Tropical-Extratropical Transition
ATMS 551
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.
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.
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
Big Impacts of ET
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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)
Hurricane Michael Example
http://meted.ucar.edu/norlat/ett/
Date: 15-20 OCT 2000
Hurricane MICHAEL
ADV LAT LON
TIME
1 30.00 -71.20 10/15/12Z
2 30.00 -71.50 10/15/18Z
3 29.90 -71.80 10/16/00Z
4 29.90 -71.90 10/16/06Z
5 29.70 -71.70 10/16/12Z
6 29.80 -71.40 10/16/18Z
7 29.90 -71.10 10/17/00Z
8 29.80 -71.00 10/17/06Z
9 29.80 -70.90 10/17/12Z
10 30.10 -70.90 10/17/18Z
11 30.40 -70.90 10/18/00Z
12 30.80 -70.80 10/18/06Z
13 31.50 -70.40 10/18/12Z
14 32.60 -69.50 10/18/18Z
15 34.20 -67.80 10/19/00Z
16 36.30 -65.50 10/19/06Z
17 39.80 -61.60 10/19/12Z
18 44.00 -58.50 10/19/18Z
19 48.00 -56.50 10/20/00Z
20 50.00 -56.00 10/20/06Z
21 51.00 -53.50 10/20/12Z
22 52.00 -50.50 10/20/18Z
WIND PR STAT
30 1007 SUBTROPICAL DEPRESSION
30 1006 SUBTROPICAL DEPRESSION
35 1005 TROPICAL STORM
35 1005 TROPICAL STORM
35 1005 TROPICAL STORM
35 1004 TROPICAL STORM
35 1003 TROPICAL STORM
45 1000 TROPICAL STORM
55 995 TROPICAL STORM
65 988 HURRICANE-1
65 988 HURRICANE-1
65 986 HURRICANE-1
65 984 HURRICANE-1
70 979 HURRICANE-1
75 983 HURRICANE-1
65 986 HURRICANE-1
75 979 HURRICANE-1
85 965 HURRICANE-2
75 966 EXTRATROPICAL STORM-1
70 966 EXTRATROPICAL STORM-1
65 968 EXTRATROPICAL STORM-1
60 970 EXTRATROPICAL STORM
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. 1052–1092.
•
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. 2634–2653.
•
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. 2613–2633.
The Differences
Observing the Transition
Movement of ET Storms
Intensity
• An ET storm generally first weakens and
then can strengthen substantially..
Climo
Tracks
of ET
Systems
Annual Frequency
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.
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.
Precipitation (in.) and cyclone track for the
extratropical transition of (a) southwest
Pacific Cyclone Audrey (1964) during the 72h 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.
ET and Predictability
• ET events are often not predicted well by
today’s synoptic models (e.g., GFS)
• ET events are often associated with periods
of poor predictability over large areas
downstream of the transition.
Anomaly correlations for NOGAPS forecasts of 500-hPa heights over the North Pacific (20°–70°N,
120°E–120°W) 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).
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
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/mmjoness.pdf
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 30°N with the kinetic energy sufficient to maintain
the circulation against frictional dissipation.
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.
• 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.
• 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 northnortheast, the southern semicircle is almost
completely clear of clouds and precipitation.
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 air…it dies.
• Only for a critical ~5 deg swath does it have
everything…upper support, ability to tap warm,
moist air.
Hurricane/ET Floyd
16-17 Sept 99
• Colle (2003, MWR) studied this with a hig
resolution MM5 simulation.
Recent Event
~12/05/2004
• http://www.atmos.washington.edu/~ovens/l
oops/wxloop.cgi?/home/disk/user_www/clif
f/transition+all
The Tropical Transition (TT)
Problem
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).
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 10–15 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.
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 15°N on occasion. In September
2001 alone, they counted 34 upper-level
vorticity maxima (averaged over a 3° × 3°
latitude–longitude box) greater than 10−5 s−1
persisting for at least 12 h while over ocean
temperatures greater than 25°C.