Hydrometeor Habits and Their Relation to the Electrification of Two Tornadic Storms as Revealed by a Polarimetric Doppler Radar

Maribel Martinez Academic Affiliation: Graduate Student, Texas Tech University

SOARS® Summer of 2003

Science Research Mentor: Alexander Ryzhkov (NSSL) Scientific Writing and Communication Mentor: Thomas Windham (UCAR)

ABSTRACT

The relationship between lightning and storm severity has been a topic of much interest to the meteorological community. Implementation of detailed radar and lightning detection networks have enabled researchers to further investigate the relationship between the electrification and microphysics of severe storms. On May 8 and 9, 2003 the research community was able to utilize two such instruments when supercells entered the coverage area of the Norman, Oklahoma (KOUN) WSR-88D radar and the Oklahoma Lightning Mapping Array (LMA). The KOUN radar, in operation as part of a proof-of-concept of an operational polarimetric WSR- 88D, allows researchers to distinguish between areas of rain and hail. The LMA measures the time-of-arrival of VHF radiation produced by individual lightning discharges and provides a more detailed look at lightning evolution. The research focused on the combination of data from these instruments. Temporal height sequences, radiation rates, and a combination of horizontal and vertical radar images were produced to determine the variations in lightning and hydrometeors before and during tornado touchdowns. Analysis of the May 8 and May 9, 2003 tornadic supercells showed that VHF radiation occurred in the frozen part of the storm and the majority of the VHF radiation occurred at the height of the updraft’s vertical extension. Although definitive results on the relationship between lightning and tornadoes can not yet be determined due to the lack of continuous data, the tools needed to further investigate the relationship are available for further research.

This work was done under the auspices of the Significant Opportunities in Atmospheric Research and Science (SOARS®) program of the University Corporation for Atmospheric Research, with funding from the National Science Foundation, the U.S. Department of Energy, the National Oceanic and Atmospheric Administration, and Goddard Space Flight Center, NASA. SOARS is a registered trademark of the University Corporation for Atmospheric Research.

SOARS®2003, Maribel Martinez, 1 INTRODUCTION The Doppler radar was introduced in the early 1950s. Since then, scientists have been able to observe and track thunderstorms in hopes of warning citizens in the storms’ paths. During the 1990s the National Weather Service’s Weather Surveillance 1988 Doppler Radar (WSR- 88Ds) network took shape in the United States. This network of 158 radars is widely used in forecasting and analyzing meteorological features (National Weather Service, 2003). The National Severe Storms Laboratory (NSSL) upgraded its Norman, Oklahoma (KOUN) WSR-88D radar in the spring of 2002 to include polarimetric capabilities as part of a proof-of-concept of an operational polarimetric WSR-88D. KOUN represents a significant advance in that it is the first in a future national network of polarimetric WSR-88D radars. Unlike current operational weather radars which only transmit in the horizontal direction, the KOUN radar simultaneously transmits both horizontal and vertical signals (Schuur et al., 2001). This advantage over conventional Doppler radar makes it capable of measuring not only reflectivity (Z), but the polarimetric variables of differential reflectivity (ZDR), specific differential phase (KDP), and cross-correlation coefficient (HV). These combination of variables allow researchers to distinguish between areas of ice, graupel, rain, and hail in clouds (Vivekanandan et al., 1999). Research done with data obtained by the KOUN polarimetric radar will be important to future operational meteorologists, especially in the area of severe weather forecasting (Schuur and Chandrasekar, 2003). The capability to classify the different hydrometeors (liquid or frozen water particles) will not only be beneficial to forecasters when they issue weather warnings but also in investigating the role of hydrometeors in electrification processes. Increasing evidence suggests that collisions between cloud ice and riming graupel/hail are what cause storm electrification strong enough to produce lightning (MacGorman and Rust, 1998). The amount of charge generated by a storm would thus vary in its microphysics and kinematics and affect the frequency and type of lightning. It has been hypothesized that large hail and tornadoes are preceded by an onset of in-cloud (IC) lightning due to large updraft surges (e.g., Lhermitte and Krehbiel 1979; Dye et al. 1999; MacGorman et al., 1989). However, datasets to test this hypothesis have been limited due to the lack of polarimetric radars and IC lightning detection networks. In December 2002, NSSL began operating the Oklahoma Lightning Mapping Array (LMA), which along with KOUN radar datasets will allow researchers to investigate hydrometeor habits and their relation to the electrification process of tornadic storms (MacGorman, 2003). This paper documents the evolution of electrification by using polarimetric variables to determine hydrometeor habits and lightning events detected by the LMA that occurred before and during the May 8 and May 9, 2003 Oklahoma City, OK tornado touchdowns. The following questions are addressed:  What are the hydrometeor habits at different elevation and azimuthal ranges?  Where do the majority of the VHF (Very High Frequency) radiation events occur relative to the different hydrometeors?  What are the hydrometeor habits and VHF radiation behavior before and during tornadic activity? Instrumentation and methodology used to complete the project are discussed. Observations and a discussion of each case are presented, followed by a summary and concluding remarks.

