Some heavy precipitation issues. Heavy precipitation at a location = intensity x longevity.

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

Mesoscale Convective Systems (MCSs)

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

Maddox et al. (1979)

Contribution of warm season MCSs clearly seen

Num

ber

of e

vent

s

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

Parker and Johnson (2000)

58%

19%

19%

Squall-lines usually multicellular

The multicell storm

Browning et al. (1976)Four cells at a single timeOr a single cell at four times:unsteady

The multicell storm

Browning et al. (1976)

Fovell and Tan (1998)

Unsteadiness =episodic entrainment owingto local buoyancy-inducedcirculations.

Storm motion matters

Doswell et al. (1996)

How a storm moves over a specificlocation determines rainfall

received

Storm motion matters

Doswell et al. (1996)

Forecasting MCS motion

(or lack of motion…)

19980714 - North Plains

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

19980714 - North Plains

“Rules of thumb”

“Why?”

“Right for the right reason?”

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”

• 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

Junker et al. (1999)

# = precip. category

70%

laye

r RH

RH > 70%

70% RH rule of thumb

Implication:Relative humiditymore skillful thanabsolute humidity

MCSs tend to follow thickness contours

Implication: vertical shear determinesMCS orientation and motion.

Thickness divergence likely impliesrising motion

Back-building towards higher CIN

Lifting takes longer where thereis more resistance

Corfidi vector method

Cell motion vs. system motion

Corfidi vector method

Propagation is vector differenceP = S - C

Therefore, S = C + Pto propagate = to cause to continue, to pass through (space)

Example

Schematic example

A multicellular squall-line

Schematic example

Cell motion as shown

Schematic example

System motion as shown

Schematic example

We wish to forecast system motionSo we need to understand what controls

cell motion and propagation

Individual cell motion

Cells tend to move at850-300 mb layer

wind speed*

Corfidi et al. (1996)

• “Go with the flow”• Agrees with

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

*Layer wind weighted towards lower troposphere,using winds determined around MCS genesis.Later some slight deviation to the right often appears

Individual cell motion

Cells tend to move at850-300 mb layer

wind speed

Cell direction comparableTo 850-300 mb layer

wind direction


Composite severe MCS hodograph

Bluestein and Jain (1985)

Selective compositealready excludes

non-severe, non-TSsqualls


Low-level jets (LLJs)are common

NoteP ~ -LLJ


Propagation vector and LLJ

• Many storm environments have a low-level jet (LLJ) or wind maximum

• Propagation vector oftenanti-parallel to LLJ


Propagation vector direction

P ~ -LLJ

Forecasting system motionusing antecedent information

Cell motion ~ 850-300 mb windPropagation ~ equal/opposite to LLJ

S = C - LLJ

Evaluation of Corfidi method

Method skillful in predictingsystem speed and direction


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 (cold pools)• Beware of boundaries!• Corfidi (2003) modified vector method


Low-level shearinfluences

storm organization& motion

Angle betweenlower & upper shear

also important(Robe and Emanuel 2001)


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Low

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5 June 2004

X = Hays,Kansas, USA

Mesoscale Convective Vortices (MCVs)

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

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 arechanging the potential temperature

Haynes and McIntyre (1987)

Rossby-Ertel potential vorticity q

Here, we simplify a little bitand focus only on the vertical direction.

The conserved quantity is mq. Holton’s version is

derivable from Rossby-Ertel’s equation, where A ishorizontal area. (Keep in mind ∆ is fixed between

two isentropes.)

Rossby-Ertel PV

For a dry adiabatic process, the mass betweentwo isentropes cannot change. Thus, the only

way to increase the cyclonic vorticity is tomove 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 threeisentropes.

We are dealing with horizontal layers.Horizontal area A is not relevant.

m1 and m2 are the initial massesresiding 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 twoisentropes, 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 representsa positive PV anomaly (+PV). Because q

has increased, is enhanced and a cycloniccirculation is induced.

In the upper layer, decreased q means -PVand an induced anticyclonic circulation.

Cyclonic vortex following squall line

Not a cleanMCV case

MCVs as PV anomalies

Raymond and Jiang (1990)

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

We are ignoring the negative PV anomaly farther aloft.


