benjamin a. schenkel ([email protected]),[email protected] lance f. bosart, and daniel keyser...
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Benjamin A. Schenkel ([email protected]),
Lance F. Bosart, and Daniel Keyser
University at Albany, State University of New York
16th Cyclone Workshop
The role of the large-scale environment in determining North Atlantic tropical cyclone size
Research Sponsored by NSF Grant AGS-0935830
Motivation: Variability of TC Size
IR Imagery Near the Time of Genesis for Small TC and Large TC
Mean 34-kt wind radius
(black circle)
Small TC
VMAX - 40 ktMean 34-kt wind radius - 133 km
Large TC
VMAX - 40 ktMean 34-kt wind radius - 102 km
While the size of the convective envelopes for each TC are different, size of the TC wind fields are initially comparable
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 2/24
Motivation: Variability of TC Size
IR Imagery at Maximum Intensity for Small TC and Large TC
Size of Charley has decreased slightly while Sandy has tripled in size
Small TC
VMAX - 125 ktMean 34-kt wind radius - 111 km
Large TC
VMAX - 100 ktMean 34-kt wind radius - 310 km
Mean 34-kt wind radius
(black circle)
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 3/24
Motivation: Variability of TC Size
IR Imagery During Extratropical Transition for Small TC and Large TC
Charley has continued to decrease in size while Sandy has continued to rapidly expand
Small TC
VMAX - 60 ktMean 34-kt wind radius - 97 km
Large TC
VMAX - 80 ktMean 34-kt wind radius - 769 km
Mean 34-kt wind radius
(black circle)
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 4/24
Motivation: Variability of TC Size
IR Imagery During Extratropical Transition for Small TC and Large TC
Small TC
VMAX - 60 ktMean 34-kt wind radius - 97 km
Large TC
VMAX - 80 ktMean 34-kt wind radius - 769 km
Mean 34-kt wind radius
(black circle)
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 5/24
Charley has continued to decrease in size while Sandy has continued to rapidly expandWhat factors determine the life cycle of TC size?
Outline
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 6/24
• Background
– Factors influencing the life cycle of TC size
• Results: Role of environment in determining TC size
– Difference in life cycle of size between average and large TCs
– Storm-relative composite analysis of average and large TCs
• Summary and conclusions
Outline
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 6/24
• Background
– Factors influencing the life cycle of TC size
• Results: Role of environment in determining TC size
– Difference in life cycle of size between average and large TCs
– Storm-relative composite analysis of average and large TCs
• Summary and conclusions
Potential Factors Influencing the Life Cycle of TC Size
• Prior work has primarily utilized idealized modeling of a limited number
of cases to determine which factors control TC size:
- Size of precursor disturbance determines TC size (i.e., large
disturbances yield large TCs; Emanuel 1987)
- Larger TCs may require environmental source of PV (e.g.,
ITCZ) to grow through convergence of environmental PV into
TC (Guinn and Schubert 1993)
- Higher environmental relative humidities can yield larger TCs
due to relatively stronger diabatic production of PV (Hill and
Lackmann 2009)
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 7/24
Potential Factors Influencing the Life Cycle of TC Size
- Larger TCs may be expected for increasing SSTs, decreasing
tropopause temperatures, and decreasing TC latitude (Chavas
and Emanuel 2013)
- Extratropically transitioning TCs may increase in size due to
expansion of TC wind field resulting from lower-tropospheric
horizontal temperature advection (e.g., Evans and Hart 2008)
- TC size may increase due to broadening of wind field that
results from eyewall replacement cycles (Maclay et al. 2008)
- TCs that make landfall over narrow landmasses (e.g., Florida)
may increase in size following landfall (Knaff et al. 2013)
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 8/24
In an effort to address this issue from a different perspective, the following
study seeks to examine whether the large-scale atmospheric
environment determines TC size using composites of reanalysis data
Potential Factors Influencing the Life Cycle of TC Size
• In spite of the potential promise of this prior work, our understanding of
the life cycle of TC size remains limited
• It is also important to note that prior work has yet to investigate what
impacts, if any, the large-scale environment has in a composite of a
large number of observed TCs
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 9/24
Motivating Questions
• What factors determine the initial size of a TC?