SOARS®2003, Maribel Martinez, 2 INSTRUMENTATION Norman, Oklahoma (KOUN) WSR-88D Polarimetric Radar The KOUN WSR-88D is currently under evaluation as part of the Joint Polarization Experiment (JPOLE) operational demonstration. Unlike most research polarimetric radars that employ an alternating horizontal and vertical scheme, KOUN transmits a simultaneous horizontal and vertical scheme (Schuur, 2001). Comparing the horizontal and vertical power returns reveal information on the size, shape, and ice density of cloud and precipitation particles. KOUN measures Z, ZDR, KDP, and HV. A combination of Z, the reflected power return, and ZDR, the ratio of the reflected horizontal and vertical power returns, is used to determine areas of hail and rain. Hail tends to tumble when it falls and thus appears more spherical to the radar. Areas of hail are indicated by larger values of Z (>55 dBZ) and lower values of ZDR (~0 dB). Rain, however, tends to become more oblate as it falls. Areas of rain are found in lower Z (<55) and higher ZDR (~1) (Doviak and Zrnic, 1993). Cross-correlation coefficient is defined as the correlation between the reflected horizontal and vertical power returns. It identifies areas where mixtures are present. Specific differential phase is the comparison of the returned phase difference between the horizontal and vertical pulses. It is a good indicator of particle orientation (Schuur, 2003). The three variables of Z, ZDR, and HV are used for classification identifications. The classification scheme includes Ground Clutter (AP), biological scatterers (birds or insects) (BS), big drops (BD), light rain (LR), moderate rain (MR), heavy rain (HR), and rain/hail (HA). Data was collected using a Volume Coverage Pattern (VCP) 11 as seen below in Figure 1. This pattern, commonly used during precipitation and severe weather events, consists of 14 elevation angles ranging from 0.5 to 19.5 degrees (Wood and Brown, 1999). A radar sequence (a collection of elevation scans) takes about five minutes to complete. Plan Position Indicator (PPI) radar images reveal the horizontal structure of the storm at a give elevation angle. Relative Height Indicator (RHI) images give a vertical representation at a specified azimuth angle. A third type of image that can be produced from the radar sequence data is the Constant Altitude Plan Position Indicator (CAPPI) images. CAPPI images are horizontal slices taken through the data at a constant height.

Figure 1: VCP-11 Pattern (Woods and Brown, 1999).

