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Long Range Lightning Nowcasting Applications for Tropical Cyclones

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Long Range Lightning Nowcasting Applications for Tropical Cyclones 19th International Lightning Detection Conference 24-25 April • Tucson, Arizona, USA 1st International Lightning Meteorology Conference 2006 26-27 April • Tucson, Arizona, USA LONG RANGE LIGHTNING NOWCASTING APPLICATIONS FOR TROPICAL CYCLONES Nicholas W.S. Demetriades1, John Molinari2, and Ronald L. Holle1 1Vaisala, Inc. Tucson, Arizona 2Department of Earth and Atmospheric Sciences The University at Albany/SUNY Albany, New York 1. INTRODUCTION (2004) found an intense lightning outbreak downshear from the center. "Downshear" The primary data sources for monitoring indicates the half of the circulation opposite the tropical cyclones beyond 625 km are (1) infrared vertical shear vector; if vertical shear is from the and visible satellite imagery from geostationary west, downshear represents the eastern half of satellites, (2) microwave and space-based radar the storm. Molinari et al. (2004) provided imagery from polar orbiting satellites and (3) evidence that this burst of lightning brought aircraft reconnaissance data. Geostationary about a downshear re-formation of the storm. satellite imagery is the only dataset that provides In addition, flash information in outer continuous monitoring of tropical cyclones far rainbands of tropical cyclones may provide from land. Unfortunately, such imagery often forecasters with a valuable diagnostic tool for does not provide detailed structural information identifying the most intense outer rainbands. in eyewalls and outer rainbands needed for For many areas that do not experience the inner nowcasting and forecasting tropical cyclones. core of a tropical cyclone, outer rainbands often The outer limit of cloud-to-ground (CG) flash contain the highest wind speeds and heaviest detection networks is 625 km from sensors in rainfall. the U.S. National Lightning Detection Network Since major questions remain about how (NLDN) and the Canadian Lightning Detection tropical cyclones form, lightning information is Network (CLDN). Prior studies found that CG potentially of significant value, both for prediction lightning information within several hundred km and understanding of storms. This paper will of the NLDN was a useful indicator of convective describe how the LLDN operates, show outbreaks in incipient and mature hurricanes, examples of spatial and temporal flash and may signal substantial deepening of the distributions in tropical cyclones, and investigate storms (Molinari et al. 1994; 1999; Lyons and the role of downshear convection in storm Keen 1994; Black and Hallett 1999; Cecil et al. development. 2002; Samsury and Orville 1994; Sugita and Matsui 2004; Demetriades and Holle 2005, 2. LONG RANGE LIGHTNING DETECTION 2006). However, most cases of hurricane NETWORK (LLDN) formation and early intensification in the North Atlantic and Pacific Oceans occur outside the NLDN and CLDN wideband sensors operate 625-km range of the NLDN. Vaisala is operating in the frequency range of about 0.5 to 400 kHz a VLF long range lightning detection network where return strokes in CG flashes radiate most (LLDN) over the North Atlantic and Pacific that strongly. The peak radiation from CG flashes detects flashes thousands of kilometers from comes near 10 kHz in the middle of the very low land and allows monitoring of tropical cyclones frequency (VLF) band. Signals in the VLF band over a much larger area than the NLDN and are trapped in the earth-ionosphere waveguide CLDN. and suffer less severe attenuation than higher The effect of vertical wind shear between frequency signals. Low frequency (LF) and VLF 850 and 200 hPa on the number of NLDN ground wave signals from CGs are attenuated flashes was investigated during the development strongly, and are almost imperceptible after a of Hurricane Danny (1997). Molinari et al. propagation distance of 500 to 1000 km. 1 However, VLF signals may be detected several storm. There is a much more defined pattern to thousand kilometers away after one or more the lightning activity in Hurricane Katrina (Fig. 2) reflections off the ground and the ionosphere. compared to its tropical storm stage (Fig. 1). Detection is best when both a lightning source The lightning pattern in category five and a sensor are on the night side of the earth, Hurricane Rita is similar to that in Katrina at that because of better ionospheric propagation intensity, except Rita had very little lightning in conditions at night. its outer rainbands (Fig. 3). The similar features The NLDN detects and processes lightning are a large eyewall lightning outbreak and signals at long distances because the standard minimal lightning activity in the central dense network sensors detect across a broad band overcast. However, these examples show a that includes all of the VLF range. Standard large variability in outer rainband lightning in NLDN sensors have been part of a long range strong hurricanes. network combining the CLDN, the Japanese Lightning Detection Network, the Meteo-France network, and the BLIDS, Benelux, and Central European networks. This combination detects CG flashes in sufficient numbers and with sufficient accuracy to identify small thunderstorm areas. The current study of tropical cyclones will use data in the North Atlantic where the nighttime detection efficiency exceeds 10% at the far reaches of the eastern Caribbean. The long range detection network is described in more detail in Murphy et al. (2006). 