Post on 22-Dec-2015
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
Corfidi et al. (1996)
Composite severe MCS hodograph
Bluestein and Jain (1985)
Selective compositealready excludes
non-severe, non-TSsqualls
Composite severe MCS hodograph
Bluestein and Jain (1985)
Composite severe MCS hodograph
Bluestein and Jain (1985)
Composite severe MCS hodograph
Low-level jets (LLJs)are common
NoteP ~ -LLJ
Bluestein and Jain (1985)
Propagation vector and LLJ
• Many storm environments have a low-level jet (LLJ) or wind maximum
• Propagation vector oftenanti-parallel to LLJ
Corfidi et al. (1996)
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
Corfidi et al. (1996)
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
Composite severe MCS hodograph
Low-level shearinfluences
storm organization& motion
Angle betweenlower & upper shear
also important(Robe and Emanuel 2001)
Bluestein and Jain (1985)
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Low
-leve
l she
ar
5 June 2004
X = Hays,Kansas, USA
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.
MCVs as PV anomalies
PV anomaly shown drifting in westerly sheared flow
Raymond and Jiang (1990)
MCVs as PV anomalies
Adiabatic ascent induced beneath anomaly
Raymond and Jiang (1990)
MCVs as PV anomalies
Ascent occurs on windward (here, east) side… destabilization
Raymond and Jiang (1990)
MCVs as PV anomalies
Westerly vertical shear implies isentropestilt upwards towards north
Raymond and Jiang (1990)
MCVs as PV anomalies
Cyclonic circulation itself results in ascent on east side
Raymond and Jiang (1990)
MCVs as PV anomalies
Combination: uplift & destabilization onwindward side AND downshear side
Raymond and Jiang (1990)
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)
Schumacher’s situation
Nocturnally-enhanced LLJ transports high e air;Flow below PV anomaly from SW
Schumacher’s situation
“Hairpin” hodograph:Sharp flow reversal above LLJ
Schumacher’s situation
South side of MCV is windward at low-levelsand downshear relative to midlevel vortex
Schumacher’s situation
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)
Evolution of the heavy rain event
600 mb vorticity, 900 mb winds & isotachs
At t - 6h (evening):
- MCV drifted west
- 900 mb winds strengthening (LLJ intensifying)
Schumacher and Johnson (2008)
Evolution of the heavy rain event
600 mb vorticity, 900 mb winds & isotachs
At time of heaviest rain (midnight):
- 900 mb jet well developed
- LLJ located east, south of MCV
Schumacher and Johnson (2008)
Evolution of the heavy rain event
600 mb vorticity, 900 mb winds & isotachs
At t + 6h (morning):
rain decreases as LLJ weakens
Schumacher and Johnson (2008)
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
Schumacher and Johnson (2005)
• 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
New cell initiation ahead of squall-lines
One possible trapping mechanism: the storm anvil
Fovell et al. (2006)
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
New cell initiation ahead of squall-lines
The waves themselves disturb the storm inflow
Fovell et al. (2006)
New cell initiation ahead of squall-lines
…and can create clouds
Fovell et al. (2006)
New cell initiation ahead of squall-lines
…some of which can develop intoprecipitating, even deep, convection
Fovell et al. (2006)
New cell initiation ahead of squall-lines
Other plausible mechanisms fornew cell initiation exist
Fovell et al. (2006)
New cell initiation ahead of squall-lines
150 km
14 k
m
Fovell et al. (2006)
New cell initiation ahead of squall-lines
150 km
14 k
m
Fovell et al. (2006)
New cell initiation ahead of squall-lines
150 km
14 k
m
Fovell et al. (2006)
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
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
end
Composite sounding(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 interactiondecreases CIN
CIN (colored) and CAPE (contours)
t - 6h:Moderate CAPE (1500 J/kg)
CIN 10-50 J/kg
Schumacher and Johnson (2008)
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
Schumacher and Johnson (2008)
Linear MCS archetypes
Parker and Johnson (2000)
TS - Trailing Stratiform58%
LS - Leading Stratiform19%
PS - Parallel Stratiform19%
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