EXAMINATION OF THE COASTAL TRANSITION ZONE IN

EXAMINATION OF THE COASTAL TRANSITION ZONE IN HURRICANE FRANCES (2004)

by BRIAN DANIEL HIRTH, M.S.

A DISSERTATION IN GEOSCIENCES Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY

Approved:

John L. Schroeder, Ph.D. Chairperson of the Committee

Christopher C. Weiss, Ph.D.

Douglas A. Smith, Ph.D.

Accepted

Peggy Gordon Miller Interim Dean of the Graduate School March, 2011

Copyright 2011, Brian Daniel Hirth

Texas Tech University, Brian D. Hirth, March 2011 ACKNOWLEDGEMENTS The completion of this research would not be possible without the support of multiple individuals. First and foremost I owe a debt of gratitude to my doctoral advisor, Dr. John Schroeder, for mentoring me through not only this research, but a wide variety of scientific endeavors during my graduate career. He has provided me the opportunity to contribute to countless field experiments while challenging me to become an independent scientist. The broad spectrum of knowledge and expertise shared by him has made me a well-rounded student of the Atmospheric Sciences and Wind Engineering. I would also like to thank Dr. Chris Weiss for serving on my doctoral committee and participating in the SMART radar deployment used for this study. Dr. Weiss has also provided me with valuable field research experience and radar expertise. Thank you to Dr. Doug Smith for serving on my committee and contributing to this research as well. Additional thanks are extended to Dr. Mike Biggerstaff and Gordon Carrie of Oklahoma University for their assistance collecting the SMART radar data during Hurricane Frances. Also, my gratitude is extended to Dr. Sylvie Lorsolo and Dr. Jeff Beck for their contribution to that SMART radar data collection effort. Thank you to Frank Merceret of the NASA Applied Meteorology Unit for acquiring and providing detailed site photos for the CCAFS/KSC towers. I would like to thank my parents, Dan and Diane Hirth, for instilling in me the work ethic and providing the encouragement and motivation needed to complete my graduate studies. Their love and support has played a vital role in the completion of this work. I am indebted to my great friend, roommate, and colleague of six years, Dr. Ian Giammanco, for sharing in the highs and lows of our graduate school endeavors.

Additionally I would like to thank Dave Kook, Pat Skinner, Frank

Lombardo and Tanya Brown who have been great friends and the source of stimulating conversation both in Lubbock and on the lonesome road for various projects.

Thank you to my girlfriend, Allison Morrison, for your motivation,

encouragement, and love through this final year of my research. Finally, thank you to all of the students and faculty I have worked with over my tenure at Texas Tech. You have taught me a great deal and made my experience enjoyable. ii

Texas Tech University, Brian D. Hirth, March 2011 TABLE OF CONTENTS ABSTRACT ....................................................................................................................... vi LIST OF TABLES ............................................................................................................ vii LIST OF FIGURES ......................................................................................................... viii CHAPTER I: INTRODUCTION ........................................................................................1 CHAPTER II: BACKGROUND AND HISTORICAL RESEARCH ................................5 2.1. General IBL Structure and Growth ...................................................................5 2.2. Hurricane Boundary Layer Offshore ..............................................................13 2.3. Hurricane Boundary Layer at the Coastal Interface .......................................15 2.4. Mesoscale and Convective Scale Influences on the Coastal IBL ...................23 2.5. Turbulence at the Coastal Interface ................................................................26 2.6. Oceanic Effects on IBL Development ............................................................28 2.7. Summary and Hypothesis ...............................................................................29 CHAPTER III: FIELD EXPERIMENT AND DATA SOURCES ...................................32 3.1. Hurricane Frances (2004) ...............................................................................32 3.2. Observational Assets .......................................................................................35 3.2.1. Shared Mobile Atmospheric Research and Teaching Radar Deployments ..............................................................................................35 3.2.2. Cape Canaveral Air Force Station/Kennedy Space Center Tower Network......................................................................................................45 3.2.3. Texas Tech University Portable Towers ..........................................47 3.2.4. NOAA/AOML HRD Research Aircraft Mission #43 ....................49 3.3. Surface Classification Datasets .......................................................................51 3.3.1. National Elevation Dataset ..............................................................51 iii

Texas Tech University, Brian D. Hirth, March 2011

3.3.2. NOAA Coastal Change Analysis Program Land Cover ..................51 CHAPTER IV: RADAR DATA PROCESSING .............................................................56 4.1. Dual-Doppler Pre-Processing .........................................................................56 4.1.1. Data Editing and Cartesian Gridding ...............................................56 4.1.2. Dual-Doppler Synthesis ...................................................................62 4.2. RHI Pre-Processing .........................................................................................65 4.2.1. Data Editing and Cartesian Gridding ...............................................65 CHAPTER V: ANALYSES OF THE COASTAL TRANSITION .................................67 5.1. Dual-Doppler Mean IBL Structure .................................................................67 5.1.1. Zone Analyses ..................................................................................67 5.1.2. Slab Analyses ...................................................................................80 5.2. Dual-Doppler Small-Scale Influences ............................................................92 5.3. RHI Mean IBL Structure ................................................................................97 5.4. RHI Small-Scale Influences ..........................................................................113 5.5. Surface Tower Analyses ...............................................................................116 5.5.1. Tower Data Validation ...................................................................116 5.5.2. Determination of Surface Exposure ...............................................118 5.5.3. Wind Speed Standardization ..........................................................122 5.5.4. Selected Tower Analyses ...............................................................123 5.5.5. Tower 313 Vertical Wind Speed Profiles ......................................131 CHAPTER VI: CONCLUSIONS AND RECOMMENDATIONS................................137 6.1. Summary .......................................................................................................137 6.2. Conclusions ...................................................................................................138 iv

