Tropical-Extratropical Transition

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Tropical-Extratropical Transition

• ATMS 551

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Extratropical Transition

• A significant number of tropical cyclones move
  into the midlatitudes and transform into
  extratropical cyclones.
• This process is generally referred to as
  extratropical transition (ET).
• During ET a cyclone frequently acquires increased
  forward motion and sometimes intensify
  substantially, so that such systems pose a
  serious threat to land and maritime activities.
• Often poorly forecast by current day numerical
  models and associated with periods of poor
  synoptic predictability over a wide area
  downstream.
• Extratropical transition occurs in nearly every
  ocean basin that experiences tropical cyclones
  with the number of ET events following a
  distribution in time similar to that of the total
  number of tropical cyclone occurrences.
• The largest number of ET events occur in the
  western North Pacific while the North Atlantic
  basin contains the largest percentage of tropical
  cyclones that undergo ET with 45 of all tropical
  cyclones undergoing ET.

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The Issue

• Tropical cyclones transform into extratropical
  cyclones as they move northward, usually between
  30 and 40 latitude.
• Interaction with upper-level troughs or
  shortwaves in the westeries, and preexisting
  baroclinic zones is an important factor in ET.
• During extratropical transition, cyclones begin
  to tilt back into the colder airmass with height,
  and the cyclone's primary energy source converts
  from the release of latent heat from condensation
  (from convection near the center) to baroclinic
  processes.
• The low pressure system eventually loses its warm
  core and becomes a cold-core system. During this
  process, a cyclone in extratropical transition
  will invariably form or connect with nearby
  fronts and/or troughs. Due to this, the size of
  the system will usually appear to increase. After
  or during transition, the storm may
  re-strengthen, deriving energy from primarily
  baroclinic processes, aided by the release of
  latent heat.
• The cyclone will also distort in shape, becoming
  less symmetric with time, but sometimes retains a
  tight, tropical-like core.

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The Other Direction As Well!

• Less frequently, an extratropical cyclone can
  transit into a tropical cyclone if it reaches an
  area of ocean with warmer waters and an
  environment with less vertical wind shear.
• The process known as "tropical transition"
  involves the usually slow development of an
  extratropic cold core vortex into a tropical
  cyclone

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Big Impacts of ET

• Severe flooding associated with the ET of
  Tropical Storm Agnes 1972
• Hurricane Hazel (1954) resulted in 83 deaths in
  the Toronto area of southern Ontario, Canada. In
  the northwest Pacific, severe flooding and
  landslides have occurred in association with ET.
• An example is the ET of Tropical Storm Janis
  (1995) over Korea, in which at least 45 people
  died and 22 000 people were left homeless.
• In one southwest Pacific ET event (Cyclone Bola)
  over 900 mm of rain fell over northern New
  Zealand).
• Another event brought winds gusting to 75 m s-1
  to New Zealand's capital city, Wellington (Hill
  1970 ), resulting in the loss of 51 lives when a
  ferry capsized.
• Extratropical transition has produced a number
  of weather-related disasters in eastern
  Australia, due to severe flooding, strong winds,
  and heavy seas e.g., Cyclone Wanda in 1974).
• Tropical systems that reintensify after ET in the
  North Atlantic constitute a hazard for Canada
  e.g., Hurricane Earl in 1998 and for northwest
  Europe. The extratropical system that developed
  from Hurricane Lili (1996) was responsible for
  seven deaths and substantial economic losses in
  Europe.
• Many of the largest NW windstorms are ET events
  (Columbus Day Storm, 1981 storm and others)

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Hurricane Michael Example
http//meted.ucar.edu/norlat/ett/

• Date 15-20 OCT 2000
• Hurricane MICHAEL
• ADV LAT LON TIME WIND PR STAT
• 1 30.00 -71.20 10/15/12Z 30 1007
  SUBTROPICAL DEPRESSION
• 2 30.00 -71.50 10/15/18Z 30 1006
  SUBTROPICAL DEPRESSION
• 3 29.90 -71.80 10/16/00Z 35 1005 TROPICAL
  STORM
• 4 29.90 -71.90 10/16/06Z 35 1005 TROPICAL
  STORM
• 5 29.70 -71.70 10/16/12Z 35 1005 TROPICAL
  STORM
• 6 29.80 -71.40 10/16/18Z 35 1004 TROPICAL
  STORM
• 7 29.90 -71.10 10/17/00Z 35 1003 TROPICAL
  STORM
• 8 29.80 -71.00 10/17/06Z 45 1000 TROPICAL
  STORM
• 9 29.80 -70.90 10/17/12Z 55 995 TROPICAL
  STORM
• 10 30.10 -70.90 10/17/18Z 65 988
  HURRICANE-1
• 11 30.40 -70.90 10/18/00Z 65 988
  HURRICANE-1
• 12 30.80 -70.80 10/18/06Z 65 986
  HURRICANE-1
• 13 31.50 -70.40 10/18/12Z 65 984
  HURRICANE-1
• 14 32.60 -69.50 10/18/18Z 70 979
  HURRICANE-1

