Some%20heavy%20precipitation%20issues

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Some heavy precipitation issues
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Heavy precipitation at a location intensity x
longevity
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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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Mesoscale Convective Systems (MCSs)
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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

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U.S. flash flood seasonality
Contribution of warm season MCSs clearly seen
Number of events
Maddox et al. (1979)
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Linear MCS archetypes(e.g., squall-lines)
58 19 19
Parker and Johnson (2000)
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Squall-lines usually multicellular
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The multicell storm
Four cells at a single time Or a single cell at
four times unsteady
Browning et al. (1976)
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The multicell storm
Unsteadiness episodic entrainment owing to
local buoyancy-induced circulations.
Browning et al. (1976)
Fovell and Tan (1998)
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Storm motion matters
How a storm moves over a specific location
determines rainfall received
Doswell et al. (1996)
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Storm motion matters
Doswell et al. (1996)
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Forecasting MCS motion

• (or lack of motion)

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19980714 - North Plains
http//locust.mmm.ucar.edu/episodes
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19980714 - North Plains
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Rules of thumb

• Why?
• Right for the right reason?

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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

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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

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70 layer RH
70 RH rule of thumb Implication Relative
humidity more skillful than absolute humidity
RH gt 70
precip. category
Junker et al. (1999)
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MCSs tend to follow thickness contours
Implication vertical shear determines MCS
orientation and motion. Thickness divergence
likely implies rising motion
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Back-building towards higher CIN
Lifting takes longer where there is more
resistance
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Corfidi vector method
Cell motion vs. system motion
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Corfidi vector method
Propagation is vector difference P S -
C Therefore, S C P to propagate to cause to
continue, to pass through (space)
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Example
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Schematic example
A multicellular squall-line
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Schematic example
Cell motion as shown
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Schematic example
System motion as shown
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Schematic example
We wish to forecast system motion So we need to
understand what controls cell motion and
propagation
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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)
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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)
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Composite severe MCS hodograph
Selective composite already excludes non-severe,
non-TS squalls
Bluestein and Jain (1985)
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Composite severe MCS hodograph
Bluestein and Jain (1985)
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Composite severe MCS hodograph
Bluestein and Jain (1985)
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Composite severe MCS hodograph
Low-level jets (LLJs) are common Note P -LLJ
Bluestein and Jain (1985)
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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)
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Forecasting system motionusing antecedent
information
Cell motion 850-300 mb wind Propagation
equal/opposite to LLJ S C - LLJ
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Evaluation of Corfidi method
Method skillful in predicting system speed and
direction
Corfidi et al. (1996)
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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

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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)
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Low-level shear
http//locust.mmm.ucar.edu/episodes
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5 June 2004
X Hays, Kansas, USA
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5 June 2004
X Hays, Kansas, USA
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Mesoscale Convective Vortices (MCVs)
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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
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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)
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Rossby-Ertel potential vorticity q
Here, we simplify a little bit and focus only on
the vertical direction. The conserved quantity is
mq. Holtons version is
derivable from Rossby-Ertels equation, where A
is horizontal area. (Keep in mind ?? is fixed
between two isentropes.)
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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
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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.
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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.
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Introduce a diabatic heat source, representing
convection. The potential temperature in the
heated region increases. This effectively moves
the isentrope ?2 downward.
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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.
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The increased q in the lower layer represents a
positive PV anomaly (PV). Because q has
increased, ? is enhanced and a cyclonic circulatio
n is induced. In the upper layer, decreased q
means -PV and an induced anticyclonic circulation.
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Cyclonic vortex following squall line
Not a clean MCV case
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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)
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MCVs as PV anomalies
PV anomaly shown drifting in westerly sheared flow
Raymond and Jiang (1990)
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MCVs as PV anomalies
Adiabatic ascent induced beneath anomaly
Raymond and Jiang (1990)
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MCVs as PV anomalies
Ascent occurs on windward (here, east) side
destabilization
Raymond and Jiang (1990)
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MCVs as PV anomalies
Westerly vertical shear implies isentropes tilt
upwards towards north
Raymond and Jiang (1990)
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MCVs as PV anomalies
Cyclonic circulation itself results in ascent on
east side
Raymond and Jiang (1990)
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MCVs as PV anomalies
Combination uplift destabilization on windward
side AND downshear side
Raymond and Jiang (1990)
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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)
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Schumachers situation
Midlevel MCV
See also Fritsch et al. (1994)
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Schumachers situation
Nocturnally-enhanced LLJ transports high ?e
air Flow below PV anomaly from SW
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Schumachers situation
Hairpin hodograph Sharp flow reversal above LLJ
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Schumachers situation
South side of MCV is windward at low-levels and
downshear relative to midlevel vortex
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Schumachers situation
Tends to result in very slow-moving, back-building
convection south of MCV
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Back-building
Ground-relative system speed 0
Schumacher and Johnson (2005) Doswell et al.
(1996)
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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)
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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)
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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)
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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)
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The South Plains nocturnal low-level jet(LLJ)
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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

