Radar and Warning Strategies for Tropical Cyclone Tornadoes

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Radar and Warning Strategies for Tropical Cyclone
Tornadoes

• 2008 NOAA in the Carolinas Hurricane Conference

Presentation by WFO Raleigh, NC
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Why TC tornadoes are especially difficult

• Needle in a haystack small, shallow feature
  within a much larger storm
• Often lack signatures seen with classic supercell
  tornadoes
• Hooks
• Appendages
• Long-lived updrafts
• Factors serving to increase instability are
  subtle
• Small areas of thinner clouds or sunshine
• Mid level dry slot can be narrow or difficult to
  see on water vapor imagery
• Lapse rates often moist adiabatic
• Storm-relative rotation can be difficult to
  detect within the fast mean layer winds within a
  TC
• Shallow and narrow supercells can be tough to
  detect if theyre too far from the radar
• Low level boundaries (favored zone for TC
  tornadogenesis) can be difficult to discern
  within a TC environment

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Recent TC Tornado Research

• Schneider and Sharp (WAF, April 2007) reviewed
  all TC tornado events from 2004 to find the best
  predicting signatures

Tornado in Hoke County
Numerous tornadoes across southern NC
Gaston (Aug. 2004)
Frances (Sep. 2004)
Tornado in N Guilford County (Stokesdale) and 79
mph gust at RDU
Tornadoes in Moore, Richmond, and Wake counties
Ivan (Sep. 2004)
Jeanne (Sep. 2004)
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Recent TC Tornado Research

• Schneider and Sharp (WAF, April 2007) findings
• All tornadoes had gate-to-gate velocity couplet
  at some point before touchdown.
• All tornadoes had at least 15 kts rotational
  velocity (Vr) most had a Vr of 20 kts or
  greater.
• Vr of 25 kts or greater had a tornado every time.
• Nearly every hook signature produced a tornado,
  but not every tornado had a hook signature.
• Enhanced velocities at 1.5 slice (7-14 kft) was
  often a precursor to a tornado (but not every
  tornado had this signature).

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TC Tornado Characteristics

• Most of the 2004 TC tornadoes were bottom-up type
  development no descending mesocyclone.
• Rotation was typically confined below 15,000 ft
  and only detectable in the lowest 2 scans
• Rotation signatures may last for only 2 or 3
  volume scans requires a quick warning decision.
  
• So, how can you get good lead time?

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Situational Awareness

• Know the environment and how it is changing
• Monitor the SPC mesoanalysis page (i.e. LCL, low
  level helicity, subtle boundaries, stability
  gradients)
• Wind profiles, via 88D VWP products, profilers,
  land and aircraft (AMDAR) soundings
• Look for boundaries in radar, satellite, and
  surface data (areas where helicity is potentially
  enhanced and surface dewpoint spread is narrower)
• North and northeast quadrants are favored areas

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Situational Awareness

• Monitor water vapor imagery for dry slot
• Can lead to reduced sky cover and more rapid
  destabilization
• Curtis (WAF, 2004) examined 13 tornado outbreaks
  associated with tropical cyclones 11 of 13
  identified with mid level dry air intrusion
• Both Ivan and Jeanne had significant dry air
  intrusion (and both events saw most severe
  occurrence of wind damage)
• Watch visible imagery for areas of sunshine or
  thinning clouds

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Jeanne September 27-28, 2004 12z
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Jeanne September 27-28, 2004 18z
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Typical TC tornado environment
From McCaul (MWR, 1991)
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Recommended Warning Criteria (used at WFO
Raleigh)

• Gate-to-gate Rotational Velocity of 15 kts
  Getting interesting pay close attention. Warn if
  there is a hook or velocity enhancement aloft
  with it or if it is far from the radar. (Good
  lead time possible, but high FAR)
• Watch for a tightening circulation to lt 3 mi
  diameter
• GTG Rotational Velocity of 20 kts Go for it
  (can get 15-20 minutes lead time).
• GTG Rotational Velocity of 20 kts hook or
  velocity enhancement aloft WARN NOW! (can get
  10-15 minutes lead time)
• Use lower thresholds when far from the radar, and
  take advantage
• of other radars and TDWRs.
• Reflectivity signatures will not be seen far from
  the radar.

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A few of our radar techniques

• At least one person only focuses on the tornado
  threat nothing else
• Sectorization based on the spiral bands
• Use reflectivity to track strongest cells
• We use VCP 121 to reduce range folding
• Lower slices scanned at different PRFs (pulse
  repetition frequencies) to reduce second trip
  echoes
• Watch cells with a history more closely
• Look at other offices radars when needed

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Gaston August 29, 2004 Harnett County History of
a tornado, plus a hook!
Max Vr was 19 kts with this cell but obviously
range folding was a problem
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Using other radars
Anson County, from KRAX
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A new finding Velocity Enhancement Signature
(VES)

• Typically found at 1.5 - 2.4 (7-14 kft)
• Located above low-level inflow region of storm
• Preceded all but one of the studied tornadoes

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Example Hurricane Jeanne
Rain bands associated with Jeanne produced
several tornadoes over the Sandhills and Piedmont
of North Carolina. Tornado touchdowns were
reported in Moore, Richmond and Wake Counties.
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Jeanne 18Z Mon Sep 27, 2004
Upper divergence to enhance lift
Boundary in place
Instability gradient boundary
Very high SRH
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Jeanne 0.5 Base Ref 2118Z Mon Sep 27, 2004
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Jeanne 4 Panel SRM 2118Z Mon Sep 27, 2004
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Jeanne 0.5 Base Ref 2124Z Mon Sep 27, 2004
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Jeanne 4 Panel SRM 2124Z Mon Sep 27, 2004
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Jeanne 4 Panel Base Ref 2145Z Mon Sep 27, 2004
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Jeanne 4 Panel SRM 2145Z Mon Sep 27, 2004
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534 pm
Vr 32 kts
Initial touchdown 541 pm
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Thank you for your attention!

• 2008 NOAA in the Carolinas Hurricane Conference