• Does TC size, on average, change over the life cycle of a TC?
• Which of the previously mentioned factors are most important in
determining the life cycle of TC size?
• What role, if any, does the large-scale atmospheric environment play in
determining TC size?
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 10/24
Motivating Questions
• What factors determine the initial size of a TC?
• Does TC size, on average, change over the life cycle of a TC?
• Which of the previously mentioned factors are most important in
determining the life cycle of TC size?
• What role, if any, does the large-scale atmospheric environment play in
determining TC size?
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 11/24
In the interest of brevity, we will only attempt to answer questions #2 and #4 during this talk
Outline
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 12/24
• Background
– Factors influencing the life cycle of TC size
• Results: Role of environment in determining TC size
– Difference in life cycle of size between average and large TCs
– Storm-relative composite analysis of average and large TCs
• Summary and conclusions
Methodology
• Objective: Examine the role of large-scale environment in determining TC size
• Mean radius of 34-kt surface wind speed from the Extended Best-Track (Demuth
et al. 2006) is used as metric for TC size for North Atlantic TCs from 1988–2012
• TCs divided into size categories based on maximum lifetime size that a TC
reaches for times that are coincident with aircraft reconnaissance data:
• Average TCs (N = 32): Third quintile of maximum TC size
• Large TCs (N = 32): Fifth quintile of maximum TC size
• Storm-relative composites are constructed from NCEP CFSR (Saha et al. 2010)
beginning at time of tropical cyclogenesis for both TC size categories
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 13/24
Analysis of Life Cycle of TC Size for Average and Large TCs
• Average and large TCs are
initially comparable in size and
grow at approximately the
same rate within first two days
following TC genesis
• Large TCs continue to grow
after day two while average TC
size remains relatively
constant
• Growth of large TCs slows
approximately six days
following TC genesis
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 14/24
Can the storm-relative composites help explain the differences in the life cycle of TC size?
Analysis of Storm-Relative Composites
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 15/24
• Large TCs are embedded
within eastern edge of region of
anomalously low heights
• Both TCs occur on
northeastern flank of
anomalous westerlies
• Anomalous westerlies are
relatively stronger and broader
in areal extent for large TCs
Analysis of Storm-Relative Composites
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 16/24
• Anomalously low heights and
winds intensify and expand in
area through day 2 particularly
for large TCs
Analysis of Storm-Relative Composites
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 17/24
• Peak magnitude and areal extent
of height and wind anomalies for
large TCs occurs on day 4 after
genesis
• Anomalously low heights may be
suggestive of more favorable
environment for convection and
growth of TC (e.g., increased
lower-tropospheric convergence,
moisture)
• Westerly anomalies may yield
deceleration of trade winds and
provide lower-tropospheric source
of cyclonic vorticity for TC growth
Cyclonic vorticity
Analysis of Storm-Relative Composites
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 17/24
• Anomalous winds to southwest
of average TC smaller in area
and may be associated with
cyclonic envelope TC
circulation rather than large-
scale feature
Analysis of Storm-Relative Composites
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 18/24
Average TC composites lose meaning by day 6 due to dissipation of large fraction of average
TC cases
• Dissipation of height and wind
anomalies by day 6 for large
TCs coincident with time at
which large TCs stop growing
according to Extended Best-
Track
• Presence of height and wind
anomalies at time of growth of
large TCs may suggest that
anomalies play a role in TC
expansion
Outline
• Background
– Factors influencing the life cycle of TC size
• Results: Role of environment in determining TC size
– Difference in life cycle of size between average and large TCs
– Storm-relative composite analysis of average and large TCs
• Summary and conclusions
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 19/24
Summary and Conclusions
Summary for Large TCs
