Analysis of Cloud-to-Ground Lightning Within 16 Landfalling Hurricanes

1. Analysis of Cloud-to-Ground Lightning Within 16 Landfalling Hurricanes Danielle Nagele

2. Previous Literature • Squires and Businger 2008 • RI outbreaks – strike density increased (decreased) rapidly before (after) reaching max. strike density • Samsury and Orville 1994 • Hugo had only 33 flashes, Jerry had 691 flashes • Majority of flashes on right side

3. Previous Literature • Molinari et al. 1999 • Findings support 3 regions • eyewall • inner bands, 20–80 km outside the eyewall • outer bands, outside of the 100-km radius • Predictive value of eyewall flashes • Lightning outbreak in core of storm can indicate intensity changes • Lyons and Keen 1994 • Lightning within convective elements of outer bands during most stages of life cycle • Lightning bursts within 100-150 km of the center may indicate intensification

4. Hypothesis and Goals • Climatology of CG lightning within hurricanes during landfall period • Reveal patterns with regards to the number/location of flashes • Hypothesis • There will be bursts of CG lightning along the coast as the right, front quadrant moves across land

5. Greater updraft strength • Increase in surface convergence due to friction cause by surface roughness • Powell 1982, Powell 1991, and Powell 1996 looked at Hurricanes Fredric, Hugo, and Andrew respectively • Enhanced convergence to the right of the center, divergence to the left • Powell 1982 – ratio of wind speeds directly inland to immediately offshore from .74 to .84 • Results in more graupel-sized particles above the freezing level and more vigorous charge seperation • Enough to create small bursts in lightning during this time

6. Hurricanes

7. Methodology • Data • NLDN lightning data (Vaisala) • Updated in 1995, increasing detection efficiency to 80-90%, 2002/2003 to 90-95% • Reliable up to 400 km away from sensor • NEXRAD WSR-88D Level II radar data (NCDC) • HURDAT satellite data (NCDC) • Six hourly best track (NHC) • H*Wind track files (HRD) • Lightning, radar, satellite data viewed on ANGEL (IDL GUI created by Dr. Kyle Wiens)

8. Methodology • Rainbands start interacting with coast - six hours after landfall • Storm center within 300 km of the coast • Storms from 1997-2007 • Landfall as a hurricane along the Gulf or Florida coasts • Landfall defined as the time center of the eye passes over land • Overlay range rings centered around the center of the eye • Overlay approximate storm track • Break storms into three regions – inner core, inner rainband, outer rainbands • Break three regions up into four storm relative quadrants • Record flashes every ten min.

9. Schematic 4 R1 - InnerCore R2 - InnerRainband 3 1 R3 - Outer Rainband 2

11. Methodology • Bursts: • Increase by 50% from one ten minute period to the next • Avg. flash rate less than five, threshold 15 • Avg. flash rate more than five, threshold 20 • Analyzed each burst that occurred inland, within 20 km of the coast • Gridded radar, examined cross sections • Height of 35 dBZ line, max. dBZ, lightning

12. Bursts • Summary: • 252 bursts in all • 190 over water • 27 inland, beyond 20 km of the coast • 37 inland, within 20 km of the coast • 3 occurred within an eyewall • Q1-100, Q2-95, Q3-32, Q4-25 • Most bursts - Katrina, Ivan, Bret, Danny • Height of 35 dBZ line • In 8 bursts, greatest height 1 or 2 scans before burst • In 12 bursts, greatest height scan during burst • In 5 bursts, greatest height 1 or 2 scans after • In 2 bursts, no correlation seen

13. Example Burst (during)

15. Burst 

17. Example Burst (before)

19. Burst 

21. Percentage of Total • Irene, 3 stdev. above mean for R2 • Irene, 2 stdev. below mean for R3 • Frances, 3 stdev. above mean for Q3 • Wilma, 3 stdev. above mean for Q4

22. Before and After Landfall

23. Patterns • Rita, Katrina, Ivan, Georges1, Danny, Bret, Charley showed similar pattern • Steady decrease in R3 lightning before landfall, slight or large increase after

24. Rita, Irene, Humberto, Dennis all had a peak in R1 flashes during landfall • Wilma had a peak ~1 hr. before and Charley ~1 hr. after • Dennis (1200) and Humberto (0600) had peak in R1 flashes at min. press.

25. Conclusions • Bursts along coast seen in right, front quadrant • Isolated, within outer bands or eyewall • Most bursts correspond with an increase in the 35 dBZ line (scan before or during burst) • Multiple bursts from same rainband occurring in regular intervals • Bursts along coast predominantly in Q1 • Only 7 of 37 in other quadrants • Of all types of bursts, largest concentration seen in Q1 and Q2 • 195 in Q1 and Q2, 57 in Q3 and Q4

26. Conclusions • Most lightning concentrated in outer band, Q1 • 7/16 storms show similar trend of a gradual decrease in flashes before landfall • On avg., stronger storms had a higher % of pos. flashes, but lower % of inner core flashes • The 3 storms that made landfall on west coast of FL, had a higher than avg. % of flashes in Q4 • Weaker storms had a greater increase in flashes after landfall • Inner core lightning may be correlated with strengthening and/or landfall

27. References • Lyons, Walter A., and Cecil S. Keen. “Observations of Lightning In Convective Supercells within Tropical Storms and Hurricanes.” Monthly Weather Review 122 (1994): 1897-1916. • Molinari, et al. “Convective Structure of Hurricanes as Revealed by Lightning Locations.” Monthly Weather Review 127 (1999): 520-534. • Powell, Mark D. “The Transition of the Hurricane Fredric Boundary-Layer Wind Field from the Open Gulf of Mexico to Landfall.” Monthly Weather Review 110 (1982): 1912 – 1932. • Samsury, Christopher E., and Richard E. Orville. “Cloud-to-Ground Lightning in Tropical Cyclones: A Study of Hurricanes Hugo (1989) and Jerry (1989).” Monthly Weather Review 122 (1994): 1887-1896. • Squires, K., and S. Businger. “The Morphology of Eyewall Lightning Outbreaks in Two Cat. 5 Hurricanes.” Monthly Weather Review 135 (2008): 1706-1726.