Baroclinic Influences on
Tropical Cyclogenesis
Ron McTaggart-Cowan, Tom
Galarneau Jr., Lance Bosart,
Rich Moore and Olivia Martius
NPS Seminar
15 August 2013
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In the Beginning ...
• Tropical cyclone (TC) development has been described
•
•
by numerous theories since the advent of regular tropical
observations in the mid-1950's
Riehl (1948) emphasizes the importance of upper-level
disturbances in enhancing the growth rate of instabilities
in the easterly trades and proposes three patterns:
• Strong polar westerlies aloft: no TC formation
• Moderate polar westerlies within 10o of the equatorial
shear line: TCs can form during interactions
• Tropical-only upper and lower-level disturbances
In Riehl's conceptual model, the isallobaric field created
by upper-level divergence enhances growth rates
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The TUTTs and TC-Genesis
• Sadler (1975) further emphasized the importance of
upper-level features by proposing outflow-enhancement
by either midlatitude troughs (b) or the tropical uppertropospheric trough (TUTT; c)
The TUTT is a time-mean
feature of the tropical uppertroposphere (200 mb). It is
also referred to as the midocean trough, and appears to
form through a combination of
radiative and dynamic
(anticyclonic wave breaking)
processes.
“TUTT cells” are isolated
vorticity maxima that break
from the base of the TUTT
(Kelley and Mock 1982).
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Textbook TC Development
• Although Gray (1968) describes elements of these
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•
theories, he focuses primarily on lower-level structures
and the initial development of TCs through a “convective
instability of the second kind” (CISK)-type process
The primary impact of the upper level flow described by
Gray is through vertical shear
However, Gray does
estimate 15% “subtropical”
(Type-B) and 1-2%
“hybrid” (Type-C)
development,
distinguished primarily by
formation latitude
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Something Different: Dianna (1984)
• Using the TS Diana (1984) case study, Bosart and Bartlo
(1991) describe the leading role that baroclinic features
play in the development of the storm:
Stage 1
An upper-level trough fractures as it
crosses the Eastern Seaboard,
leaving an isolated cyclonic PV
anomaly over Florida, upstream of
the remnant low-level baroclinic
zone.
SLP isobars (solid, 8 mb) and 1000-500 mb
thickness (dashed, 6dam) at 0000 UTC 7
September 1984 (Bosart and Bartlo 1991).
Anticyclogenesis behind the
progressive trough leads to cool, dry
northerlies along the coast that
enhance surface heat fluxes behind
the offshore front.
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Something Different: Dianna (1984)
• Using the TS Diana (1984) case study, Bosart and Bartlo
(1991) describe the leading role that baroclinic features
play in the development of the storm:
Stage 2
The upper-level isolated cyclonic PV
(cold) anomaly drifts eastward:
cyclonic vorticity advection by the
thermal wind yields deep ascent
along uplifted isentropic surfaces.
Surface sensible and latent heat
fluxes are maximized as the
northeasterly flow behind the front
wraps into the developing lowerlevel center.
Surface sectional map for 1200 UTC 8 September 1948 show SLP (solid, 2 mb) and
temperature (dashed, 2oC). Stations are plotted conventionally (Bosart and Bartlo 1991).
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Something Different: Dianna (1984)
• Using the TS Diana (1984) case study, Bosart and Bartlo
(1991) describe the leading role that baroclinic features
play in the development of the storm:
Stage 3
Heights at 300
mb (solid, 4
dam), and PV
(dashed) for
0000 UTC 9
September
(top) and 0000
UTC 10
September
1984 (bottom).
Figure 19 from
Bosart and
Bartlo (1991).
The cold dome aloft collapses as
diabatic heating from the
convectively-active cyclone actively
erodes the PV feature aloft.
Southwesterly geostrophic flow
between an upstream trough and
the retreating ridge leads to
enhancement of the poleward
outflow channel as TS Diana
develops a warm core.