• Wind field of large and average TCs are initially comparable in size
• Large TCs embedded within relatively larger and stronger region of anomalously
low heights and anomalous westerlies
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 20/24
Negative height anomalies Wind anomalies
TC
Summary for Average TCs
Negative height anomalies Wind anomalies
TC
Day of Genesis Day of Genesis
Summary and Conclusions
Summary for Large TCs
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 21/24
Negative height anomalies Wind anomalies
TC
Summary for Average TCs
Negative height anomalies Wind anomalies
TC
2 Days After Genesis2 Days After Genesis
• Anomalous low heights and anomalous westerlies grow in magnitude and areal
extent over time
Summary and Conclusions
Summary for Large TCs
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 22/24
Negative height anomalies Wind anomalies
TC
Summary for Average TCs
Negative height anomalies Wind anomalies
TC
4 Days After Genesis4 Days After Genesis
• Peak magnitude of anomalous low heights and anomalous westerlies occurs day 4
after genesis for both TC types
• Broad region of anomalously low heights may favor convection and growth of size
of large TCs
Summary and Conclusions
Summary for Large TCs
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 22/24
Negative height anomalies Wind anomalies
TC
Summary for Average TCs
Negative height anomalies Wind anomalies
TC
4 Days After Genesis4 Days After Genesis
• Westerly anomalies are suggestive of deceleration of trade winds and generation of
anomalous cyclonic vorticity for large TCs
• Average TCs associated with anomalous cyclonic envelope of winds rather than
extending across the basin like for large TCs
Cyclonic vorticity
Summary and Conclusions
Summary for Large TCs
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 23/24
Negative height anomalies Wind anomalies
Summary for Average TCs
Negative height anomalies Wind anomalies
4 Days After Genesis6 Days After Genesis
TCAverage TC composites lose meaning by day 6 due to dissipation of large fraction of average
TC cases
• Growth of large TC size slows substantially at day 6 after genesis coincident with
the dissipation of height and wind anomalies
• Strength and timing of height and wind anomalies relative to life cycle of large TC
size may be suggestive of role of anomalies in expansion of TC size
Cyclonic vorticity
TC
Questions Raised…
• What role, if any, do the anomalously low heights and anomalous westerlies play in
creating a conducive environment for large TCs?
• Which large-scale phenomena (e.g., convectively coupled equatorial waves) are
responsible for the anomalously low heights and westerly winds?
• How do the potential impacts of the large-scale environment on TC size fit with the
factors that were previously discussed in the background?
• Are there times in which these anomalously low heights and anomalous westerlies
favor the existence of multiple smaller TCs rather than one large TC?
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 24/24
Additional Slides
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 2/24
Additional Slides
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 2/24
Additional Slides
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 2/24
Additional Slides
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 2/24
• Upper-tropospheric warm
anomalies to west of TC
associated possibly induced by
enhanced convection
• Lower-tropospheric
environment anomalously
moist to the east of TC rather
than to the west
West East
West East
Analysis of Storm-Relative Composites
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 2/24
Next, we will examine the vertical structure of the environment averaged meridionally over the shaded region
for the large TC composites
Vertical Structure of the Environment for Large TCs
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 2/24
• Upper-tropospheric warm
anomalies to west of TC
associated possibly induced by
enhanced convection
West East
Summary and Conclusions
Summary for Large TCs
Background Results ConclusionsMotivation
Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 2/24
Negative height anomalies Wind anomalies
TC
Summary for Average TCs
Negative height anomalies Wind anomalies
TC
4 Days After Genesis4 Days After Genesis
• Anomalous low heights and anomalous westerlies continue growing magnitude and
areal extent through day 4 after genesis for both size categories
• Anomalous low heights to west of TC associated with upper-tropospheric warm
anomalies without any substantial moisture anomalies