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A Few References

• Sarah C. Jones, Patrick A. Harr, Jim Abraham,
  Lance F. Bosart, Peter J. Bowyer, Jenni L. Evans,
  Deborah E. Hanley, Barry N. Hanstrum, Robert E.
  Hart, François Lalaurette, Mark R. Sinclair,
  Roger K. Smith and Chris Thorncroft. 2003 The
  Extratropical Transition of Tropical Cyclones
  Forecast Challenges, Current Understanding, and
  Future Directions. Weather and Forecasting Vol.
  18, No. 6, pp. 10521092.
• Patrick A. Harr, Russell L. Elsberry and Timothy
  F. Hogan. 2000 Extratropical Transition of
  Tropical Cyclones over the Western North Pacific.
  Part II The Impact of Midlatitude Circulation
  Characteristics. Monthly Weather Review Vol.
  128, No. 8, pp. 26342653.
• Patrick A. Harr and Russell L. Elsberry. 2000
  Extratropical Transition of Tropical Cyclones
  over the Western North Pacific. Part I Evolution
  of Structural Characteristics during the
  Transition Process. Monthly Weather Review Vol.
  128, No. 8, pp. 26132633.

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The Differences
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Observing the Transition
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Movement of ET Storms
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Intensity

• An ET storm generally first weakens and then can
  strengthen substantially..

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Climo Tracks of ET Systems
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Annual Frequency
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ET and Precipitation

• During an ET event the precipitation expands
  poleward of the center and is typically maximum
  to the left (right) of the track in the Northern
  (Southern) Hemisphere. The change in the
  structure of the precipitation field from the
  more symmetric distribution in a tropical cyclone
  to the asymmetric distribution during ET can be
  attributed to increasing synoptic-scale forcing
  of vertical motion associated with midlatitude
  features such as upper-level PV anomalies or
  baroclinic zones.
• Ets have been associated with extraordinarily
  large precipitation amounts and associated
  flooding.

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Precipitation and ET

• DiMego and Bosart (1982a) diagnosed the
  contributions to the vertical motion during the
  ET of Agnes (1972) and showed how the forcing of
  vertical motion evolves from an almost symmetric
  forcing due to diabatic heating during the
  tropical phase to an asymmetric quasigeostrophic
  forcing during ET.
• A majority of the precipitation associated with
  ET occurs poleward of the center of the decaying
  tropical cyclone. Harr and Elsberry (2000)
  identified this as a region of warm frontogenesis
  north and east of the tropical cyclone center.
  Harr and Elsberry (2000) showed that the ascent
  and frontogenesis in the warm frontal region had
  a gentle upward slope. This suggests that warm,
  moist air in the southerly flow ahead of the
  tropical cyclone center ascends along the gently
  sloping warm front, allowing the region of
  precipitation to extend over a large area ahead
  of the tropical cyclone.

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Precipitation (in.) and cyclone track for the
extratropical transition of (a) southwest Pacific
Cyclone Audrey (1964) during the 72-h period
ending 2300 UTC 14 Jan 1964 (taken from Bureau of
Meteorology 1966 ) and (b) North Atlantic
Hurricane Hazel (1954) during the 24-h period
ending 0600 UTC 16 Oct 1954 adapted from Palmén
(1958) . Solid circles mark the track of
Hurricane Hazel in 3-h increments from 0900 UTC
15 Oct to 0600 UTC 16 Oct 1954.
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ET and Predictability

• ET events are often not predicted well by todays
  synoptic models (e.g., GFS)
• ET events are often associated with periods of
  poor predictability over large areas downstream
  of the transition.

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Anomaly correlations for NOGAPS forecasts of
500-hPa heights over the North Pacific (2070N,
120E120W) during Aug 1996. Each panel
represents a specific forecast interval, as
labeled. The extratropical transition events that
occurred during the month are marked in (a).
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• The 500-hPa height (contours) and mean sea level
  pressure (shaded) from the NOGAPS model for
  Typhoon David (1997). Top row analyses at 0000
  UTC 18 Sep (left), 0000 UTC 19 Sep (middle), and
  0000 UTC 20 Sep (right). Middle row
  corresponding forecasts initialized at 0000 UTC
  16 Sep. Bottom row corresponding forecasts
  initialized at 0000 UTC 17 Sep

