Hurricane Eye Formation

1. Hurricane Eye Formation Jonathan Vigh NCAR Earth Systems Laboratory & Advanced Study Program Research Review 10:00 AM 27 May 2010 FL1-2033 NCAR is sponsored by the National Science Foundation

2. The Hurricane Problem • Hurricane Katrina • at least 1836 deaths • $81 billion in damage • a “mega-disaster” • Just the worst part of a very bad stretch: • 3 major U.S. landfalls in 2004 • Charley – 35 dead, $14 billion • Frances – 42 dead, $9 billion • Ivan- 92 dead, $18 billion • Jeanne – 3000 dead, $7 billion • 4 major U.S. landfalls in 2005 • Dennis – 54 dead, $2.2 billion • Katrina – 1836 dead, $80 billion • Rita – 62 dead, $10 billion • Stan – 1000-2000 dead • Wilma – 23 dead, $26 billion 6500+ deaths, ~$160 billion

3. Katrina became the cause célèbre to warn the world of the imminent perils of anthropogenic global warming Sparked acrimonious debate within the subfields of tropical cyclone and climate researchers • A bevy of funding unleashed: • Hurricane Forecast Improvement Project • Goals: reduce forecast errors by 50% over all lead times • 10 year project • $17 million funding in 2009 • $25 million funding in 2010 • HWRF, data assimilation, verification, observations

4. Katrina was NOT the worst case 60 hours before landfall, track near New Orleans identified with high confidence intensity forecasts were at least Category 3 with even greater potential • Evacuations were largely successful • roughly 90% of the city evacuated before storm • about 100,000 chose to stay or did not have the means to evacuate • mortality rates of those who stayed were on the order of 1% • death toll on Mississippi Coast was greater than in Camille

5. Worst case scenarios • A storm which experiences a sudden and unexpected change in track • A storm which rapidly intensifies from a TS or Cat 1 to a Cat 3+ the day before landfall • A storm which undergoes an unexpected increase in size (e.g. Katrina 2005) • Evacuation times for certain vulnerable areas are 36-48+ hours • New Orleans Houston Tampa • Florida Keys New York City • Evacuation routes cutoff well before storm • Evacuation incomplete in surge-prone areas

6. Track surprises • reduced through steady improvement in track forecasting • overwarning (warned area 3X area of hurricane force winds) • storm coming at oblique angle (e.g. Charley 2004) still troublesome • Intensity surprises • Average 48-h hour intensity error is 14 kt (2008) • Probability of detection of the Rapid Intensification Index ranged from 15-59% (Kaplan et al 2010) • BUT False Alarm Rate was 71-85%! • Size surprises • Inadequate size information to even verify against • Possibility for mass casualties (>5000 dead) is not 0%

7. Malkus (1958a) – intensity limited to moderate tropical storm intensity until the storm forms an eye Mundell (1990) found that 87% of all rapid intensifications commenced when the central pressure was between 987 and 962 hPa - this is the range of pressures when the eye appeared on satellite and radar imagery Kaplan et al (2010) found that ~50% of all rapid intensification events began when the storm was at tropical storm intensity (35-60 kt)

8. Eye and eyewall integral

9. Jorgensen 1984b

10. Intensity Intensification Strong link found between a storm’s intensification rate and the timing of various stages of eye formation

11. Intensity Change Stratified by Eye Success • Little lasting intensity change for eyes which form and dissipate • Rapid intensity changes in most cases in which the eye persists • If failures are removed, storms intensify most rapidly right near the time of ‘uninhibited’ eye formation

12. Radius of Maximum Winds • Best Track rmfound to have large high bias! • ~ half of storms undergo large contraction in the 24-h before eye formation • Eye formation appears to halt the contraction ofrm

13. Role of Convective Morphology Heymsfield et al (2004)

14. ASP Research Goals • Explore eye-intensification link in greater detail • Timing and control of warm core development • Fundamental question: • Is eye formation a stochastic process brought on by convection? • Or is it a natural attractor of the dynamical system sometimes inhibited by unfavorable environment? • Investigate mechanisms of eyewall formation • Role of boundary layer • Role of convective morphology

15. Strategies • Observations • Release structure and intensity data set to community • “better” radius of maximum wind -> size change • Extend and upgrade the Willoughby-Rahn flight level data set • inertial stability and temperature tendency -> wind profiles • Identify precursors to eye formation • Modeling • Analytical and numerical approach (intermediate complexity) • Advanced Hurricane WRF • Sensitivity study of environmental influences • Eyewall formation processes • Diagnosis of eyes formed in “real” modeled storms • Theory • better understand causes of subsidence • develop improved analytic framework

17. First eye formation Flight level winds reduced to surface equivalents Minimum central pressure Radius of maximum wind, eye radius Max temperature and DP in eye, outside temp –> 700 hPa equivalent DP temperature depression, baroclinity Land mask for BT

18. What is the radius of maximum winds?? Best Track RMW found to be biased 30-90% above the lower bound of flight level RMW

19. 1-day window where things can happen quickly – seems quicker than Dvorak model Banding noted in 43% of all storms