schemes “Achilles’ heels” of regional-scale models

    • Improved convective parameterizations help simulating accurate propagation (Anderson et al. 2007; Bukovsky et al. 2006)

  • Supercells often produce intense but not heavy rainfall

    • Form in highly sheared environments

    • Tend to move quickly, not stay in one place


U s flash flood seasonality
U.S. flash flood seasonality

Contribution of

warm season MCSs

clearly seen

Number of events

Maddox et al. (1979)


Linear mcs archetypes e g squall lines
Linear MCS archetypes(e.g., squall-lines)

58%

19%

19%

Parker and Johnson (2000)



The multicell storm
The multicell storm

Four cells at a single time

Or a single cell at four times:

unsteady

Browning et al. (1976)


The multicell storm1
The multicell storm

Unsteadiness =

episodic entrainment owing

to local buoyancy-induced

circulations.

Browning et al. (1976)

Fovell and Tan (1998)


Storm motion matters
Storm motion matters

How a storm moves over a specific

location determines rainfall

received

Doswell et al. (1996)


Storm motion matters1
Storm motion matters

Doswell et al. (1996)


Forecasting mcs motion

Forecasting MCS motion

(or lack of motion…)


19980714 north plains
19980714 - North Plains

http://locust.mmm.ucar.edu/episodes



Rules of thumb

“Rules of thumb”

“Why?”

“Right for the right reason?”


Some common rules of thumb ingredients
Some common “rules of thumb” ingredients

  • CAPE (Convective Available Potential Energy)

  • CIN (Convective Inhibition)

  • Precipitable water

  • Vertical shear - magnitude and direction

  • Low-level jet

  • Midlevel cyclonic circulations


Some common rules of thumb
Some common “rules of thumb”

  • MCSs tend to propagate towards the most unstable air

  • 1000-500 mb layer mean RH ≥ 70%

  • MCSs tend to propagate parallel to 1000-500 mb thickness contours

  • MCSs favored where thickness contours diverge

  • MCSs “back-build” towards higher CIN

  • Development favored downshear of midlevel cyclonic circulations


70 layer rh
70% layer RH

70% RH rule

of thumb

Implication:

Relative humidity

more skillful than

absolute humidity

RH > 70%

# = precip. category

Junker et al. (1999)


Mcss tend to follow thickness contours
MCSs tend to follow thickness contours

Implication: vertical shear determines

MCS orientation and motion.

Thickness divergence likely implies

rising motion


Back building towards higher cin
Back-building towards higher CIN

Lifting takes longer where there

is more resistance


Corfidi vector method
Corfidi vector method

Cell motion vs. system motion


Corfidi vector method1
Corfidi vector method

Propagation is vector difference

P = S - C

Therefore, S = C + P

to propagate = to cause to continue, to pass through (space)



Schematic example
Schematic example

A multicellular squall-line


Schematic example1
Schematic example

Cell motion as shown


Schematic example2
Schematic example

System motion as shown


Schematic example3
Schematic example

We wish to forecast system motion

So we need to understand what controls

cell motion and propagation


Individual cell motion
Individual cell motion

  • “Go with the flow”

  • Agrees with previous observations (e.g, Fankhauser 1964) and theory (classic studies of Kuo and Asai)

Cells tend to move at

850-300 mb layer

wind speed*

*Layer wind weighted towards lower troposphere,

using winds determined around MCS genesis.

Later some slight deviation to the right often appears

Corfidi et al. (1996)


Individual cell motion1
Individual cell motion

Cell direction comparable

To 850-300 mb layer

wind direction

Cells tend to move at

850-300 mb layer

wind speed

Corfidi et al. (1996)


Composite severe mcs hodograph
Composite severe MCS hodograph

Selective composite

already excludes

non-severe, non-TS

squalls

Bluestein and Jain (1985)


Composite severe mcs hodograph1
Composite severe MCS hodograph

Bluestein and Jain (1985)


Composite severe mcs hodograph2
Composite severe MCS hodograph

Bluestein and Jain (1985)


Composite severe mcs hodograph3
Composite severe MCS hodograph

Low-level jets (LLJs)

are common

Note

P ~ -LLJ

Bluestein and Jain (1985)


Propagation vector and llj
Propagation vector and LLJ

• Many storm environments

have a low-level jet (LLJ) or

wind maximum

• Propagation vector often

anti-parallel to LLJ

Propagation vector direction

P ~ -LLJ

Corfidi et al. (1996)


Forecasting system motion using antecedent information
Forecasting system motionusing antecedent information

Cell motion ~ 850-300 mb wind

Propagation ~ equal/opposite to LLJ

S = C - LLJ


Evaluation of corfidi method
Evaluation of Corfidi method

Method skillful in predicting

system speed and direction

Corfidi et al. (1996)


Limitations to corfidi method
Limitations to Corfidi method

  • Wind estimates need frequent updating

  • Influence of topography on storm initiation, motion ignored

  • Some storms deviate significantly from predicted direction (e.g., bow echoes)

  • P ~ -LLJ does not directly capture reason systems organize (shear) or move (coldpools)

  • Beware of boundaries!