PV anomaly shown drifting in westerly sheared flow



Adiabatic ascent induced beneath anomaly



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



Westerly vertical shear implies isentropestilt upwards towards north



Cyclonic circulation itself results in ascent on east side



Combination: uplift & destabilization onwindward side AND downshear side


Composite analysis of MCV heavy rain events

Schumacher and Johnson (2008)600 mb vorticity (color), heights and winds.Map for scale only

• 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

Schumacher’s situation

Midlevel MCV

See alsoFritsch et al. (1994)


Nocturnally-enhanced LLJ transports high e air;Flow below PV anomaly from SW


“Hairpin” hodograph:Sharp flow reversal above LLJ


South side of MCV is windward at low-levelsand downshear relative to midlevel vortex


Tends to result in very slow-moving,back-building convection south of MCV

Back-building

Schumacher and Johnson (2005)Doswell et al. (1996)

Ground-relative system speed ~ 0

Evolution of the heavy rain event

600 mb vorticity, 900 mb winds & isotachs

At t - 12h (afternoon):

- MCV located farther west

- 900 mb winds fairly light

Schumacher and Johnson (2008)



At t - 6h (evening):

- MCV drifted west

- 900 mb winds strengthening (LLJ intensifying)




At time of heaviest rain (midnight):

- 900 mb jet well developed

- LLJ located east, south of MCV




At t + 6h (morning):

rain decreases as LLJ weakens


The South Plains nocturnal low-level jet

(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

• 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 of wind speed vs. height for days in which nocturnal LLJ appeared at Ft. Worth, TX

-wind speed max just below 1 km MSL (about 800 m AGL) at midnight and 6AM local time

- note increased low-level shear

Bonner (1968) observations 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& predictability

Carbone et al. (2002)

Hovmoller diagrams reveal westward- propagating MCSs

Note “envelope” of several systems with “connections”

MCV role in predictability

Carbone et al. (2002)

“Training lines” of cells


• 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

Sun and Lee (2002)

Record 619 mm in 15 h at Ganghwa, Korea

X

Lee et al. (2008)

shear

2-3 April 2006

2-3 April 2006

QuickTime™ and aPNG decompressor

are needed to see this picture.

Why did new cells appearahead of the mature line?

Effectively “speeds up” (earlier rain) & “slows down” (prolongs rain)”

New cell initiation ahead of squall-lines

Fovell et al. (2006)

Unsteady multicellularityexcites internal gravity waves


One possible trapping mechanism: the storm anvil


Trapping mechanism

• 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


The waves themselves disturb the storm inflow



…and can create clouds



…some of which can develop intoprecipitating, even deep, convection



Other plausible mechanisms fornew cell initiation exist



150 km

14 k

m


Importance of antecedentsoil moisture conditions

(Generally not captured well by models)

Tropical Storm Erin (2007)

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

http://upload.wikimedia.org/wikipedia/commons/d/d5/Erin_2007_track.png

TS Erin well inland

QuickTime™ and aAnimation decompressor

are needed to see this picture.

Kevin Kloesel, University of Oklahoma

Erin’s redevelopmentover Oklahoma

Emanuel (2008)http://www.meteo.mcgill.ca/cyclone/lib/exe/fetch.php?id=start&cache=cache&media=wed2030.ppt

Erin inland reintensification

• 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

Emanuel (2008)

Summary

• 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

Composite sounding(at heavy rain location)

• Southwesterly winds beneath MCV

• Westerly winds at midlevels push MCV eastward

• Southerly shear at midlevels across MCV


MCV-shear interactiondecreases CIN

CIN (colored) and CAPE (contours)

t - 6h:Moderate CAPE (1500 J/kg)

CIN 10-50 J/kg


MCV-shear interactiondecreases CIN

CIN (colored) and CAPE (contours)

t - 6h:Moderate CAPE (1500 J/kg)

CIN 10-50 J/kg

t - 0h:CAPE unchanged

CIN disappeared owingto MCV uplift


Linear MCS archetypes

Parker and Johnson (2000)

TS - Trailing Stratiform58%

LS - Leading Stratiform19%

PS - Parallel Stratiform19%

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Some heavy precipitation issues. Heavy precipitation at a location = intensity x longevity.

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