3. TROPICAL CYCLONE LIGHTNING NOWCASTING APPLICATIONS Tropical depressions and tropical storms are generally more prolific lightning producers than hurricanes. Lightning activity in these weaker systems does not show a preferential spatial Figure 1. CG lightning (red symbols) pattern. Figure 1 shows lightning in Tropical detected within Tropical Storm Katrina Storm Katrina when it was northeast of Cuba as between 1415 and 1445 UTC 24 August 2005. a minimal tropical storm with sustained winds of Infrared satellite image from 1445 UTC. 40 miles per hour (64 km per hour). Katrina was producing a large amount of lightning during this 30-minute time period in a number of convective clusters that covered most of the cloud mass of the storm. In hurricanes, lightning shows preferential spatial patterns. The eyewall usually contains a weak flash maximum. There is a well-defined minimum in flashes from 80 to 100 km outside the eyewall maximum due to stratiform rain processes that dominate most of the central dense overcast (Molinari et al. 1999). The outer rain bands typically contain a strong maximum in flash density. Figure 2 shows the lightning activity within Katrina after it strengthened to a category five hurricane on the Saffir-Simpson Scale. Lightning in Hurricane Katrina shows this pattern consisting of a large eyewall lightning outbreak, very little lightning in the central dense Figure 2. Same as Figure 1, between overcast, and a tremendous amount of lightning 1845 and 1915 UTC 28 August 2005. Infrared in the outer rainbands surrounding most of the satellite image from 1915 UTC. 2 Figure 5. Long range lightning data and infrared satellite image on 5 September 2003. Lightning is from 1253 to 1553 UTC. Yellow dots are flashes detected during the first two hours, and red are from the latest hour. Infrared Figure 3. Same as Figure 1, except for satellite image is from 1553 UTC. Hurricane Rita between 0615 and 0645 UTC 22 September 2005. Infrared satellite image toward the east of the center of Fabian (yellow) from 0645 UTC. to the north of Fabian’s center (red). In tropical depressions and storms, intense rainbands often 4. OUTER RAINBAND LIGHTNING contain the highest wind speeds and heaviest rainfall. In order to properly issue tropical Lightning activity can help identify the most depression and storm advisories and warnings, intense outer rainbands in a tropical cyclone. it is critical to be able to identify the location and These rainbands often contain flooding rains evolution of these features. and strong, gusty winds. Lightning can also show the evolution of growth and dissipation of 5. EYEWALL LIGHTNING OUTBREAKS outer rainbands as they rotate around the periphery of the storm. Figure 4 shows CG 5.1 Eyewall lightning patterns lightning detected within Hurricane Ivan as it moved toward the U.S. coast in the Gulf of The LLDN has detected numerous eyewall Mexico. Lightning activity delineates an intense lightning outbreaks in hurricanes over the outer rainband on the east side of Ivan. Atlantic and Eastern Pacific during the past Figure 5 shows the evolution of an outer several years. The larger outbreaks tend to rainband as it rotates counterclockwise around occur on relatively small time and space scales. the center of Hurricane Fabian on 5 September Lightning bursts in hurricane eyewalls 2003. Lightning indicated that the most intense sometimes rotate cyclonically around the center outer rainband rotated from a position located of circulation for some distance before they dissipate. However, eyewall lightning outbreaks that were studied in several hurricanes since 2002 show that these outbreaks tend to preferentially occur on one side of the hurricane track. Several hours may separate consecutive bursts of eyewall lightning or these bursts may occur continually for 24-48 hours. Figure 6 shows lightning detected along the track of Hurricane Frances on 3 September 2004. The track of Frances during this 24-hour period is shown by the yellow and red line. Three eyewall lightning outbreaks can be identified as Frances Figure 4. Same as Figure 1, except for moved toward the northwest. All three Hurricane Ivan between 1345 and 1415 UTC 15 outbreaks occurred on the northeast side of the September 2004. Infrared satellite image from storm track and are shown sequentially in 1415 UTC. shades of green, orange and red. 3 Impressive eyewall lightning signatures were change in structure and intensity. Lightning also produced by Hurricane Rita as it became a helps to identify intense convective cores with category five hurricane on 21 September 2005. larger updraft speeds embedded in the eyewalls At times, Rita produced significant eyewall of hurricanes. These lightning outbreaks may lightning that surrounded almost the entire be strongly, and perhaps uniquely, associated eyewall (Fig.
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