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PV Analysis of TC-Genesis
• Key conceptual elements proposed by Bosart and Bartlo
•
(1991), new to TC formation theories, include:
• The impact of sustained deep ascent on mid-level
moisture, permitting convection upshear of the
developing centre and reducing downdraft formation
• Strong surface heat fluxes behind the baroclinic zone
enhance instability and enhances upright convection
at the southwestern end of the front
Montgomery and Farrell (1993) employ the conceptual
model developed for TS Diana to investigate the
interaction between upper- and lower-level PV anomalies
during the initial stage of development – before a
circulation of sufficient strength for CISK or WISHE-type
feedbacks exists.
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PV Analysis of TC-Genesis
• Montgomery and Farrell (1993) propose that there is a
range of admissable shears for a given environment,
which will allow an upper-level PV disturbance to
constructively interact (couple) with a developing lowerlevel feature to produce rapid growth rates
• Without an upper-level PV
•
anomaly growth rates are
smaller, but lower-level baroclinicity can promote sufficient
ascent for development
Coupling is most effective in
moist neutral environments,
where balanced vertical
motions are enhanced
Cross-section of PV through a developing
vortex in a moist environment (Fig. 4b from
Montgomery and Farrell 1993).
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Composite Structures
• Bracken and Bosart (2000) identify two dominant upper•
level patterns in the western and eastern North Atlantic
Shear and QG forcing for ascent in both configurations
900 mb
Stream & Div
200 mb
QG Forcing
1) Cyclonic lower-level ζ
2) Forcing for deep ascent
(destabilization; cap removal;
stretching; humidification;
convective organization)
3) Local shear reduction near
ascent maximum
Storm-centered composite
streamlines (lines) and divergence
(shading) for the Bahamas (top)
and Cape Verde (bottom) regions in
the left columns. The Sutcliffederived QG forcing for ascent in
each domain is shown on the right
(Bracken and Bosart 2000).
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Shear Reduction
• Davis and Bosart (2003) show that
•
•
•
reduction of the environmental shear
is critical to high latitude late-season
TC formation from precursors of
different initial intensities
Baroclinic growth rate is small, but
the features focus latent heating by
moistening and destabilization
Shear is reduced in the heating
regions by effective vertical
redistribution of PV
Elimination of cyclonic PV anomaly
aloft reduced zonal PV gradient
Ctrl
Dry
Efficiency
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Introducing the TT Paradigm
• Davis and Bosart (2004) formalize these processes and
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coin the phrase “tropical transition” (TT)
The concept of “weak” and “strong” precursors is also
developed by Davis and Bosart (2004):
• WEC: convective organization by the precursor
progressively weakens shear and strengthens flow
• SEC: extratropical
cyclogenesis creates a
mesoscale vortex that is selfsustaining through WISHE
Convection along a bent-back
front is most effective for shear
reduction (diabatic occlusion)
Combined radar/satellite image for Hurricane Michael ~ 1200 UTC 15 October 2000.
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TC-Genesis as Occlusion
• Hulme and Martin (2009) show that the upstream position
•
of convection along the bent-back front is crucial to the
development of a strong vortex
Compare TT process directly to occlusion with an
enhanced diabatic component
PV and PV Tendencies
Diabatic
Advective
Convection along bentback front:
1) creates low-level PV
that is transported into the
centre (possible LLJ)
2) enhances frontal
strength by downdrafts
3) eliminates PV aloft to
create a PV “notch”
4) enhances advective
reduction of the local
gradient by backing aloft
Layer-mean (330-340 K) PV (solid) and PV tendencies (dark negative, light positive) for in a WRF
simulation of Hurricane Karen at 1800 UTC 11 October 2001 (Hulme and Martin 2009).
