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Benjamin A. Schenkel ( bschenkel@albany.edu ) , Lance F. Bosart, and Daniel Keyser

The role of the large-scale environment in determining North Atlantic tropical cyclone size. Benjamin A. Schenkel ( bschenkel@albany.edu ) , Lance F. Bosart, and Daniel Keyser University at Albany, State University of New York 16 th Cyclone Workshop.

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Benjamin A. Schenkel ( bschenkel@albany.edu ) , Lance F. Bosart, and Daniel Keyser

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  1. The role of the large-scale environment in determining North Atlantic tropical cyclone size Benjamin A. Schenkel (bschenkel@albany.edu), Lance F. Bosart, and Daniel Keyser University at Albany, State University of New York 16th Cyclone Workshop Research Sponsored by NSF Grant AGS-0935830

  2. Motivation • Background • Results • Conclusions Motivation: Variability of TC Size IR Imagery Near the Time of Genesis for Small TC and Large TC Large TC Small TC Mean 34-kt wind radius (black circle) VMAX - 40 kt Mean 34-kt wind radius - 133 km VMAX - 40 kt Mean 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 Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 2/24

  3. Motivation • Background • Results • Conclusions Motivation: Variability of TC Size IR Imagery at Maximum Intensity for Small TC and Large TC Large TC Small TC Mean 34-kt wind radius (black circle) VMAX - 125 kt Mean 34-kt wind radius - 111 km VMAX - 100 kt Mean 34-kt wind radius - 310 km Size of Charley has decreased slightly while Sandy has tripled in size Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 3/24

  4. Motivation • Background • Results • Conclusions Motivation: Variability of TC Size IR Imagery During Extratropical Transition for Small TC and Large TC Large TC Small TC Mean 34-kt wind radius (black circle) VMAX - 60 kt Mean 34-kt wind radius - 97 km VMAX - 80 kt Mean 34-kt wind radius - 769 km Charley has continued to decrease in size while Sandy has continued to rapidly expand Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 4/24

  5. Motivation • Background • Results • Conclusions Motivation: Variability of TC Size IR Imagery During Extratropical Transition for Small TC and Large TC Large TC Small TC Mean 34-kt wind radius (black circle) VMAX - 60 kt Mean 34-kt wind radius - 97 km VMAX - 80 kt Mean 34-kt wind radius - 769 km What factors determine the life cycle of TC size? Charley has continued to decrease in size while Sandy has continued to rapidly expand Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 5/24

  6. Motivation • Background • Results • Conclusions 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 Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 6/24

  7. Motivation • Background • Results • Conclusions 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 Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 6/24

  8. Motivation • Background • Results • 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) Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 7/24

  9. Motivation • Background • Results • Conclusions Potential Factors Influencing the Life Cycle of TC Size • Larger TCs may be expected for increasing SSTs, decreasing tropopause temperatures, and decreasing TC latitude (Chavasand 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) Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 8/24

  10. Motivation • Background • Results • Conclusions 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 • 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 Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 9/24

  11. Motivation • Background • Results • Conclusions 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? Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 10/24

  12. Motivation • Background • Results • Conclusions 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 Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 11/24

  13. Motivation • Background • Results • Conclusions 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 Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 12/24

  14. Motivation • Background • Results • 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 Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 13/24

  15. Motivation • Background • Results • Conclusions 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 large-scale response of environment to TC passage similar to TC Yuri Can the storm-relative composites help explain the differences in the life cycle of TC size? Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 14/24

  16. Motivation • Background • Results • Conclusions 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 large-scale response of environment to TC passage similar to TC Yuri Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 15/24

  17. Motivation • Background • Results • Conclusions Analysis of Storm-Relative Composites • Anomalously low heights and winds intensify and expand in area through day 2 particularly for large TCs large-scale response of environment to TC passage similar to TC Yuri Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 16/24

  18. Motivation • Background • Results • Conclusions Analysis of Storm-Relative Composites • 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 large-scale response of environment to TC passage similar to TC Yuri Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 17/24

  19. Motivation • Background • Results • Conclusions Analysis of Storm-Relative Composites • Anomalous winds to southwest of average TC smaller in area and may be associated with cyclonic envelope TC circulation rather than large-scale feature large-scale response of environment to TC passage similar to TC Yuri Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 17/24

  20. Motivation • Background • Results • Conclusions Analysis of Storm-Relative Composites • 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 Average TC composites lose meaning by day 6 due to dissipation of large fraction of average TC cases large-scale response of environment to TC passage similar to TC Yuri Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 18/24

  21. Motivation • Background • Results • Conclusions 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 Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 19/24

  22. Motivation • Background • Results • Conclusions Summary and Conclusions Summary for Large TCs Summary for Average TCs Negative height anomalies Negative height anomalies Wind anomalies Wind anomalies TC TC Day of Genesis Day of Genesis • 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 Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 20/24

  23. Motivation • Background • Results • Conclusions Summary and Conclusions Summary for Large TCs Summary for Average TCs Negative height anomalies Negative height anomalies Wind anomalies Wind anomalies TC TC 2 Days After Genesis 2 Days After Genesis • Anomalous low heights and anomalous westerlies grow in magnitude and areal extent over time Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 21/24

  24. Motivation • Background • Results • Conclusions Summary and Conclusions Summary for Large TCs Summary for Average TCs Negative height anomalies Negative height anomalies Wind anomalies Wind anomalies TC TC 4 Days After Genesis 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 Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 22/24

  25. Motivation • Background • Results • Conclusions Summary and Conclusions Summary for Large TCs Summary for Average TCs Negative height anomalies Negative height anomalies Wind anomalies Wind anomalies Cyclonic vorticity TC TC 4 Days After Genesis 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 Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 22/24

  26. Motivation • Background • Results • Conclusions Summary and Conclusions Summary for Large TCs Summary for Average TCs Average TC composites lose meaning by day 6 due to dissipation of large fraction of average TC cases Negative height anomalies Negative height anomalies Wind anomalies Wind anomalies TC Cyclonic vorticity TC 6 Days After Genesis 4 Days After Genesis • 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 Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 23/24

  27. Motivation • Background • Results • Conclusions 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? Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 24/24

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

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

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

  31. Motivation • Background • Results • Conclusions 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 • East • West large-scale response of environment to TC passage similar to TC Yuri • East • West Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 2/24

  32. Motivation • Background • Results • Conclusions Analysis of Storm-Relative Composites large-scale response of environment to TC passage similar to TC Yuri Next, we will examine the vertical structure of the environment averaged meridionally over the shaded region for the large TC composites Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 2/24

  33. Motivation • Background • Results • Conclusions Vertical Structure of the Environment for Large TCs • Upper-tropospheric warm anomalies to west of TC associated possibly induced by enhanced convection • East • West large-scale response of environment to TC passage similar to TC Yuri Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 2/24

  34. Motivation • Background • Results • Conclusions Summary and Conclusions Summary for Large TCs Summary for Average TCs Negative height anomalies Negative height anomalies Wind anomalies Wind anomalies TC TC 4 Days After Genesis 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 Role of environment in determining TC size Benjamin A. Schenkel University at Albany, SUNY 2/24

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