  • Corfidi (2003) modified vector method


Composite severe mcs hodograph4
Composite severe MCS hodograph

Low-level shear

influences

storm organization

& motion

Angle between

lower & upper shear

also important

(Robe and Emanuel 2001)

Bluestein and Jain (1985)


Low-level shear

http://locust.mmm.ucar.edu/episodes


5 june 2004
5 June 2004

X = Hays,

Kansas, USA


5 june 20041
5 June 2004

X = Hays,

Kansas, USA



Potential vorticity
Potential vorticity

Simplest form (see Holton. Ch. 4):

absolute vorticity/depth is conserved for dry adiabatic processes.

Equivalent to angular momentum conservation;

stretching increases vorticity.

This is a special case of Rossby-Ertel PV


Rossby ertel potential vorticity q
Rossby-Ertel potential vorticity q

incorporating:

3D vorticity vector, potential temperature gradient

and Coriolis expressed as a vector (function of z only)

In this formulation,

mass x q is conserved between two isentropes

even (especially!) if diabatic processes are

changing the potential temperature

Haynes and McIntyre (1987)


Rossby ertel potential vorticity q1
Rossby-Ertel potential vorticity q

Here, we simplify a little bit

and focus only on the vertical direction.

The conserved quantity is mq. Holton’s version is

derivable from Rossby-Ertel’s equation, where A is

horizontal area. (Keep in mind ∆ is fixed between

two isentropes.)


Rossby ertel pv
Rossby-Ertel PV

For a dry adiabatic process, the mass between

two isentropes cannot change. Thus, the only

way to increase the cyclonic vorticity  is to

move the object equatorward (decreasing f)

OR decrease its horizontal area A.

Now, consider a more relevant example…


Start with a stably stratified environment,

with no initial horizontal variation.

Define two layers, bounded by these three

isentropes.

We are dealing with horizontal layers.

Horizontal area A is not relevant.


m1 and m2 are the initial masses

residing in these two layers.

q1 and q2 are the initial PVs.

mq can be transported horizontally but not vertically.

So m1q1 and m2q2 will not change.


Introduce a diabatic heat source, representing convection.

The potential temperature in the heated region

increases. This effectively moves the isentrope 2

downward.


Now there is less mass in the lower isentropic layer,

and more mass in the upper layer.

Because mq is conserved between any two

isentropes, q has increased in the lower layer

because m has decreased there.

q has NOT been advected vertically.


The increased q in the lower layer represents

a positive PV anomaly (+PV). Because q

has increased,  is enhanced and a cyclonic

circulation is induced.

In the upper layer, decreased q means -PV

and an induced anticyclonic circulation.



Mcvs as pv anomalies
MCVs as PV anomalies

MCV is a midlevel positive PV anomaly that can be created by a squall-line.

We are ignoring the negative PV anomaly farther aloft.

Raymond and Jiang (1990)


Mcvs as pv anomalies1
MCVs as PV anomalies

PV anomaly shown drifting in westerly sheared flow

Raymond and Jiang (1990)


Mcvs as pv anomalies2
MCVs as PV anomalies

Adiabatic ascent induced beneath anomaly

Raymond and Jiang (1990)


Mcvs as pv anomalies3
MCVs as PV anomalies

Ascent occurs on windward (here, east) side… destabilization

Raymond and Jiang (1990)


Mcvs as pv anomalies4
MCVs as PV anomalies

Westerly vertical shear implies isentropes

tilt upwards towards north

Raymond and Jiang (1990)


Mcvs as pv anomalies5
MCVs as PV anomalies

Cyclonic circulation itself results in ascent on east side

Raymond and Jiang (1990)


Mcvs as pv anomalies6
MCVs as PV anomalies

Combination: uplift & destabilization on

windward side AND downshear side

Raymond and Jiang (1990)


Composite analysis of mcv heavy rain events
Composite analysis of MCV heavy rain events

• Based on 6 cases

poorly forecasted by models

• Composite at time of

heaviest rain (t = 0h)

• Heaviest rain in early

morning

• Heaviest rain south of

MCV in 600 mb trough

600 mb vorticity (color), heights and winds.