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Tropical Cyclogenesis - Climatology
• McTaggart-Cowan et al. (2008) use the conceptual model
of TT to create a pair of “metrics” that define the
baroclinic nature of the formation environment:
• Lower-level thickness asymmetry (Th) represents the
strength of remnant frontal structures that can lead to
lower-level ascent and PV generation/advection
• Upper-level Q-vector
Fraction
convergence (Q) represents Category
Non-baroclinic
40%
the combined effects of
Low-Level
13%
Baroclinic
destabilization, moistening
Trough 16%
and convective organization Transient
Interaction
by an upper-level feature
Weak TT
13%
Strong TT
Percentage of TC developments following each
pathway in the 1948-2004 North Atlantic dataset.
15%
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Tropical Cyclogenesis - Climatology
• The latent trajectory model (LTM) technique used by
•
McTaggart-Cowan et al. (2008) classifies environments
by their evolution over the 36 h prior to TC development;
however, it is not
extensible to other
seasons or basins
A linear discriminant
analysis (LDA) allows for
~90% correct reclassification of the original
dataset without the
restrictions of the LTM
North Atlantic TC in metric space, where storms (dots)
are colour-coded by the LTM classification and LDA
classification regions are shaded: a dot on the samecoloured background is a correct reclassification.
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Tropical Cyclogenesis - Climatology
Metric values from a set of
reanalysis products (NCEP,
ERA-40, ERA-Interim, JRA) are
combined to compute metrics
over the ocean regions at 6hourly intervals
This allows both for the
construction of a climatological
description of local
environments, and for individual
TC classification by sampling of
the categorized state
Mean 400-200 mb PV (shaded) and 1000700 mb thickness (black lines at 6 dam
intervals) for the development of Typhoon
Irving (1992) in (a). In (b), the associated
pathway classifications are plotted from 50oS
to 50oN as shown in the legend.
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Environment Climatology
The non-baroclinic
pathway environment
dominates (a,b) near the
equator except where
local jets are associated
with low level
baroclinicity (c,d).
Frequency of occurrence of pathways as identified on each panel.
The early- and late-summer seasons are defined as MJJ and ASO for
the Northern Hemisphere, NDJ and FMA in the Southern
Hemisphere. The 2 PVU contour on the 350 K (equatorward) and
330 K (poleward) surfaces are shown with heavy dark lines.
The trough induced
environment (e,f) is
most common
downshear of the TUTT
axes, where isolated PV
features are extruded
from breaking Rossby
waves in the mid-ocean
regions.
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Environment Climatology
Weak TT environments
(g and h) are most
common along TUTT
axes, where subtropical
fronts, precursor
vortices or SST
gradients can provide
lower-level baroclinicity.
Frequency of occurrence of pathways as identified on each panel.
The early- and late-summer seasons are defined as MJJ and ASO for
the Northern Hemisphere, NDJ and FMA in the Southern
Hemisphere. The 2 PVU contour on the 350 K (equatorward) and
330 K (poleward) surfaces are shown with heavy dark lines.
Strong TT environments
(i and j) occur at higher
latitudes, although
breaking Rossby waves
enhance their frequency
downshear of the TUTT
axes, primarily in the
Northern Hemisphere.
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Baroclinic Pathways
• The geographical separation of the classes suggests that
the classifications are physically relevant, since there was
no spatial information used to define the categories
Low latitude and warm SST
TUTT cells
AEJ
Fractured PV
Coastal TCs
TUTT axes
Continental
troughs
Non-baroclinic
Low Level
Baroclinic
Trough Induced
Weak TT
Strong TT
Tropical cyclone
development
locations for the full
climatology, displayed
for all categories (a),
and colour-coded by
pathway classification
in all other panels.
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Baroclinic Pathways
• While the non-baroclinic pathway dominates in all basins,
the secondary development pathway varies regionally:
overall, 30% of events display some baroclinic influence
Highlights
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The North Atlantic has
the most varied set of
baroclinic development types
The trough induced
pathway is most
prominent in the West
Pacific
The North Indian
Ocean experiences
numerous weak TT
formations
Frequency of development by pathway (bar group) in each basin (bar colour).