Texas Tech University, Brian D. Hirth, March 2011 6.3. Recommendations for Future Work..............................................................141 LIST OF REFERENCES .................................................................................................145 APPENDIX A: NOAA C-CAP LAND COVER CLASSIFICATION SCHEME ..........150 APPENDIX B: CCAFS/KSC AND TTU TOWER TIME HISTORIES........................155

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Texas Tech University, Brian D. Hirth, March 2011 ABSTRACT Understanding the structure of the coastal internal boundary layer (IBL) during the landfall of a tropical cyclone has important ramifications on operational forecasting, structural design, and post-storm damage assessment.

Despite these

important issues, it is unclear how the structure of the IBL evolves at the coastline on micro- and meso-scales during a landfalling hurricane. Knowledge of the vertical kinematic structure within tropical cyclones over water has improved greatly through aircraft reconnaissance missions and the advent of GPS dropsondes and the Stepped Frequency Microwave Radiometers.

Unfortunately, reconnaissance and research

aircraft are limited to over-water missions resulting in a poor understanding of vertical kinematic structure near the coastal interface where changes in IBL structure are expected due to changes in coastal geometry and surface roughness. Additionally, IBL structure may evolve due to the passage of convective precipitation and associated downdrafts. A unique observational dataset was collected from the coastal transition zone in the onshore flow region of Hurricane Frances (2004) over Cape Canaveral, FL. Single- and dual-Doppler radar data collected by the Shared Mobile Atmospheric Research and Teaching radars provide the ability to discern horizontal and vertical mean IBL structure over a complex coastal interface while assessing the influence of a variable underlying surface and the passage of transient convective wind gusts. Additional wind speed data were collected by a meso-network of surface towers operated by the Cape Canaveral Air Force Station and Kennedy Space Center along with a portable surface tower deployed by Texas Tech University. Radar and tower data analyses reveal that IBL mean structure over the Cape Canaveral remains quite consistent during the landfall of Hurricane Frances, though IBL growth is suppressed when compared to empirical growth models. Additionally, transient convective gusts commonly perturb the mean structure at the top of the IBL, though the higher momentum associated with these gusts is typically not able to descend to the surface within an established IBL. vi

Texas Tech University, Brian D. Hirth, March 2011 LIST OF TABLES 3.1. Specifications for the SMART Radars. Adopted from Biggerstaff et al. (2005) .........................................................................................................................36 3.2. SR1 and SR2 data collection specifications during Hurricane Frances. .....................37 3.3. Deployment locations of both SMART-Radars during Hurricane Frances ................37 3.4. SMART-Radar scanning strategies utilized during the landfall of Hurricane Frances.......................................................................................................39 3.5. SMART-Radar data inventory during the landfall of Hurricane Frances...................40 3.6. Dual-Doppler periods considered for analyses. .........................................................41 3.7. Inventory of available CCAFS/KSC tower data collected during hurricane Frances. Green rows indicate towers that are used. Red rows indicate towers that were used but also possess wind instrumentation at multiple vertical levels. Grey rows indicate towers that collected relevant data, but were not used due to complex nearby terrain. ......................................................47 3.8. Deployment Information for the TTU portable tower at Space Coast Regional Airport, FL. .................................................................................................48 3.9. C-CAP land cover classifications and corresponding roughness length assignments. ...............................................................................................................55 4.1. Radius of influence values used at various gridpoint ranges from a given radar for Cartesian interpolation.................................................................................61 5.1. DD zone surface characterization statistics. ...............................................................68 5.2. Differences in dual-Doppler slab horizontal wind speed and direction at 80.8° and -80.55°. .....................................................................................................90 5.3. Empirical IBL height compared to gridded RHI “kinks” for the various roughness transitions across the Cape Canaveral region..........................................106 5.4. Roughness lengths of homogeneous surface types (Adopted from Wieringa 1993, Table VIII). .....................................................................................120 5.5. Qualitatively assigned segment roughness length (zo) values for the CCAFS/KSC and TTU towers for relevant wind directions. ...................................121 vii

