Heavy Convective Rainfall Forecasting: A Look at Elevated Convection, Propagation, and Precipitation Efficiency March 2006

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Heavy Convective Rainfall Forecasting A Look
at Elevated Convection, Propagation, and
Precipitation EfficiencyMarch 2006
Ted Funk Science and Operations Officer WFO
Louisville, KY theodore.funk_at_noaa.gov
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Rainfall potential at any one location depends on

• Moisture availability, transport, and
  replenishment
• Rainfall rate/intensity
• Areal coverage of precipitation
• Motion and speed of precipitation
• Precipitation propagation
• Precipitation efficiency

Flash flood potential depends on

• Rainfall amount
• Topography
• Urbanization
• Land use
• Soil type
• Antecedent conditions

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Elevated Convection Overview

• Occurs above a frontal inversion, isolated from
  surface diabatic effects
• MCS forms 100-200 km north of surface boundary
  given significant isentropic lift and elevated
  convective instability
• Boundary layer CAPE 0 ambient LI values gt 0
  max-?e CAPE (most unstable CAPE aloft) gt 0
• LLJ directed toward entrance region of ULJ to
  north of MCS
• Sloped frontogenetical forcing often present in
  lower levels
• Common in spring and late summer/early fall in
  the Midwest

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Elevated Convection Example 28 April 1994
Sounding at Norman, OK 00 UTC 28 April 1994
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Composites of Elevated Convection (21 cases,
1993-98)

• Average active MCS location 160 km north of sfc
  front near
• Exit region of LLJ (frontogenetical forcing)
• Low-level ?e gradient zone and within max
  low-level ?e advection
• Low level moisture convergence max
• Southern portion of sfc-500 mb mean RH
• Right entrance region of ULJ
  From Moore et al., 2003, WAF

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Elevated Convection Cross-Section Composite

• Cross-section looking east to west into page
• Low-level WAA and convergence lead to
  frontogenetical forcing and resultant direct
  thermal circulation
• Enhances large scale ageo circulation associated
  with right entrance of ULJ

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Types of MCS Propagation

• Forward
• Fast or slow forward propagation
• Short duration heavy rain potential bow
  echo/severe threat
• Given significant trailing stratiform precip,
  still could pose a flood threat, depending on
  speed, soil conditions, and terrain
• Backward
• MCS appears to move backward due to new cell
  development on upwind flank individual cells may
  move slowly forward
• Prolonged heavy rain and flash flood threat
• Regenerative
• Cells within MCS move forward, but new cells
  and/or MCSs develop upwind and move forward over
  same location
• Prolonged heavy rain and flash flood threat

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(No Transcript)
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MCS Movement and Propagation

• Movement of cells (MBEs) within MCS is the sum of
  2 components
• Advective comp mean motion of existing cells in
  MCS cells move 90 of speed and slightly right
  of mean 850-300 mb wind
• Propagation comp given by rate/location of new
  cell formation relative to existing cells
  produces apparent movement due to new development
  on one flank proportional (but opposite in sign)
  to speed direction of LLJ

VCL mean flow in cloud layer (V850 V700
V500 V300) / 4 VPROP propagation
component VMBE MBE movement
Propagation rate strongly dependent on LLJ. The
stronger the LLJ, the more MCS will deviate from
mean wind
From Corfidi, Merritt, and Fritsch, 1996 Wea.
Fcstg.
VMBE VCL - VLLJ
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MCS Movement and Propagation

• Environments of backbuilding MCSs and rapidly
  moving bow echoes can look similar, despite very
  different propagation. Propagation often occurs
  in the direction of the greatest system-relative
  low-level convergence
• Potential to produce a strong downdraft,
  mesohigh, and cold pool at the surface (via
  evaporative cooling/dry air entrainment)
  distinguishes a fast from slow moving system
• Strong cold pool causes a fast-moving gust
  front and greatest S-R convergence on the leading
  edge of the MCS, resulting in rapid forward
  propagation, faster than that predicted from the
  mean wind
• Weak cold pool potential for slow movement/cell
  training

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MCS Movement and Propagation
QUASI-STATIONARY PORTION
PROGRESSIVE PORTION
From Corfidi
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Weak Mesohighs and Backbuilding MCSs

• Weak mesohighs/cold pools are associated with
• Modest positive pressure anomalies just slightly
  greater than ambient pressure thus, there is
  only weak isallobarically driven flow
• Absence of diurnal heating and steep low-level
  lapse rates thus, prolonged heavy rainfall more
  favored at night and in elevated situations
• High mean surface-500 mb RH (little or no dry air
  aloft), which reduces ability to form strong
  downdrafts
• Weak mid-level (700-500 mb) flow limits
  vertical momentum transfer to surface via
  downdrafts

