Lightning and Tropical Tornadogenesis

Transcription

1 Lightning and Tropical Tornadogenesis Zachary Hargrove Atmospheric Sciences The University of North Carolina at Asheville One University Heights Asheville, North Carolina USA Faculty Advisor: Dr. Christopher Hennon Abstract Proceedings of the Big South Undergraduate Research Symposium (BigSURS) 2012 Winthrop University, Rock Hill SC April 13-14, 2012 Tropical cyclones can produce significant numbers of tornadoes as they make landfall. Unfortunately, tornadogenesis (formation of tornadoes) in tropical cyclones is extremely difficult to predict. The velocity radar signatures of these spin- ups are often brief and many times the tornado has touched down and lifted by the time a warning is issued. The relationship between lightning concentration and tornadogenesis in tropical supercells is investigated. Since lightning is usually tempered in tropical systems, concentrated areas in individual cells could give forecasters a localized region to focus on in the warning process. Tornadoes spawned by Tropical Storm Lee (2011), using a new lightning data set called the Earth Networks Total Lightning Network (ENTLN), were analyzed. No conclusive overall relationships were discovered between the two variables, though two of the cases exhibited an apparent relationship. Keywords: Tornadogenesis, Lightning, Tropical Cyclones, Hurricanes 1. Introduction It is widely documented that land falling tropical cyclones often spawn tornadoes 1,2,3,4. In previous work, McCaul 3 observed that 59% of all hurricanes that made landfall over the U.S. produced at least one tornado, with some occurring in the outer rain bands well before landfall (based on landfalling hurricanes over the period of 1948 through 1986). These tornadoes are often difficult to detect on radar, which makes it challenging to issue warnings for them 5. Tornadoes are often observed in areas of intense convection 6. Lightning is also typically observed with intense convection 7. In fact, past work has discovered a relationship between lightning and classic supercell tornado formation 8. However, lightning is often less prevalent in tropical cyclones when compared with continental deep convection 9. One possible explanation is that the updrafts in the individual tropical storm cells are not as intense as classic supercells. Also, tropical cyclones form in tropical air masses where the freezing level is typically higher. Because there is less ice availability, charge separation may not be as likely. The goal of this research is to determine the relationship between lightning activity and tornadogenesis in a landfalling tropical cyclone. The results presented here could be useful in alerting forecasters to higher probabilities of tornado formation. If a relationship between concentrated lightning activity and tropical tornadogenesis were detected, warning lead-time for these brief spin-ups could possibly increase. Tropical Storm Lee (2011) was selected as a case study for this project. The broad circulation was classified as a tropical depression on September 1 st and meandered in the warm Gulf of Mexico waters with weak steering currents aloft 10. Early in its development, Lee was nearly stationary before eventually making landfall in Louisiana on September 4 th as a tropical storm with 45 mph sustained winds 11. McCaul 3 defined any tropical tornado outbreak producing at least 24 tornadoes as severe. During its life, Lee spawned 30 confirmed tornadoes in the continental United States. Later in the evening of the 4 th, Lee was downgraded to a tropical depression 12, and sometime during the morning on the 5 th, the storm made the transition to an extra-tropical low-pressure system 13.