Texas Tech University, Brian D. Hirth, March 2011 LIST OF FIGURES 2.1. Schematic diagram of internal boundary layer development and its modulation of an upstream wind profile downstream of a surface discontinuity. The location of the surface discontinuity is denoted by the vertical dashed grey line, the top of the internal boundary layer is denoted by the solid blue line, and the top of the fully adjusted layer is denoted by the solid red line. The profile “kink” is identified, marking the top of the internal boundary layer. ........................................................................6 2.2. Dependence of IBL growth (in terms of normalized IBL height) on (a) Richardson number and (b) surface roughness. Length scales are normalized by the quantity Δθ/γ. Adopted from Venkatram (1977). ........................7 2.3. Schematic representation of the IBL h(x) and inner equilibrium layer hs(x) downstream of a step change in roughness (zo), temperature (θo) and heat or moisture flux (Fo). Streamline displacement, δ, is also shown. Adopted from Garratt (1990).........................................................................8 2.4. Cross-section of terrain and instrumented tower locations for the Echols and Wagner (1972) High Island, TX experiment. Adopted from Echols and Wagner (1972). ....................................................................................................9 2.5. Wind profiles obtained at the beach tower showing the kink (horizontal tick mark) associated with the internal boundary layer. The kinks have been annotated by a red oval to highlight the general decreasing trend in height with increasing wind speed and increased stability (nighttime). Adopted from Echols and Wagner (1972). .................................................................9 2.6. Plot of internal boundary layer height (zi) with downwind fetch (x) for varying upwind roughness lengths (zo’) and downwind roughness lengths (zo). Adopted from Deaves (1981). .............................................................11 2.7. IBL profiles derived from Equation 2.1 using various c and bIBL values with ZOR values of (A) 0.001 m, (B) 0.03 m, and (C) 0.5 m.....................................12 2.8. Mean hurricane wind speed profiles for the eyewall and outer vortex regions, normalized by the 700 hPa (flight-level) wind speed. Adopted from Franklin et al. (2003). .......................................................................................14 2.9. Schematic summarizing the general change in the vertical wind profile and the typical low-level jet envelope with respect to radial distance for a tropical cyclone. Radial distance from the eye increases from left to right. Inflow layer depth is also denoted. Adopted from Giammanco (2010). .......................................................................................................................15 viii


2.10. Landfall composite wind speed (m s-1) analyses for the 10-meter level for Hurricane Alicia at 0730 UTC on 18 August 1983. Adopted from Powell (1987). ...........................................................................................................17 2.11. (A) Landfall surface wind field analysis from Powell and Houston (1996) showing coastal discontinuity when compared to (B) the landfall wind analysis from the simulation of Liu et al. (1997) which does not....................19 2.12. Time versus height plots of MIPS Doppler profiler parameters for the moments of (A) signal-to-noise ratio (SNR, dB), (B) total vertical motion (W, (m s-1), and (C) spectrum width (σv, m s-1). Adopted from Knupp et al. (2006). ..................................................................................................22 2.13. (A) Isotach analysis from 0553 UTC of the horizontal velocity component within the vertical plan that passes over the SMART-R and MIPS along the direction 265°-85°, roughly parallel to the wind direction at 400 m AGL. Contours in m s-1. (B) Vertical wind speed (m s-1) profiles for the cross-sections depicted in (A). (C) Same as (A), but for 1355 UTC. (D) Vertical wind speed (m s-1) profiles for the crosssections depicted in (C). Adopted from Knupp et al. (2006). ..................................23 2.14. (A) High-resolution image of Doppler velocity during Hurricane Fran (1996). Near the radar (left) at altitudes of ~100m AGL, peak and trough wind speed values are ~40 m s-1 and ~10 m s-1, respectively. Further from the radar (right) peak and trough wind speed values alternate from ~25 m s-1 to ~55 m s-1. (B) Schematic representation of observed shear- and wind-parallel boundary layer rolls. Highmomentum air (red) is brought to the surface in the downward legs of the rolls, which air slowed near the surface is brought aloft in the upward legs. Adopted from Wurman and Winslow (1998). ....................................25 2.15. Example of superposition of scales of motion in the HBL of Hurricane Frances (2004). (A) Vertical cross-section of residual radial velocity. The solid line indicates the height at which the data presented in (B) were extracted. (B) The thin line represents the individual residual velocity data at 350 m AGL, and the dashed line outlines the larger scales of motion superimposed on the signal. Adopted from Lorsolo et al. (2008). ..................................................................................................................26 2.16. Surface momentum exchange quantities as a function of the 10-m wind speed (U10). Vertical bars represent estimate ranges of 95% confidence. Quantities include (A) friction velocity (m s-1), (B) roughness length (zo) and (C) drag coefficient. Ref. 9 represents the curves of Charnock (1955) and Ref. 13 represents the curves of Large and Pond (1981). Adopted from Powell et al. (2003). ..........................................................................28 ix


