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
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
Number of events
U.S. flash flood seasonality
Contribution of
warm season MCSs
clearly seen
Maddox et al. (1979)
Linear MCS archetypes
(e.g., squall-lines)
58%
19%
19%
Parker and Johnson (2000)
Storm motion matters
Doswell et al. (1996)
Forecasting MCS motion
(or lack of motion…)
19980714 - North Plains
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
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
Implication: vertical shear determines
MCS orientation and motion.
Thickness divergence likely implies
rising motion
Back-building towards higher
CIN
Lifting takes longer where there
is more resistance
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
System motion as shown
Schematic example
We wish to forecast system motion
So we need to understand what controls
cell motion and propagation
Individual cell motion
Cells tend to move at
850-300 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 at
850-300 mb layer
wind speed
Corfidi et al. (1996)
Cell direction comparable
To 850-300 mb layer
wind direction
Composite severe MCS
hodograph
Low-level jets (LLJs)
are common
Note
P ~ -LLJ
Bluestein and Jain (1985)
Propagation vector and LLJ
• Many storm environments
have a low-level jet (LLJ) or
wind maximum
Propagation vector direction
• Propagation vector often
anti-parallel to LLJ
P ~ -LLJ
Corfidi et al. (1996)
Forecasting system motion
using antecedent information
Cell motion ~ 850-300 mb wind
Propagation ~ equal/opposite to LLJ
S = C - LLJ
Evaluation of Corfidi method
Method skillful in predicting
system 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 shear
influences
storm organization
& motion
Angle between
lower & upper shear
also important
(Robe and Emanuel 2001)
Bluestein and Jain (1985)
http://locust.mmm.ucar.edu/episodes
Low-level shear
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 are
changing the potential temperature
Haynes and McIntyre (1987)
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
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
Combination: uplift & destabilization on
windward side AND downshear side
Raymond and Jiang (1990)
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
Tends to result in very slow-moving,
back-building convection south of MCV
Back-building
Ground-relative
system speed ~ 0
Schumacher and Johnson (2005)
Doswell et al. (1996)
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
• 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
Hovmoller diagrams
reveal westwardpropagating MCSs
Note “envelope” of
several systems
with “connections”
Carbone et al. (2002)
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 backbuilding
Schumacher and Johnson (2005)
X
Record 619 mm in 15 h at
Ganghwa, Korea
shear
Lee et al. (2008)
Sun and Lee (2002)
2-3 April 2006
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
…and can create clouds
Fovell et al. (2006)
New cell initiation ahead of
squall-lines
…some of which can develop into
precipitating, even deep, convection
Fovell et al. (2006)
New cell initiation ahead of
squall-lines
150 km
Fovell et al. (2006)
Tropical Storm Erin (2007)
http://en.wikipedia.org/wiki/Image:Erin_2007_track.png
Erin’s redevelopment
over 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