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North Atlantic Basin
• Tropical cyclones forming in the he North Atlantic basin
follow a broad range of baroclinic development pathways
The non-baroclinic
pathway is strongly
peaked between July
and September
Baroclinic pathways
have a flatter
distribution during the
hurricane season
The relative
importance of the
baroclinic pathways
(particularly weak and
strong TT) is thus
maximal during the
shoulder seasons
Distribution of TC development events by month (indigo, early
summer; violet, late summer).
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West Pacific Basin
• The West Pacific is the most prolific basin, representing
>30% of all TC formation events
The West Pacific warm
pool effectively
eliminates lower-level
baroclinicity via
differential fluxes
The short weak TT
season coincides with
the subtropical western
North Pacific monsoon,
during which the TUTT
shifts westward
TUTT cells appear to
play an important role
in the trough induced
pathway
Distribution of TC development events by month (indigo, early
summer; violet, late summer).
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Arabian Sea (North Indian Subbasin)
• The Arabian Sea is the only region in which the nonbaroclinic pathway is not clearly dominant
Easterly shear and
poleward displacement
of the monsoon trough
suppress development
during the monsoon
The Somali jet induces
cold upwelling in the
western Arabian Sea
and generates cyclonic
shear at lower levels
During monsoon break
periods, westerly
troughs interact with
lower level baroclinicity
for strong TT
Distribution of TC development events by month (indigo, early
summer; violet, late summer).
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TC Development Efficiency (Yield)
Late-summer increase
in non-baroclinic yield (a
and b) is likely a result
of increased SSTs.
Late-summer yield of
low level baroclinic TCs
(c and d) is highest off
the west African coast in
response to warming
SSTs beneath the AEJ.
Development efficiency (shading) by pathway and season as
indicated on each panel. The efficiency levels correspond
approximately to per-event periods of 0.2=1.5yr, 0.5=6mo, 1=3mo,
2=2mo, 4=1mo, and 6=2wks. The 26.5oC SST isotherm is shown
with a dashed line.
Despite the rarity of
trough induced
environments, TUTT
cells appear to be highly
efficient TC generators
(e and f).
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TC Development Efficiency (Yield)
Elevated climatological
shear along the TUTT
axes restricts efficient
weak TT formation
regions to the edges of
the basins (g and h).
Development efficiency (shading) by pathway and season as
indicated on each panel. The efficiency levels correspond
approximately to per-event periods of 0.2=1.5yr, 0.5=6mo, 1=3mo,
2=2mo, 4=1mo, and 6=2wks. The 26.5oC SST isotherm is shown
with a dashed line.
Troughs dropping
equatorward across
North America lead to
increased late-summer
yields following both the
weak TT (g and h) and
strong TT (i and j)
pathways in the western
North Atlantic basin.
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Metric-Space Analysis
• Since each point in space and each
TC development event has its own
metrics, the analysis can be recast
from geographical to metric space
• The structure of the metric-space
climatologies suggests both preferred
environments (a) and pathways (b)
• Extensions of event frequencies into
strong TT and low level baroclinic
categories are indicative of extreme
events in these environments
Environmental climatology (a) and TC developments (b) recast in metric
space. Classification regions are colour-coded, with the class centroids
indicated by squares. The kernel density estimate for development
events (contours in b) is computed using the cell area in (a).
Efficiency
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Metric-Space Analysis
• The non-baroclinic
•
environment does not
represent the optimal state for
TC development: an upperlevel disturbance is beneficial
(Montgomery and Farrell
1993, Bracken and Bosart
2000)
Minimum yield along the low
level baroclinic / strong TT
boundary represents the
negative impact of
intermediate shear values
Development efficiency in metric space. Classification regions are colour-coded, with the class centroids
indicated by squares. The yield is computed as the ratio of the development KDE (b) to the
environmental frequency (a). A value of unity corresponds to the “average” yield, while higher and lower
values represent more and less effective development positions in metric space, respectively.