3.1. NHC Best Track positions and intensity for the lifetime of Hurricane Frances between 25 August – 8 September 2004. ....................................................34 3.2. KMLB WSR-88D base reflectivity image of the landfall of Hurricane Frances from 5 September 2004 at 0432 UTC. The black dotted line indicates a five-minute extrapolated track from the Hurricane Research Division between 4 September 1800 UTC – 5 September 1600 UTC. ....................34 3.3. SR1 deployed at Merritt Island Airport, FL prior to the landfall of Hurricane Frances (Photo credit: Kevin Scharfenberg). ...........................................36 3.4. Deployment locations of SR1 and SR2 and resultant dual-Doppler lobes (dashed circles). Large thin circles identify the maximum range for each radar, and the thick line indicates the track of Frances with designated timestamps. The location of the KMLB WSR-88D radar is also shown for reference. .............................................................................................................37 3.5. KMLB WSR-88D base reflectivity (dBZ) images at (A) 1910 UTC on 4 September, (B) 1952 UTC on 4 September, (C) 2154 UTC on 4 September, (D) 2350 UTC on 4 September, (E) 0308 UTC on 5 September, (F) 0351 UTC on 5 September, (G) 0510 UTC on 5 September, and (H) 0552 UTC on 5 September. The magenta box outlines the Cape Canaveral region for reference. ....................................................42 3.6. Same as Figure 3.4, but for (A) 1400 UTC, (B) 1415 UTC, (C) 1442 UTC, and (D) 1503 UTC on 5 September. ...............................................................44 3.7. CCAFS/KSC single level towers used (green circles), multiple level towers used (red circles), towers dismissed due to terrain complexity (grey circles), location of TTU portable tower (red flag), and locations of SMART-Radars and resulting dual-Doppler lobes. ..................................................46 3.8. Aerial photo of the deployment location of the TTU portable tower with respect to SR2 at Space Coast Regional Airport, Titusville, FL (Image source: Google Earth). .............................................................................................48 3.9. Raw wind speed (m s-1; blue line) and direction (degrees; green dots) time histories from the TTU portable tower at Space Coast Regional Airport in Titusville, FL from 4 September 1200 UTC – 6 September 0000 UTC..................................................................................................................49

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Texas Tech University, Brian D. Hirth, March 2011 3.10. (A) NOAA 43 flight path through Hurricane Frances on 4 September from 1820 – 2350 UTC. The green (red) dot indicates the start (end) of the mission at 1820 (2350). The red line defines the HRD highresolution interpolated center track during the mission. (B) Flight-level extrapolated sea-level pressure versus storm-relative radius through the mission. The red line indicates the mean pressure profile, excluding data represented by the green line, corresponding the portion of the mission made over the Florida mainland in green in (A). ......................................................50 3.11. NED 1/3 arc-second (~10 m horizontal resolution) elevation (m) data for Cape Canaveral and the adjacent Florida mainland. .................................................53 3.12. (A) C-CAP land cover classification and (B) corresponding C-CAP derived roughness length (m) over Cape Canaveral and the adjacent Florida mainland. ......................................................................................................54 4.1. Aerial photographs of the deployment locations of (top) SR1 and (bottom) SR2 (Image source: Google Earth). ...........................................................57 4.2. Radial velocity (m s-1) sweeps at the 0.8° tilt from 5 September at 0300 UTC showing (A) raw data from SR1, (B) edited data from SR1, (C) raw data from SR2 and (3) edited data from SR2 .....................................................59 4.3. Sample REORDER input script used to convert data from SR1 to Cartesian coordinates. ...............................................................................................60 4.4. Plan view perspective of the Cartesian domain to be used for all DD analyses (bold green bounds). The locations of SR1 and SR2 and DD lobes are also shown. ................................................................................................61 4.5. Sample CEDRIC script used for DD synthesis..........................................................64 4.6. Minimum height (m) used for the DD analyses grid shown in Figure 4.4 based on 0.5° tilt from both SMART radars. The clear area in the middle of the domain denotes the data void baseline. ..............................................64 4.7. SR2 (A) raw and (B) gridded RHI radial velocity (m s-1) for 140623 UTC on 5 September. ........................................................................................................66 5.1. DD domain mean analyses zones. .............................................................................68 5.2. Contributing Zone 2 profiles for 0300 – 0315 UTC 5 September, with the plus-or-minus one standard deviation bound (bold cyan lines) and final mean (bold yellow line) shown. ................................................................................70

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Texas Tech University, Brian D. Hirth, March 2011 5.3. Normalized DD horizontal (A) wind speed (m s-1) and (B) wind direction (°) mean profiles for the eight DD analysis periods in Zone 1. The mean profile is denoted by the bold magenta line. .............................................................71 5.4. Same as Figure 5.3, but for Zone 2. ...........................................................................71 5.5. Same as Figure 5.3, but for Zone 3. ...........................................................................72 5.6. Same as Figure 5.3, but for Zone 4. ...........................................................................72 5.7. Same as Figure 5.3, but for Zone 5. ...........................................................................73 5.8. Same as Figure 5.3, but for Zone 6. ...........................................................................73 5.9. Same as Figure 5.3, but for Zone 7. ...........................................................................74 5.10. Normalized wind speed composites for all analyses zones and times. ....................75 5.11. Constant altitude normalized wind speed composites for all analyses zones and times. ........................................................................................................77 5.12. Same as Figure 5.10, but for wind direction. ...........................................................79 5.13. Same as Figure 5.11, but for wind direction. ...........................................................79 5.14. Example of the slab analyses domain for the 0300 – 0315 UTC period. The area bound in green represents the dual-Doppler analyses domain and the area bound in magenta represents the slab domain. .....................................81 5.15. Dual-Doppler vertical cross-sections of (A) horizontal wind speed (m s1 ) and (B) wind direction (°) for 1945 – 2000 UTC on 4 September. IBL profiles are provided for various roughness lengths beginning at the first encounter of land for onshore flow. (C) Qualitatively derived surface roughness length (m) and (D) elevation (m) along the cross section is also shown. An inset of the region and representative dual-Doppler slab is also provided. ........................................................................................................82 5.16. Same as Figure 5.15, but for 2145 – 2200 UTC on 4 September. ...........................83 5.17. Same as Figure 5.15, but for 2345 – 0000 UTC on 4 – 5 September. .....................84 5.18. Same as Figure 5.15, but for 0300 – 0315 UTC on 5 September. ...........................85 5.19. Same as Figure 5.15, but for 0500 – 0515 UTC on 5 September. ...........................86