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Strong cold pool/rapid movement/severe winds
northeast part of line. Weaker cold
pool/training/heavy rainfall southwest part of
line.
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Characteristics of Regenerative Convection

• Steering flow carries new echoes away from
  regeneration area
• Watch for intersection of low-level jet with
  pre-existing synoptic or storm-generated boundary
• Consider whether regeneration will be fast enough
  to balance cell movement
• Approaching shortwave causes surface pressure
  falls, which may enhance local low-level flow
  that supplies storm

From Kelsch 2001
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Northern Iowa Flash Flood 14-15 Sept 2004
Winds and fronts at sfc (green/white) 850 mb
(red/blue) 850 mb temps at 03z 14Sep2006
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Northern Iowa Flash Flood 14-15 Sept 2004
300 mb winds, isotachs, and ageo winds at 06z
14Sep2006
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Northern Iowa Flash Flood 14-15 Sept 2004
850-700 mb 2-D frontogenesis at 06z 14Sep2006
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Northern Iowa Flash Flood 14-15 Sept 2004
2148 utc 14Sep2004 to 1115 utc 15Sep2004
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Precipitation Efficiency
Ratio of precipitation that occurs at surface
over lifetime of an MCS to water vapor (moisture)
ingested into MCS updraft during same period
Storm developing stage Input very high, no
output (sfc precip) Mature stage Water vapor
still being supplied within updraft heavy rain
reaches sfc (output) Dissipation stage Storm
rains itself out (no input, only output) The
taller the output curve versus input curve, the
greater the precip efficiency
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Factors Affecting Precipitation Efficiency
Moderate CAPE values (lt2000 J/kg) long,
skinny () area on sounding to promote slow
vertical acceleration increases drop residence
time in warm part of cloud less condensate loss
near top of storm
Low centroid storm (highest reflectivity in lower
half of cloud, especially below 0 C)
Vertically deep warm cloud layer (Tcloud gt0 C
roughly gt3-4km Midwest, gt4km Southeast), higher
cloud liquid water content enhances
collision-coalescence process
High mean relative humidity (gt70) throughout
sounding moisture plume aloft (less dry air
entrainment)
High precipitable water values (1.5-2.5
inches...warm season)
Strong storm-relative low-level inflow of high
theta-e air and mixing ratios to enhance moisture
convergence along a boundary (replenishment)
Weak-to-moderate winds and wind shear in
mid/upper levels yields slower system movement
and decreased entrainment
Broad spectrum of cloud droplet sizes to enhance
collision-coalescence (occurs when air mass has
long trajectory over water, e.g., inflow from
GulfMex)
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Factors Affecting Precipitation Efficiency
Equal reflectivity values may not mean equal
rainfall rates
Not all reflectivity is created equally
Better collision-coalescence
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Factors Affecting Precipitation Efficiency

• Low-centroid storm
• Moderate CAPE, weak shear, deep moisture
• More efficient rainfall production than sheared
  severe storms, BUT severe storms which organize
  into MCS can become efficient in time

Tropical Z-R relationship better? Will cause
over-estimate of precip for high centroid storms.
From Kelsch 2001
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May 7, 2000 Flash Flood Union, MO
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Reflectivity Cross Section Union, MO 5/7/00
Low-centroid storms trained over the same
location (cross-section line) causing extensive
flash flooding
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Reflectivity Cross Section Union, MO 5/7/00
0418 - 0831 utc May 7, 2000
Low-centroid storms trained from left-to-right
(west-to-east) over the same location
(cross-section line) causing extensive flash
flooding
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Beware of persistent, moist low-level flow into a
boundary where training cells exhibit low
centroids and deep warm cloud depths!!
The Bottom Line
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Summary

• Analyze each situation closely from synoptic to
  convective scale. No two situations are the
  same despite what may be similar patterns.
• Understand the processes on various scales which
  produce heavy rainfall and how they may evolve
  given the recognized environment.
• Does environment favor high rainfall rates, fast
  or slow moving convection, regeneration, isolated
  or widespread convection, etc.?
• Model guidance can be used as a first guess, but
  understand model limitations and biases and
  modify your forecast accordingly.

This presentation is available at http//www.crh.
noaa.gov/images/lmk/ppt/ Download these 3 files
into same directory Heavy_Rainfall_Fcstg_DSM_SvrW
x_Conf_Mar2006.ppt animate_backprop_ir.AVI
animate_union_ir.AVI