2 2. Methodology Lightning data for Tropical Storm Lee were obtained from the Earth Networks Total Lightning Network (ENTLN) for September 4-5, 2011 as the cyclone was making landfall and progressing inland. The ENTLN 14 dataset was developed especially for lightning research within tropical cyclones; it provided the first-ever continuous lightning track of a major hurricane (Hurricane Irene 2011). Developed in 2009, the project uses 400 sensors in the continental United States and 100 more sensors in the Atlantic Basin. The sensors capture the complete waveform for every flash and are wideband. These specifications help in detecting intra-cloud flashes as well as cloud to ground strikes. Radar data were obtained from the National Climatic Data Center (NCDC) for two hours before and two hours after each tornado touchdown. Five individual tornadic cells were analyzed as well as two non-tornadic cases. For the two non-tornadic cases, a four-hour time frame was chosen in which the cells appeared to be the most intense. Lightning data were retrieved within ten kilometers and five minutes of a given latitude and longitude at a given time. The time was based on the time stamp of the radar scan and the latitude and longitude coordinates were based on the apparent center of the convective cell. Please refer to figure 1 for a radar image of one of the tornadic cases at the approximate time of tornadogenesis. All analyzed cases took place on September 4 th, as the timing of Lee s evolution from tropical cyclone to extratropical cyclone on the 5 th is somewhat uncertain. Hourly flash rates for each cell were analyzed. The times of tornadogenesis were estimated from the Storm Prediction Center (SPC) reports and radar images. 3. Individual Cases 3.1. tornadic cases 1 and 2 Case 1 was a cell that produced a tornado in Baldwin County, AL. Figure 1-1 shows the lightning vs. time data for this case. The tornado touched down around 0825 UTC on September 4 th coincident with a brief spike of lightning flashes. One interesting observation was that there was a dip in activity around the time of genesis before spiking again. Case 2 (Figure 1-2) produced two tornadoes. One was spawned just northwest of the Gulfport, MS airport and one formed in the town of Saucier, MS. The Gulfport airport tornado was spawned during a noticeable spike of lightning activity, but the Saucier tornado seemed to form as the flashes were subsiding. The same circulation, however, could have produced these two tornadoes and it could be argued that genesis should only be attributed to the first tornado. The airport tornado was spawned at around 0542 UTC on September the 4 th and the Saucier tornado was spawned shortly after at approximately 0600 UTC. Large differences in flash rates for these two storms were observed. Case 2 created strikes per hour while case 1 exhibited a flash rate of strikes per hour. In fact, case 2 exhibited the highest hourly flash rates of any of the studied cells, while case 1 produced the 4 th highest hourly flash rates observed. These two cases showed the most noticeable relationships between tornadogenesis and lightning concentration. The storms also produced tornadoes shortly after they made landfall and both spawned tornadoes in the early morning hours tornadic cases 3, 4, and 5 Cases 3, 4, and 5 showed little, if any relationship between tornadogenesis and lightning concentration. Lightning vs. time data are shown in Figure 1-3 to Figure 1-5. Hourly flash rates were meager in each cell, exhibiting 157, , and 75 strikes per hour respectively. Figure 1 shows that there was little lightning activity during the approximate times of tornadogenesis for any of these three cases. It is interesting to note that cases 3 and 5 show spikes in lightning activity later in their life cycle, well after the tornadoes had been spawned. For case 4, however, we only see a spike in flash activity well before the cell made landfall. All three of these storms produced tornadoes shortly after they made landfall, either very early in the morning or late at night.

3 Figure 1. A graphical representation of lightning strikes vs. time for each tornadic case. Dark bold lines represent estimated landfall of the individual cell and the shaded ovals represent the estimated times of tornadogenesis.

4 3.3. non-tornadic cases The study did not find any conclusive differences between the non-tornadic cells and the tornadic ones. Figure 2 shows the lightning vs. time data for the non-tornadic cells. Case 6 showed the second largest hourly flash rate of any cell analyzed (541.5 per hour) while case 7 exhibited a meager 130 strikes per hour. Lightning activity in case 7 seemed to increase sharply shortly after its second landfall (the cell made a landfall over some barrier islands before making a second landfall on Louisiana s main land), while lightning activity in case 6 seemed to decrease and almost cease after it made landfall. After case 7 was on land for a long period of time, lightning spiked again. Therefore, no definitive conclusions can be made about individual cell landfall and lightning relationship. Figure 2. A graphical representation of lightning strikes vs. time for each non-tornadic case. Dark bold lines represent estimated landfall of the individual cell. Note that case 7 made two landfalls. 4. Discussion The investigation did not discover any definitive relationships between tornadogenesis and lightning. Cases 1 and 2 suggest some relationship between the two variables, but none of the other cases showed much, if any relationship. Hourly flash rates (shown in figure 3) did not suggest any discernable patterns between tornadic and non-tornadic cells. The study did support some conclusions from previous work. Gentry 2 found that the majority of tropical tornadoes were spawned in the right front quadrant of the cyclone and that most of these tornadoes formed shortly after their parent cell made landfall. Each of the tornadic cases presented here meet these criteria. One interesting observation, however, was that most of the tornadoes produced during Lee s landfall were spawned in the late night or early morning hours. Gentry had found that most tropical tornadoes were spawned during the afternoon hours, from noon to 6pm. This research does not necessarily refute these findings (it is a small sample size), but it is interesting that many of Tropical Storm Lee s tornadoes were formed in the late night and early morning hours. Gentry s work examined the climatology of many different tropical cyclones over numerous years, while this study only sampled convective cells on two individual days of the life of one tropical storm. In other work, McCaul et al. 4 analyzed the tornado swarm produced by Tropical Storm Beryl in Beryl produced 37 confirmed tornadoes around the southeast. Interestingly, the tornadic cell during Beryl s landfall that produced the highest flash rate only generated 310 total flashes during a 5.5-hour period. This study presented a