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Texas Tech University, Brian D. Hirth, March 2011 5.20. Constant altitude horizontal slices of (A) horizontal wind speed (m s-1) and (B) horizontal wind direction (°) through the dual-Doppler slab generated for 1945 – 2000 UTC on 4 September. ....................................................88 5.21. Same as Figure 5.20, but for 2145 – 2200 UTC on 4 September. ...........................88 5.22. Same as Figure 5.20, but for 2345 – 0000 UTC on 4 – 5 September. .....................89 5.23. Same as Figure 5.20, but for 0300 – 0315 UTC on 5 September. ...........................89 5.24. Same as Figure 5.20, but for 0500-0515 UTC on 5 September. ..............................90 5.25. Single volume slab analyses of horizontal (A) wind speed (m s-1) at 2145 UTC, (B) wind direction at 2145 UTC, (C) wind speed at 2150 UTC, (D) wind direction at 2150 UTC, (E) wind speed at 2155 UTC, and (F) wind direction at 2155 UTC on 4 September. Slab domain is shown for reference. ...................................................................................................................93 5.26. Single volume slab analyses of SR2 reflectivity (dBZ) for (A) 2145 UTC, (B) 2150 UTC, and (C) 2155 UTC on 4 September. Slab domain is shown for reference ...............................................................................................94 5.27. Same as Figure 5.25, but for residual wind speed and direction. ............................95 5.28. Single volume slab analyses of horizontal (A) wind speed (m s-1) at 0300 UTC, (B) wind direction at 0300 UTC, (C) wind speed at 0305 UTC, (D) wind direction at 0305 UTC, (E) wind speed at 0310 UTC, and (F) wind direction at 0310 UTC on 5 September. Slab domain is shown for reference. ...................................................................................................................98 5.29. Same as Figure 5.26, but for residual wind speed and direction .............................99 5.30. Aerial photograph of SR2 and various reference distances along the 90° radial. ......................................................................................................................101 5.31. SR2 composite RHI gridded mean radial velocity (m s-1) for (A) RHI-1 and (B) RHI-2. IBL profiles, starting at x = 14 km, are presented for roughness length values of 0.01 m (solid black line), 0.03 m (solid green line), and 0.10 m (solid cyan line). The along-radial (C) elevation (m) and (D) C-CAP derived roughness length (m) are also provided for reference. SR2 is located at x = 0 km. ...................................................................102 xiii

Texas Tech University, Brian D. Hirth, March 2011 5.32. Mean RHI near-surface horizontal radial velocity slice represented as a percent difference from the x = 14 km value.......................................................................104 5.33. SR2 composite RHI gridded radial velocity profiles at 4, 8, 12, and 16 km range for (A) RHI-1 and (B) RHI-2. .................................................................104 5.34 IBL profiles derived from Equation 2.3 using the ZOR values described in Table 5.3 for the various roughness transitions across the Cape Canaveral region. The x-distance is 90° from SR2. The solid vertical bar represents x = 4 km from SR2. The boxes represent the observed gridded radial velocity kinks for each transition.....................................................106 5.35. SR2 composite RHI gridded maximum radial velocity (m s-1) for (A) RHI-1 and (B) RHI-2. .............................................................................................107 5.36. Same as Figure 5.35, but for radial velocity standard deviation (m s-1). ...............109 5.37. Same as Figure 5.35, but for gust factor. ...............................................................110 5.38. Same as Figure 5.35 but for turbulence intensity. .................................................110 5.39. Same as Figure 5.35, but for mean spectrum width (m s-1). ..................................112 5.40. Same as Figure 5.35, but for “pseudo” turbulence intensity derived from spectrum width and radial velocity. ........................................................................112 5.41. SR2 individual RHI radial velocity (m s-1) scans collected at (A) 135522 UTC, (B) 140405 UTC, (C) 140738 UTC, (D) 141047 UTC, (E) 141404 UTC, (F) 142009 UTC, (G) 142229 UTC, and (H) 142457 UTC on 5 September. The x-axis denotes x-distance from SR2 (km) and y-axis denotes height (m)...................................................................................................114 5.42. SR2 gridded RHI radial velocity residuals (m s-1) taken from (A) 135522 UTC, (B) 140405 UTC, (C) 140738 UTC, (D) 141047 UTC, (E) 141404 UTC, (F) 142009 UTC, (G) 142229 UTC, and (H) 142457 UTC on 5 September. The x-axis denotes x-distance from SR2 (km) and y-axis denotes height (m)...................................................................................................115 5.43. Comparison of tower 313 five-minute wind speed (top) and wind direction (bottom) data collected by the northeast (3131) and southeast (3132) pointed boom arms at the 16.5 m level. ......................................................117 5.44. TTU time histories of (A) raw and standardized wind speed (m s-1) and (B) raw wind direction and derived roughness length (m). ....................................124 xiv