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Discussion
• Our appreciation for the influence of baroclinic features
•
•
on TC development has been increasing since the early
theories on TC formation, some of which have led quite
directly to our current understanding of these events
While developments involving large-amplitude baroclinic
features are not the norm from a global perspective, they
represent an important subset and can be regionally
dominant
Consistent with previous results involving case studies,
idealized simulations, limited climatologies, it appears
that the presence of an upper-level baroclinic disturbance
of limited amplitude is required to create the optimal
environment for TC development
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References
Bracken, W. E., and L. F. Bosart, 2000: The role of synoptic-scale flow during tropical cyclogenesis
over the North Atlantic Ocean. Mon. Wea. Rev., 353-376.
Bosart, L. F. and J. A. Bartlo, 1991: Tropical storm formation in a baroclinic environment. Mon.
Wea. Rev., 119, 1979-2013.
Davis, C. A. and L. F. Bosart, 2003: Baroclinically influenced tropical cyclogenesis. Mon. Wea.
Rev., 131, 2730-2747.
Davis, C. A. and L. F. Bosart, 2004: The TT problem: forecasting the tropical transition of cyclones.
Bull. Amer. Meteor. Soc., 85, 1657-1662.
Gray, W. M., 1968: Global view of the origin of tropical disturbances and storms. Mon. Wea. Rev.,
96, 669-700.
Hulme, A. L. and J. E. Martin, 2009: Synoptic- and frontal-scale influences on tropical transition
events in the Atlantic basin. Part II: Tropical transition of Hurricane Karen. Mon. Wea. Rev., 137,
3636-3650.
Kelley, W. E. and Mock, D. R., 1982: A diagnostic study of upper tropospheric cold lows over the
western North Pacific. Mon. Wea. Rev., 110, 471-480.
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References
McTaggart-Cowan, R., G. D. Deanne, L. F. Bosart, C. A. Davis and T. J. Galarneau Jr., 2008:
Climatology of tropical cyclogenesis in the North Atlantic (1948-2004). Mon. Wea. Rev., 136, 12841304.
McTaggart-Cowan, R., T. J. Galarneau Jr., L. F. Bosart, R. W. Moore and O. Martius, 2013: A global
climatology of baroclinically influenced tropical cyclogenesis. Mon. Wea. Rev., (in press).
Montgomery, M. T., and B. F. Farrell, 1993: Tropical cyclone formation. J. Atmos. Sci., 50, 285-310.
Riehl, H., 1948: On the formation of typhoons. J. Meteor., 5, 247-264.
Sadler, J., 1975: A role of the tropical upper tropospheric trough in early season typhoon
development. Mon. Wea. Rev., 104, 1266-1278.
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Baroclinic Pathways
• While the non-baroclinic pathway dominates in all basins,
the secondary development pathway varies regionally:
overall, 30% of events display some baroclinic influence
North Atlantic
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Smaller fraction of
“traditional” events
than all other basins
Frequent baroclinic
trough penetrations
promote TT paths
Strong temperature
gradients in the Cape
Verde region promote
low-level baroclinic
pathway
Frequency of development by pathway (bar group) in each basin (bar colour).
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Baroclinic Pathways
• While the non-baroclinic pathway dominates in all basins,
the secondary development pathway varies regionally:
overall, 30% of events display some baroclinic influence
North Indian


Upwelling beneath the
Somali jet creates
SST gradients in the
Arabian Sea for
baroclinic pathways
Breaks in the
monsoon allow trough
penetration and a
temporarily enhanced
relative frequency of
baroclinic pathways
Frequency of development by pathway (bar group) in each basin (bar colour).
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Baroclinic Pathways
• The geographical separation of the classes suggests that
the classifications are physically relevant, since there was
no spatial information used to define the categories
Non-baroclinic
Low Level
Baroclinic
Trough Induced
Weak TT
Strong TT
Tropical cyclone
development
locations for the full
climatology, displayed
for all categories (a),
and colour-coded by
pathway classification
in all other panels.