benjamin a. schenkel ( [email protected] ) , lance f. bosart, and daniel keyser

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

mailto:[email protected]

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


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


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


Large TC



(black circle)




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


Large TC



(black circle)




IR Imagery During Extratropical Transition for Small TC and Large TC

Small TC


Large TC



(black circle)



Charley has continued to decrease in size while Sandy has continued to rapidly expandWhat factors determine the life cycle of TC size?

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

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)




- 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)



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


• 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



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?



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?



In the interest of brevity, we will only attempt to answer questions #2 and #4 during this talk

Outline



• Background






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



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



Can the storm-relative composites help explain the differences in the life cycle of TC size?

Analysis of Storm-Relative Composites



• 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




• Anomalously low heights and

winds intensify and expand in

area through day 2 particularly

for large TCs




• 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




• Anomalous winds to southwest

of average TC smaller in area

and may be associated with

cyclonic envelope TC

circulation rather than large-

scale feature




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








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



Negative height anomalies Wind anomalies

TC

Summary for Average TCs Negative height anomalies Wind anomalies

TC

Day of Genesis Day of Genesis






TC


TC

2 Days After Genesis2 Days After Genesis

• Anomalous low heights and anomalous westerlies grow in magnitude and areal

extent over time






TC


TC


• 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






TC


TC


• 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








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?



Additional Slides



Additional Slides



• 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




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



• Upper-tropospheric warm

anomalies to west of TC

associated possibly induced by

enhanced convection

West East






TC


TC


• 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

benjamin a. schenkel ( [email protected] ) , lance f. bosart, and daniel keyser

Documents

tc size benjamin

large tc vmax

large tc size of charley

tc sizedifference

tc wind fields

sizesmall tc vmax

expandsmall tc vmax

large tcsmall tc vmax