Map for scale only

Schumacher and Johnson (2008)


Schumacher s situation
Schumacher’s situation

Midlevel MCV

See also

Fritsch et al. (1994)


Schumacher s situation1
Schumacher’s situation

Nocturnally-enhanced LLJ transports high e air;

Flow below PV anomaly from SW


Schumacher s situation2
Schumacher’s situation

“Hairpin” hodograph:

Sharp flow reversal above LLJ


Schumacher s situation3
Schumacher’s situation

South side of MCV is windward at low-levels

and downshear relative to midlevel vortex


Schumacher s situation4
Schumacher’s situation

Tends to result in very slow-moving,

back-building convection south of MCV


Back building
Back-building

Ground-relative

system speed ~ 0

Schumacher and Johnson (2005)

Doswell et al. (1996)


Evolution of the heavy rain event
Evolution of the heavy rain event

At t - 12h (afternoon):

- MCV located farther west

- 900 mb winds fairly light

600 mb vorticity, 900 mb winds & isotachs

Schumacher and Johnson (2008)


Evolution of the heavy rain event1
Evolution of the heavy rain event

At t - 6h (evening):

- MCV drifted west

- 900 mb winds strengthening (LLJ intensifying)

600 mb vorticity, 900 mb winds & isotachs

Schumacher and Johnson (2008)


Evolution of the heavy rain event2
Evolution of the heavy rain event

At time of heaviest rain (midnight):

- 900 mb jet well developed

- LLJ located east, south of MCV

600 mb vorticity, 900 mb winds & isotachs

Schumacher and Johnson (2008)


Evolution of the heavy rain event3
Evolution of the heavy rain event

At t + 6h (morning):

rain decreases as LLJ weakens

600 mb vorticity, 900 mb winds & isotachs

Schumacher and Johnson (2008)



South plains llj
South Plains LLJ

  • Enhanced southerly flow over South Plains

  • Most pronounced at night

  • Responsible for moisture advection from Gulf & likely a major player in nocturnal thunderstorms and severe weather


  • Bonner (1968)

  • - LLJ occurences

  • meeting certain

  • criteria

  • most frequent in

  • Oklahoma

  • - most frequent at

  • night


Explanations for llj
Explanations for LLJ

  • Oscillation of boundary layer friction (mixing) responding to diurnal heating variation

  • Vertical shear responding to diurnally varying west-east temperature gradients owing to sloped topography

  • Cold air drainage down the Rockies at night

  • Topographic blocking of some form



  • Bonner (1968) observations in which nocturnal LLJ appeared at Ft. Worth, TX

  • of Ft. Worth wind at height

  • of wind max.

  • • wind weaker, more

  • southerly during afternoon

  • • nighttime wind stronger,

  • more from southwest,

  • elevation lower


Episodes of MCSs in which nocturnal LLJ appeared at Ft. Worth, TX

& predictability

Hovmoller diagrams

reveal westward-

propagating MCSs

Note “envelope” of

several systems

with “connections”

Carbone et al. (2002)


Mcv role in predictability
MCV role in predictability in which nocturnal LLJ appeared at Ft. Worth, TX

Carbone et al. (2002)


Training lines of cells
“Training lines” of cells in which nocturnal LLJ appeared at Ft. Worth, TX

• In Asia, stationary front

could be the Mei-Yu

(China), Baiu (Japan) or

Changma (Korea) front

• Motion along the front

and/or continuous back-

building

Schumacher and Johnson (2005)


Record 619 mm in 15 h at Ganghwa, Korea in which nocturnal LLJ appeared at Ft. Worth, TX

X

shear

Lee et al. (2008)

Sun and Lee (2002)


2 3 april 2006
2-3 April 2006 in which nocturnal LLJ appeared at Ft. Worth, TX


2 3 april 20061
2-3 April 2006 in which nocturnal LLJ appeared at Ft. Worth, TX


Why did new cells appear ahead of the mature line
Why did new cells appear in which nocturnal LLJ appeared at Ft. Worth, TXahead of the mature line?

Effectively “speeds up” (earlier rain)

& “slows down” (prolongs rain)”


New cell initiation ahead of squall lines
New cell initiation ahead of squall-lines in which nocturnal LLJ appeared at Ft. Worth, TX

Unsteady multicellularity

excites internal gravity waves

Fovell et al. (2006)


New cell initiation ahead of squall lines1
New cell initiation ahead of squall-lines in which nocturnal LLJ appeared at Ft. Worth, TX

One possible trapping mechanism: the storm anvil

Fovell et al. (2006)


Trapping mechanism
Trapping mechanism in which nocturnal LLJ appeared at Ft. Worth, TX

  • Trapping can occur when a layer of lower l2 resides over a layer with higher values

  • More general Scorer parameter (c = wave speed)

