Tropical-Extratropical Transition
ATMS 551
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.
Tropical Versus Extratropical Cyclones:
The Differences
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.
ET
• 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.
Climo
Tracks
of ET
Systems
Annual Frequency
Shaded are ETs
Transition
• 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
are important factors in ET.
• During extratropical transition, cyclones begin to
tilt back into the colder air mass 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.
ET
• 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 and the size of the system will
usually appear to increase.
• After or during transition, the storm may restrengthen, 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.
Satellite
Sequence
Superstorm Sandy: An Excellent
Example
Some 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.
• 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.
ET Impacts
•
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)
http://oregonstate.edu/~readw/October1
962.html
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”-TT
involves the usually slow development of an
extratropical cold core vortex into a tropical
cyclone
Observing the Transition
ET and Precipitation
• During an ET event the precipitation expands poleward of the
center.
• 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.
ET and Predictability
• ET events are often not predicted well by
today’s synoptic models (e.g., GFS)
• The interaction of ET’s with the midlatitude
jet stream/baroclinic zone can result in the
production of Rossby wave trains/packets
that can have a large impact on downstream
flow.
• ET events are often associated with periods
of poor predictability over large areas
downstream of the transition.
MOTIVATION: IMPROVED WEATHER FORECASTS
Impacts on Numerical Model Performance The poleward
movement of a
tropical cyclone
often results in a
period of
reduced forecast
accuracy in
operational
numerical
Hurricane
Maria
global weather
prediction
models
TY Nabi
TY Saola
Variability among ensemble members as a
measure of the predictability downstream
from an ET event
09/09
08/09
07/09
Standard deviation among GFS ensemble members from the model
integration initiated at 1200 UTC 6 September 2005 increases downstream
of the extratropical transition of TY Nabi
06/09
05/09
04/09
03/09
02/09
01/09
31/08
30/08
29/08
Impacts on Forecasting
The movement of a tropical cyclone into the midlatitudes often
induces a large downstream Rossby-wave like response. These
large-amplitude circulation features are often mispositioned, which
contributes to errors in the forecast timing and interaction between
the decaying tropical cyclone and the midlatitude circulation into
which it is moving.
Maemi
Isabel
Maemi
Fabian
Source: NCEP/EMC model verification web page
Isabel
Impacts on Predictability
A “plume” of increased
std. dev. propagates
downstream of the
extratropical transition
forecast position. Local
maxima occur in the base
of each successive trough
of the ET-induced wave
train. Largest variability
exists over North
America.
TY Tokage
http://agora.ex.nii.ac.jp/digital-typhoon/
TY Tokage
ET Time
GFS ensemble membe
+00 0000 UTC 16 Sep 2003
500 hPa height (m) at a
240 m interval
Hurricane Isabel
GFS 500 hPa
ensemble
+108 h
VT 1200 UTC 20
Sep 03
Interaction With Midlatitude
Troughs
• Correct phasing is crucial.
• If the TC is too far east with little upper support 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.
The Details of ET
• The transformation stage of the extratropical
transition of a tropical cyclone was 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.
Step 1
• Step 1, 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 north-northeast, the
southern semicircle is almost completely
clear of clouds and precipitation.
Hurricane/ET Floyd
16-17 Sept 99
• Colle (2003, MWR) studied this with a high
resolution MM5 simulation.
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-1 on 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.
PRES: Often Associated with
ET
• Predecessor Rainfall Events (PRE) in
Tropical Cyclones
Conceptual Model: LOT PRE (SR/AR TC)
UL Jet
LL θe-Ridge
Axis
PREs
See inset
ML
Streamlines
TC Rainfall
Revised and updated from Fig.
13 of Bosart and Carr (1978)
Representative TC Tracks
Conceptual Model (More Detailed Inset)
UL Jet
LL θe-Ridge
Axis
Mountain
Axes
LL Temp/ Moisture
Boundary
UL Jet
LL θe-Ridge
Axis
PREs
PREs
Idealized LL
Winds
ML
Streamlines
TC
Rainfall
TC Tracks
Frequency of Occurrence
• Bosart Stutdy: 1998 to 2006
• 47 PRE were identified, which were tied to a total of 21
TC
• About 1/3 of all Atlantic Basin TC that made U.S. Landfall
for this period had PREs
PRE Statistics
Agnes PRE
Separation Distance
1086 ± 482 km
Median: 935 km
Bosart and Carr (1978) conceptual
model of antecedent rainfall indirectly
associated with TC Agnes (from 1972)
PRE Statistics (Continued)
Agnes PRE
Separation Distance
1086 ± 482 km
Median: 935 km
Event Duration
14 ± 7 h
Median: 12 h
Bosart and Carr (1978) conceptual
model of antecedent rainfall indirectly
associated with TC Agnes (from 1972)
Major Recent Example: Joaquin
The End