Texas Tech University, Brian D. Hirth, March 2011 5.45. Same as Figure 5.44, but for tower 511. ................................................................125 5.46. Select surface towers used for analyses (Image source: Google Earth). ..............126 5.47. Raw 30-minute mean time histories of (A) wind speed (m s-1) and (B) wind direction (°) for 1200 UTC 4 September thru 1200 UTC 5 September. ..............................................................................................................127 5.48. Standardized 30-minute mean wind speed (m s-1) time histories for 1200 UTC 4 September thru 1200 UTC 5 September. ....................................................129 5.49. (A) One-second/30-minute gust factor time histories and (B) linear regression of the gust factor time histories in (A) for 1200 UTC 4 September thru 1200 UTC 5 September. Mean roughness length values over the 24-hour period for each tower are provided in the legend for reference. .................................................................................................................130 5.50. Distances of Tower 313 from the coast at varying angles from true north.............131 5.51. IBL profiles for zo = 0.25 m (bold blue line), 10% of the zo =0.25 m profile (bold red line), zo = 0.10 m (dashed black line), zo = 0.03 m (dashed green line), zo = 0.01 m, dashed cyan line. Tower 313 is shown with it various measuring heights for reference. .....................................................132 5.52. Tower 313 30-min mean vertical wind speed (m s-1) profiles (black line) for (A) 04/1200 UTC, (B) 04/1500 UTC, (C) 04/1800 UTC, (D) 05/2100 UTC, (E) 05/0000 UTC, (F) 05/0300 UTC, (G) 05/0600 UTC, (H) 05/0900 UTC, (I) 05/1200 UTC. The green line depicts the log-law profile using the 16.5 m mean wind speed and zo = 0.25 m. Horizontal solid red bars indicate the seven observation levels. ..............................................133 5.53. Tower 313 lower and upper profile derived roughness length (m). .......................136 5.54. Normalized composite vertical wind speed verses log height for tower 313 between 1200 UTC 4 September and 1200 UTC 5 September. Error bars at each level represent one standard deviation. Horizontal solid red bars indicate tower observation heights. .................................................................136 B.1. Time histories of (top) raw and standardized wind speeds (m s-1) and (bottom) raw wind direction and derived roughness lengths (m) for tower 001. ...............................................................................................................157 B.2. Same as B.1, but for tower 002 ...............................................................................158 B.3. Same as B.1, but for tower 003 ...............................................................................159 xv

Texas Tech University, Brian D. Hirth, March 2011 B.4. Same as B.1, but for tower 006 ...............................................................................160 B.5. Same as B.1, but for tower 019 ...............................................................................161 B.6. Same as B.1, but for tower 022 ...............................................................................162 B.7. Same as B.1, but for tower 108 ...............................................................................163 B.8. Same as B.1, but for tower 110 ...............................................................................164 B.9. Same as B.1, but for tower 211 ...............................................................................165 B.10. Same as B.1, but for tower 300 ..............................................................................166 B.11. Same as B.1, but for tower 303 ..............................................................................167 B.12. Same as B.1, but for tower 311 ..............................................................................168 B.13. Same as B.1, but for tower 313 ..............................................................................169 B.14. Same as B.1, but for tower 421 ..............................................................................170 B.15. Same as B.1, but for tower 511 ..............................................................................171 B.16. Same as B.1, but for tower 512 ..............................................................................172 B.17. Same as B.1, but for tower 513 ..............................................................................173 B.18. Same as B.1, but for tower 714 ..............................................................................174 B.19. Same as B.1, but for tower 1007 ............................................................................175 B.20. Same as B.1, but for tower 1605 ............................................................................176 B.21. Same as B.1, but for tower 1612 ............................................................................177 B.22. Same as B.1, but for tower 2008 ............................................................................178 B.23. Same as B.1, but for tower 2016 ............................................................................179 B.24. Same as B.1, but for tower 2202 ............................................................................180 B.25. Same as B.1, but for tower TTU (999) ...................................................................181

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CHAPTER I INTRODUCTION The damage potential from landfalling hurricanes in the United States continues to rise as the coastal population steadily increases. In the years between 1960 – 2008, the number of housing units built on the Atlantic coast has increased 98%, while an increase of 246% has occurred on the Gulf coast (Wilson and Fischetti 2010). A consequence of this steady growth is an upswing in damage generated by hurricane landfalls in these coastal regions. The Atlantic hurricane seasons of 2004 and 2005 accrued over $150 billion in insured damage (Pielke et al. 2008), while the 2008 hurricane season accounted for an additional $50 billion. These staggering numbers exemplify the necessity for increased education towards developing mitigatory measures to reduce the adverse effects of landfalling hurricanes.