  • Lowered l2 can result from decreased stability or creation of a jet-like wind profile

    • Storm anvil does both


New cell initiation ahead of squall lines2
New cell initiation ahead of squall-lines in which nocturnal LLJ appeared at Ft. Worth, TX

The waves themselves disturb

the storm inflow

Fovell et al. (2006)


New cell initiation ahead of squall lines3
New cell initiation ahead of squall-lines in which nocturnal LLJ appeared at Ft. Worth, TX

…and can create clouds

Fovell et al. (2006)


New cell initiation ahead of squall lines4
New cell initiation ahead of squall-lines in which nocturnal LLJ appeared at Ft. Worth, TX

…some of which can develop into

precipitating, even deep, convection

Fovell et al. (2006)


New cell initiation ahead of squall lines5
New cell initiation ahead of squall-lines in which nocturnal LLJ appeared at Ft. Worth, TX

Other plausible mechanisms for

new cell initiation exist

Fovell et al. (2006)


New cell initiation ahead of squall lines6
New cell initiation ahead of squall-lines in which nocturnal LLJ appeared at Ft. Worth, TX

14 km

150 km

Fovell et al. (2006)


New cell initiation ahead of squall lines7
New cell initiation ahead of squall-lines in which nocturnal LLJ appeared at Ft. Worth, TX

14 km

150 km

Fovell et al. (2006)


New cell initiation ahead of squall lines8
New cell initiation ahead of squall-lines in which nocturnal LLJ appeared at Ft. Worth, TX

14 km

150 km

Fovell et al. (2006)


Importance of antecedent soil moisture conditions

Importance of antecedent in which nocturnal LLJ appeared at Ft. Worth, TXsoil moisture conditions

(Generally not captured well by models)


Tropical storm erin 2007
Tropical Storm Erin (2007) in which nocturnal LLJ appeared at Ft. Worth, TX

http://en.wikipedia.org/wiki/Image:Erin_2007_track.png


Ts erin well inland
TS Erin well inland in which nocturnal LLJ appeared at Ft. Worth, TX

Kevin Kloesel, University of Oklahoma


Erin s redevelopment over oklahoma
Erin’s redevelopment in which nocturnal LLJ appeared at Ft. Worth, TXover Oklahoma

Emanuel (2008)

http://www.meteo.mcgill.ca/cyclone/lib/exe/fetch.php?id=start&cache=cache&media=wed2030.ppt


Erin inland reintensification
Erin inland reintensification in which nocturnal LLJ appeared at Ft. Worth, TX

  • Hot and wet loamy soil can rapidly transfer energy to atmosphere

  • Previous rainfall events left Oklahoma’s soil very wet

  • Need to consider antecedent soil moisture and soil type

Emanuel (2008)

see also

Emanuel et al. (2008)


Soil t as erin passed
Soil T as Erin passed in which nocturnal LLJ appeared at Ft. Worth, TX

Emanuel (2008)


Summary
Summary in which nocturnal LLJ appeared at Ft. Worth, TX

  • A critical view of some ideas, tools relevant to heavy precipitation forecasting

  • Emphasis on factors operational models do not handle particularly well

    • CAPE & CIN, MCS development and motion, surface and boundary layer conditions


end in which nocturnal LLJ appeared at Ft. Worth, TX


Composite sounding at heavy rain location
Composite sounding in which nocturnal LLJ appeared at Ft. Worth, TX(at heavy rain location)

• Southwesterly winds

beneath MCV

• Westerly winds at

midlevels push MCV

eastward

• Southerly shear at

midlevels across MCV

Schumacher and Johnson (2008)


Mcv shear interaction decreases cin
MCV-shear interaction in which nocturnal LLJ appeared at Ft. Worth, TXdecreases CIN

CIN (colored) and CAPE (contours)

t - 6h:

Moderate CAPE (1500 J/kg)

CIN 10-50 J/kg

Schumacher and Johnson (2008)


Mcv shear interaction decreases cin1
MCV-shear interaction in which nocturnal LLJ appeared at Ft. Worth, TXdecreases CIN

CIN (colored) and CAPE (contours)

t - 6h:

Moderate CAPE (1500 J/kg)

CIN 10-50 J/kg

t - 0h:

CAPE unchanged

CIN disappeared owing

to MCV uplift

Schumacher and Johnson (2008)


Linear mcs archetypes
Linear MCS archetypes in which nocturnal LLJ appeared at Ft. Worth, TX

TS - Trailing Stratiform

58%

LS - Leading Stratiform

19%

PS - Parallel Stratiform

19%

Parker and Johnson (2000)


http://locust.mmm.ucar.edu/episodes in which nocturnal LLJ appeared at Ft. Worth, TX


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