Benjamin A. Schenkel Lance F. Bosart, and Daniel Keyser University at Albany, State University of New York 16 th Cyclone Workshop The role of the large-scale environment in determining North Atlantic tropical cyclone size Research Sponsored by NSF Grant AGS

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 kt Mean 34-kt wind radius km Large TC VMAX - 40 kt Mean 34-kt wind radius km While the size of the convective envelopes for each TC are different, size of the TC wind fields are initially comparable BackgroundResultsConclusionsMotivation 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 kt Mean 34-kt wind radius km Large TC VMAX kt Mean 34-kt wind radius km Mean 34-kt wind radius (black circle) BackgroundResultsConclusionsMotivation 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 kt Mean 34-kt wind radius - 97 km Large TC VMAX - 80 kt Mean 34-kt wind radius km Mean 34-kt wind radius (black circle) BackgroundResultsConclusionsMotivation 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 kt Mean 34-kt wind radius - 97 km Large TC VMAX - 80 kt Mean 34-kt wind radius km Mean 34-kt wind radius (black circle) BackgroundResultsConclusionsMotivation 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 expand

Outline BackgroundResultsConclusionsMotivation 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 BackgroundResultsConclusionsMotivation 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) BackgroundResultsConclusionsMotivation 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) BackgroundResultsConclusionsMotivation 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 BackgroundResultsConclusionsMotivation 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? BackgroundResultsConclusionsMotivation 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? BackgroundResultsConclusionsMotivation Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 11/24

Outline BackgroundResultsConclusionsMotivation 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 BackgroundResultsConclusionsMotivation 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 large-scale response of environment to TC passage similar to TC Yuri 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 BackgroundResultsConclusionsMotivation Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 14/24

Analysis of Storm-Relative Composites large-scale response of environment to TC passage similar to TC Yuri BackgroundResultsConclusionsMotivation 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 large-scale response of environment to TC passage similar to TC Yuri BackgroundResultsConclusionsMotivation 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 large-scale response of environment to TC passage similar to TC Yuri BackgroundResultsConclusionsMotivation 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 large-scale response of environment to TC passage similar to TC Yuri BackgroundResultsConclusionsMotivation 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 large-scale response of environment to TC passage similar to TC Yuri BackgroundResultsConclusionsMotivation 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 BackgroundResultsConclusionsMotivation 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 BackgroundResultsConclusionsMotivation 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

Summary and Conclusions Summary for Large TCs BackgroundResultsConclusionsMotivation 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 Genesis Anomalous low heights and anomalous westerlies grow in magnitude and areal extent over time

Summary and Conclusions Summary for Large TCs BackgroundResultsConclusionsMotivation 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 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 BackgroundResultsConclusionsMotivation 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 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 BackgroundResultsConclusionsMotivation 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 Genesis 6 Days After Genesis TC Average 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? BackgroundResultsConclusionsMotivation Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 24/24

Additional Slides large-scale response of environment to TC passage similar to TC Yuri BackgroundResultsConclusionsMotivation Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 2/24

Additional Slides large-scale response of environment to TC passage similar to TC Yuri BackgroundResultsConclusionsMotivation Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 2/24

Additional Slides large-scale response of environment to TC passage similar to TC Yuri BackgroundResultsConclusionsMotivation Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 2/24

Additional Slides large-scale response of environment to TC passage similar to TC Yuri BackgroundResultsConclusionsMotivation 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 large-scale response of environment to TC passage similar to TC Yuri BackgroundResultsConclusionsMotivation Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 2/24

Vertical Structure of the Environment for Large TCs large-scale response of environment to TC passage similar to TC Yuri BackgroundResultsConclusionsMotivation 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 BackgroundResultsConclusionsMotivation 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 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