Of

foremost importance is better understanding of the hurricane wind field distribution itself, both in the horizontal and vertical dimensions. Hurricane wind field structure becomes increasingly complex at the coastal interface as abrupt frictional changes at the surface are occurring when flow transitions from water to land, or vice versa. Unfortunately, pre-existing fixed operational surface observing systems generally fail very early in tropical cyclone landfall events and the current WSR-88D radar network is inherently limited in the wind speed and wind direction information it can provide. The immediate coastline becomes a vital region with respect to hurricane mitigation efforts because augmented man-made development, including high-rise structures, are located there. Extreme care must be taken when developing building codes for this complex coastal region, which in turn requires full understanding of the hurricane wind field affecting this zone. Because of this, increased coastal observations during landfall events are necessary and essential. Over the past decade, the scientific community has made great strides in understanding the vertical and horizontal structure of hurricane winds over water. Instrumented aircraft have collected valuable wind data using tools such as on-board Doppler radars, GPS dropsondes, and Stepped Frequency Microwave Radiometers. 1

Texas Tech University, Brian D. Hirth, March 2011 These instruments have allowed for the development of relationships between wind speed and direction at flight-level and the ocean surface (Hock and Franklin 1999, Franklin et al. 2003, Uhlhorn et al. 2007), and have helped identify prominent features within the hurricane boundary layer (HBL) including low-level wind maxima (Kepert 2006, Giammanco 2010). Additionally, these observations have shed light on how the ocean surface amalgamates at differing wind speeds, which in turn impacts the resultant friction generated by the ocean surface affecting the shape of the HBL wind speed profile (Powell et al. 2003, Donelan et al. 2004, Zachry 2009). However, as robust as these datasets and findings may be, these observations are generally taken well offshore any coastal regions, and therefore provide little description of what is occurring where societal impacts are the greatest. To date, a thorough study detailing mean horizontal and vertical HBL structure at the coastal interface has never been conducted. Several empirical and wind tunnel studies exist describing the modeled effect of wind flow over various discontinuities in surface roughness (Antonia and Luxton 1972, Venkatram 1977, Deaves 1981) and resultant internal boundary layer (IBL) development.

These

studies, however, were ideally limited to simplified surface roughness changes. These studies were also not geared towards extreme wind events such as hurricanes at landfall. Tower observations collected near the coastline during hurricane landfalls have provided some valuable insights as to the structure of turbulence near the coastal interface and have confirmed the existence of IBL development (Schroeder 1999, Schroeder et al. 2003, Howard 2004, Yu et al. 2008). These observations, though, are largely restricted in vertical coverage and represent sparsely-spaced point measurements. The need for a comprehensive observational dataset at the coastal interface during a hurricane landfall cannot be understated.

Data providing full

horizontal and vertical coverage are necessary to further scientific efforts towards mitigating adverse societal effects of landfalling hurricanes in the coastal transition region. The research presented herein will utilize observational data collected by two mobile research radars and several instrumented towers during a hurricane landfall to investigate: 2

Texas Tech University, Brian D. Hirth, March 2011 •

Mean IBL structure at the coastal interface and how this structure compares to previous work and theory;

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General variability of mean IBL structure at the coastal interface as a function of wind speed and radial distance from the storm center;

•

The effect of complex surface roughness on mean IBL structure;

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Small-scale influences and modulation of IBL structure as a result of transient (convective) features within the HBL flow.

Volumetric data collected by two Shared Mobile Atmospheric Research and Teaching (SMART) radars will be used to provide high-spatial and temporal resolution wind speed and direction data coverage over a large portion of Cape Canaveral, FL during the landfall of Hurricane Frances (2004).

The radar

observations will be supplemented with tower data collected by the Cape Canaveral Air Force Station and Kennedy Space Center (CCAFS/KSC) meteorological tower network and an additional portable research tower deployed by the Texas Tech University (TTU) Hurricane Research Team in the region. The primary analyses presented will consist of interrogation of dual-Doppler synthesized wind fields generated by both SMART radars spread over an 11 hour period. Additional analyses will utilize repetitive range-height indicator data collected by a single radar over a 70 minute period.

The supporting surface tower data will be used to investigate

horizontal wind variability across the region, and will provide some insights regarding the vertical wind speed structure in the lowest 150 m from a single tower located in close proximity to the coast. The process used to conduct this research is described in the chapters that follow. Chapter II provides a background of historical research related to this study and lays the groundwork for this work. This chapter also defines the hypothesis to be tested by this research. Chapter III discusses the field experiment employed and describes all data sources used in this study. Chapter IV describes the procedures necessary to prepare data for analyses while Chapter V contains a synopsis of the multiple analyses conducted. Chapter VI offers conclusions to the important findings

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Texas Tech University, Brian D. Hirth, March 2011 of this research, and also poses recommendations for continuing and advancing the results presented herein.