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Hovmoeller plot for forecast from 9 September
2003, 12 UTC root mean square difference (RMSD)
of ensemble forecasts with perturbations averaged
over 40 - 50 N for 200 hPa (left) and 500 hPa
(right). The high values of RMSD spread
downstream from Typhoon Maemi (black dot) at both
levels.
European Centre for Medium Range Weather
Forecasts (ECMWF) Ensemble Prediction System
EPS http//www.onr.navy.mil/obs/reports/docs/06/m
mjoness.pdf
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ET Energetics

• Palmén (1958) compared the ET of Hurricane Hazel
  with a typical extratropical cyclone in terms of
  their sources and sinks of energy. He found that
  an extraordinary amount of kinetic energy was
  exported to the midlatitude westerlies from the
  region of the decaying tropical cyclone, which
  led him to estimate that only two to three
  disturbances such as Hazel would provide the
  entire Northern Hemisphere north of 30N with the
  kinetic energy sufficient to maintain the
  circulation against frictional dissipation.

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The Details of ET

• The physical mechanisms associated with the
  transformation stage of the extratropical
  transition of a tropical cyclone were simulated
  with a mesoscale model by Ritchie and Elsberry
  (2001, MWR).
• There appears to be three steps in the
  transformation, which compares well with
  available observations.
• During step 1 of transformation when the tropical
  cyclone is just beginning to interact with the
  midlatitude baroclinic zone, the main
  environmental factor that affects the tropical
  cyclone structure appears to be the decreased sea
  surface temperature. The movement of the tropical
  cyclone over the lower sea surface temperatures
  results in reduced surface heat and moisture
  fluxes, which weakens the core convection and the
  intensity decreases.

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• During step 2 of transformation, the low-level
  temperature gradient and vertical wind shear
  associated with the baroclinic zone begin to
  affect the tropical cyclone.
• Main structural changes include the development
  of cloud-free regions on the west side of the
  tropical cyclone, and an enhanced rain region to
  the northwest of the tropical cyclone center.
  Gradual erosion of the clouds and deep convection
  in the west through south sectors of the tropical
  cyclone appear to be from subsidence.

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• Step 3. Even though the tropical cyclone
  circulation aloft has dissipated, a broad
  cyclonic circulation is maintained below 500 mb.
  Whereas some precipitation is associated with the
  remnants of the northern eyewall and some
  cloudiness to the north-northeast, the southern
  semicircle is almost completely clear of clouds
  and precipitation.

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Hurricane/ET Floyd16-17 Sept 99

• Colle (2003, MWR) studied this with a high
  resolution MM5 simulation.

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Interaction With Midlatitude Troughs

• Correct phasing is crucial.
• If the TC is too far east with little upper
  support (and cold water) it dies.
• If TC too west, it is west of the upper support
  and is facing lots of cold dry airit dies.
• Only for a critical 5 deg swath does it have
  everythingupper support, ability to tap warm,
  moist air.
• ET events can spawn Rossby wave packets that
  propagate far downstream
• Ryan Torn Experiments
• http//www.atmos.washington.edu/torn/research/ets
  ens.php

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Recent Event12/05/2004

• http//www.atmos.washington.edu/ovens/loops/wxloo
  p.cgi?/home/disk/user_www/cliff/transitionall

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The Tropical Transition (TT) Problem
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More important than one might expect

• Nearly half of the Atlantic tropical cyclones
  from 2000 to 2003 depended on an extratropical
  precursor (26 out of 57).
• Many of these disturbances had a baroclinic
  origin and were initially considered cold-core
  systems.
• A fundamental dynamic and thermodynamic
  transformation of such disturbances was required
  to create a warm-core tropical cyclone. This
  process is referred to as tropical transition
  (TT).

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TT

• Tropical cyclogenesis associated with
  extratropical precursors often takes place in
  environments that are initially sheared, contrary
  to conditions believed to allow tropical cyclone
  formation.
• The adverse effect of vertical wind shears
  exceeding 1015 m s-1on the formation of
  low-latitude storms is well documented (DeMaria
  et al. 2001).
• However, a beneficial role of vertical shear,
  hypothesized to organize convection, was
  indicated by the statistical analysis of Bracken
  and Bosart (2000) for 24 developing cases in the
  northern Caribbean Sea.

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TT

• Davis and Bosart (1986, BAMS) showed that
  apparent that PV debris extruded from the
  midlatitude jet is common over the warm oceans of
  the subtropical Atlantic, even as far south as
  15N on occasion. In September 2001 alone, they
  counted 34 upper-level vorticity maxima (averaged
  over a 3 3 latitudelongitude box) greater
  than 10-5 s-1 persisting for at least 12 h while
  over ocean temperatures greater than 25C.