5 tornadic case that produced approximately strikes per hour (case 2). Comparing the flash rates from McCaul s selected cases and this study s selected cases, we see that Lee was a much more prolific lightning producer than Beryl. However, it is important to note that Beryl produced more and stronger tornadoes. One hypothesis about the lack of relationship could be related to the vertical motion characteristics in tropical supercells. In other work, Perez et al. 8 show that strong deep layer updrafts play a large role in lightning/tornado coupling when it comes to classic Great Plains supercells with violent tornadoes. Tropical supercells typically do not have updrafts as intense in the deep layer, and therefore, may not exhibit the same relationship 15. The lack of ice in tropical cyclones may also have something to do with the general lack of charge separation and lightning. 5. Conclusions Figure 3. Hourly flash rates for each case. This study s purpose was to determine if there was any relationship between lightning and tornadogenesis in tropical cyclones. If a relationship between these two variables were found, concentrated lightning clusters could provide a valuable tool for forecasters during a landfalling event, as many of these tornadoes are a challenge to locate on radar. Lightning trends and flash rates were examined when attempting to determine these relationships in Tropical Storm Lee. The Baldwin County and Gulfport airport cases (cases 1 and 2) were the two cells that most closely show a relationship between lightning and tropical tornadogenesis. Based on the lightning data shown in figure 1, there is an apparent relationship between the lightning flashes and tornado development. However, the other three tornadic cells examined are lacking any relationship whatsoever. For the non-tornadic cells, there was no discernible difference in the lightning distribution between them and the tornadic cases examined. Future research could assess more storms and more cases to glean other possible patterns. This study was not even able to inspect every tornado generated by Tropical Storm Lee. Future studies could examine other individual cyclones to examine whether similar results are observed. If other studies were to find a relationship, then the research could be expanded to multiple tropical cyclones throughout history. Other patterns could also be explored with regard to positive vs. negative lightning strokes in tropical supercells. The study of lightning with respect to tropical tornadogenesis is still a relatively untouched subject and these possible future studies could be valuable to the understanding of any relationships between the two variables. 6. Acknowledgments The author would like to express his appreciation to Steve Prinzivalli and the Earth Networks Total Lightning Network (ENTLN), without whose data the study would not have been possible. The author would also like to

6 acknowledge the assistance given from Dr. Christopher Hennon of the University of North Carolina at Asheville. Dr. Christopher Godfrey of the University of North Carolina at Asheville provided Fortran support. Thanks to the National Weather Service offices in Mobile, AL and Slidell, LA as well as the Storm Prediction Center (SPC) in Norman, OK for storm report and damage survey information. Additional appreciation also goes to the National Climatic Data Center (NCDC) for providing the radar data archives. 7. References 1. Gentry, R. C., 1983: Genesis of tornadoes associated with hurricanes. Mon. Wea. Rev., 111, McCaul, E. W., Jr., 1991: Buoyancy and shear characteristics of hurricane-tornado environments. Mon. Wea. Rev., 119, McCaul, E. W., Jr., D. E. Buechler, S. J. Goodman, and M. Cammarata, 2004: Doppler radar and lightning network observations of a severe outbreak of tropical cyclone tornadoes. Mon. Wea. Rev., 132, Novlan, D. J., and W. M. Gray, 1974: Hurricane-spawned tornadoes. Mon. Wea. Rev., 102, Schneider, D., and S. Sharp, 2007: Radar Signatures of Tropical Cyclone Tornadoes in Central North Carolina. Wea. Forecasting, 22, Doswell, C. A. III, S. J. Weiss, and R. H. Johns, 1993: Tornado forecasting a review. Proc., Tornado Symp. III. C. Church, Ed., Amer. Geophys. Union, (in press). 7. Zipser, Edward J., Kurt R. Lutz, 1994: The Vertical Profile of Radar Reflectivity of Convective Cells: A Strong Indicator of Storm Intensity and Lightning Probability?. Mon. Wea. Rev., 122, Perez, Antony H., Louis J. Wicker, Richard E. Orville, 1997: Characteristics of Cloud-to-Ground Lightning Associated with Violent Tornadoes. Wea. Forecasting, 12, Samsury, Christopher E., Richard E. Orville, 1994: Cloud-to-Ground Lightning in Tropical Cyclones: A Study of Hurricanes Hugo (1989) and Jerry (1989). Mon. Wea. Rev., 122, Brown, D., L. Avila, cited 2011: Tropical Depression Thirteen Special Discussion Number 1. [Available online at Stewart, S., cited 2011: Tropical Storm Lee Discussion Number 12. [Available online at Brennan, M., cited 2011: Tropical Depression Lee Discussion Number 14. [Available online at Kong, K., cited 2011: Remnants of Lee Advisory Number 15. [Available online at emnants.] 14. Prinzivalli, Seve, Earth Networks, 2011, personal communication. 15. Suzuki, O., N. Hiroshi, O. Hisao, and N. Hiroshi, 2000: Tornado-producing mini supercells associated with Typhoon Mon. Wea. Rev., 128,