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Texas Tech University, Brian D. Hirth, March 2011 CHAPTER II BACKGROUND AND HISTORICAL RESEARCH This summary will discuss several previous modeling and surface-based observational studies investigating the internal boundary layer (IBL) and the structure of hurricane boundary layer (HBL) wind. Because a considerable number of historic IBL investigations exist, this review will focus on those providing relevant background to onshore flow at the coastal (smooth-to-rough) transition. Initially, a review of general IBL characteristics and research will be presented followed by a look at HBL structure both offshore and onshore, respectively. Previous studies investigating the existence of smaller-scale features within the HBL will also be discussed, including how these features may potentially modulate mean IBL structure. Observations of turbulence characteristics in the near-shore environment in landfalling tropical cyclones will be explored as well as a contribution and complication of the near-shore shoaling region and breaking waves on IBL development. Finally, hypotheses for this research will be presented.

2.1 General IBL Structure and Growth An IBL forms within a region of horizontal advection across a discontinuity in one or more surface properties. These discontinuities are generally defined as step changes in surface roughness, heat flux and/or temperature. While the rate of IBL growth (increase in depth with increase in downwind distance from a surface discontinuity) will vary depending on the surface property under investigation, historic research supports the notion that an IBL will exhibit continual growth until the entire depth of the boundary layer has fully adjusted to the new underlying surface.

The most common surface property related to IBL development is

roughness, and the IBL itself is generally characterized by kinematic changes with height and downwind distance.

Using these characterizations, the IBL can be

identified by finding ‘kinks’ in the vertical profile of wind speed (Antonia and Luxton 1972), denoting the intersection point of the upwind and newly-modified downwind 5

Texas Tech University, Brian D. Hirth, March 2011 profiles. A boundary layer vertical wind profile taken within a developing IBL consists of three layers. As seen in the general schematic for IBL growth in neutral stability presented in Figure 2.1, a shallow layer of the downwind fetch has fully adjusted to the new underlying roughness regime while the majority of depth consists of a transition layer where a gradual shift from the downstream to upstream logarithmic wind speed profile occurs with increasing height.

Above the IBL

(defined by the ‘kink’), the wind speed profile is unaffected, and is representative of the profile upwind of the roughness discontinuity.

Figure 2.1. Schematic diagram of internal boundary layer development and its modulation of an upstream wind profile downstream of a surface discontinuity. The location of the surface discontinuity is denoted by the vertical dashed grey line, the top of the internal boundary layer is denoted by the solid blue line, and the top of the fully adjusted layer is denoted by the solid red line. The profile “kink” is identified, marking the top of the internal boundary layer. This schematic is not to scale.

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Texas Techh University,, Brian D. Hirth, H March 2011 Initiall laboratory studies of thhe IBL weree confined too small scalees and fetchees w within the surrface layer and a generallyy focused onn discontinuiities in surfaace roughnesss inn neutral th hermal stabiility.

Laterr studies beegan to lookk at longerr fetches annd

inncorporated thermal effeects, noting stability s playys an importtant role in IB BL growth in i w weak kinemaatic conditioons (Garratt 1990). Thhis thermal effect was of particulaar siignificance for f industriaal interests near n coastal regions whhere the IBL L had a direcct im mpact on th he dispersionn of pollutaants. A connvective theermal IBL model m run by b V Venkatram (1 1977) isolateed and altereed specific boundary b layyer parameteers to monitoor thheir indepen ndent effect on o IBL grow wth. It was found that the t IBL grew w faster for a laarger Richarrdson numbber (increased thermal instability), and also for a largeer rooughness disscontinuity (Figure ( 2.2).. Increases in these parrameters werre also linkeed too increases in n near-surfacce turbulencce intensitiess.

Figure 2.2. Dependence D e of IBL groowth (in term ms of normaalized IBL height) h on (aa) R Richardson number n and (b) surface roughness. r Length scalles are norm malized by thhe quuantity Δθ/γγ. Adopted from f Venkattram (1977).

Their studies aim med to develop an empirical e reelationship between thhe m magnitude off the upstreaam flow, theermal effectts, and a surrface disconntinuity to thhe grrowth rate of o the IBL. Similar to Figure F 2.1, Garratt G (19900) developedd a schematiic foor IBL grow wth, noting a streamlinee displacemeent above thhe IBL and emphasizinng thhat at any giiven downw wind distancee from a surrface disconttinuity, ~90% % of the IBL w was 7


Figure 2.3. Schematic representation of the IBL h(x) and inner equilibrium layer hs(x) downstream of a step change in roughness (zo), temperature (θo) and heat or moisture flux (Fo). Streamline displacement, δ, is also shown. Adopted from Garratt (1990).

actually a transitional regime where flow was affected but not fully adjusted to the new underlying surface (Figure 2.3). Observational experiments provided “real-life” measurements where a collocated discontinuity of roughness and thermal stability existed. Among the first surface-based observational studies of IBL growth was conducted by Echols and Wagner (1972) near High Island, TX to investigate the effect of sea-breeze circulations on the transport of pollutants. The experiment utilized two, regularly instrumented (from 1 to 27 m) towers; the first located 90 m from the waterfront, to capture the IBL transition, and the second ~5 km inland, to measure the fully adjusted flow (Figure 2.4). As previously noted, the top of the IBL was defined as the height the modified vertical wind profile intersected that of the upwind logarithmic profile (Figure 2.5). The sea breeze wind speeds associated with this study were relatively weak (