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• Bonner (1968)
• - LLJ occurences
• meeting certain
• criteria
• most frequent in
• Oklahoma
• - most frequent at
• night

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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

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• 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

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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
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Episodes of MCSs predictability
Hovmoller diagrams reveal westward- propagating
MCSs
Note envelope of several systems with
connections
Carbone et al. (2002)
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MCV role in predictability
Carbone et al. (2002)
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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
Schumacher and Johnson (2005)
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Record 619 mm in 15 h at Ganghwa, Korea
X
shear
Lee et al. (2008)
Sun and Lee (2002)
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2-3 April 2006
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2-3 April 2006
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Why did new cells appearahead of the mature line?
Effectively speeds up (earlier rain) slows
down (prolongs rain)
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New cell initiation ahead of squall-lines
Unsteady multicellularity excites internal
gravity waves
Fovell et al. (2006)
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New cell initiation ahead of squall-lines
One possible trapping mechanism the storm anvil
Fovell et al. (2006)
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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

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New cell initiation ahead of squall-lines
The waves themselves disturb the storm inflow
Fovell et al. (2006)
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New cell initiation ahead of squall-lines
and can create clouds
Fovell et al. (2006)
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New cell initiation ahead of squall-lines
some of which can develop into precipitating,
even deep, convection
Fovell et al. (2006)
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New cell initiation ahead of squall-lines
Other plausible mechanisms for new cell
initiation exist
Fovell et al. (2006)
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New cell initiation ahead of squall-lines
14 km
150 km
Fovell et al. (2006)
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New cell initiation ahead of squall-lines
14 km
150 km
Fovell et al. (2006)
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New cell initiation ahead of squall-lines
14 km
150 km
Fovell et al. (2006)
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Importance of antecedentsoil moisture conditions
(Generally not captured well by models)
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Tropical Storm Erin (2007)
http//en.wikipedia.org/wiki/ImageErin_2007_track
.png
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TS Erin well inland
Kevin Kloesel, University of Oklahoma
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Erins redevelopmentover Oklahoma
Emanuel (2008) http//www.meteo.mcgill.ca/cyclone/
lib/exe/fetch.php?idstartcachecachemediawed20
30.ppt
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Erin inland reintensification

• Hot and wet loamy soil can rapidly transfer
  energy to atmosphere
• Previous rainfall events left Oklahomas soil
  very wet
• Need to consider antecedent soil moisture and
  soil type

Emanuel (2008) see also Emanuel et al. (2008)
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Soil T as Erin passed
Emanuel (2008)
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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

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end
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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)
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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)
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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 owing to
MCV uplift
Schumacher and Johnson (2008)
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Linear MCS archetypes
TS - Trailing Stratiform 58 LS - Leading
Stratiform 19 PS - Parallel Stratiform 19
Parker and Johnson (2000)
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http//locust.mmm.ucar.edu/episodes