SOARS®2003, Maribel Martinez, 3 Oklahoma Lightning Mapping Array (LMA) The Oklahoma LMA was installed in October of 2002 and successfully began mapping in December of 2002. Unlike the National Lightning Detection Network (NLDN) which only detects cloud-to-ground (CG) lightning, the LMA is able to detect both IC and CG lightning. This three-dimensional mapping allows researchers to not only investigate how lightning relates to severe weather, but the possibility of assimilating lightning data into weather forecast models as well (MacGorman, 2003). The network operates much like other LMA systems developed by the New Mexico Institute of Mining and Technology. Ten stations located across central Oklahoma detect VHF radiation generated by lightning channels. These pulses, measured within less than a millionth of a second, are detected in the 6 Mhz bandwidth of television channel 3 centered at 63 MHz. The data is then communicated to the central analyzer located in Norman, OK. The peak signal times are recorded within a 50 ns resolution using a 20 MHz digitizer phase locked to the one pulse per second output of the GPS receiver. For an active storm, the LMA detects about 500-1000 radiation impulses per second (Rison et al., 1999). Lightning is able to be mapped out in three-dimensions to a range of 75 km and two- dimensions to a range of 200 km (MacGorman, 2003). Figure 2 below shows the setup for the Oklahoma LMA. The green ring shows the three-dimensional capability of the lightning mapping network.

Figure 2: KOUN WSR-88D radar 100 km range ring (red) and the Oklahoma LMA 75 km range ring (green) in the state of Oklahoma (Schuur and Chnadrasekar, 2002).

DATA ANALYSIS METHODS The majority of analysis procedures were done with programs written for use with the Interactive Data Language (IDL) software. Radar PPI, RHI, and CAPPI images were produced for the times when LMA data was available. Impulse data used for the study was restricted with x and y maximum and minimum limits to insure analysis of only LMA activity that occurred within the storm.

SOARS®2003, Maribel Martinez, 4 Overall electrification observations were made by producing impulse rates with time, five minute height distributions, and detailed height distributions over time. Impulse rates were computed by counting the number of VHF radiation that occurred every minute. With time, this reveals the variation of lightning production. Impulse rates coupled with preliminary severe weather reports give an idea of how electrically active a storm was prior, leading to, and after it produced severe weather. Five minute height distributions show where the majority of impulses were concentrated in a five minute window. Histograms using a bin size of 1000 were calculated to determine the height distributions. This allowed impulses to be counted every km from zero to 15 km. Detailed height distributions were also created to get a better picture of the electrification processes. Histograms using a bin size of 500 were calculated for every minute of data. The data was contoured in IDL with a contour interval of 100. Hydrometeor and electrification observations were made by determining where the majority of VHF radiation points were located during a specific radar scan sequence. CAPPI radar displays and LMA data images were able to be produced and combined. LMA data used for these comparisons were restricted to plus or minus one-half km to the CAPPI image.

OBSERVATIONS AND DISCUSSIONS May 8, 2003 A tornado hit the Moore, OK community on May 8, 2003. It injured more than 180 people and damaged between 500-1500 homes. The public had a 21-minute tornado warning lead time, which is well above the average of 15 minutes (National Weather Service Forecast Office Norman, OK, 2003). The time series for May 8, 2003 is seen in Figure 3. LMA data was available for analysis from 2150-2200 UTC and 2210-2220 UTC. The radar data analysis includes the sequence from 2153-2158.

2150-2200 UTC This time period was twenty minutes before a tornado was reported on the ground. The storm exhibited tornado signatures threatening enough for forecasters to issue a tornado warning just a minute prior to this time period. In Figure 3 the radiation rates varied from 20000-36000 with a sharp decrease in events beginning at 2153. It is at this time that the rotation signature increased and a tornado became imminent according to weather office reports (National Weather Service Forecast Office Norman, OK, 2003). Two minutes later rates picked up again. The time of this increase was coincident with a report of 0.88 and 0.75 inch hail. At 2158 the rates decreased yet again. The time height distributions, Figure 4, show that the majority of the VHF radiation events occurred at altitudes from 7-9 km. From 2155 to 2157, events decreased at these mid-altitudes but did increase at higher and lower altitudes. This time period coincided with the report of hail. Radar images from 2153-2158 give an idea of what is occurring at these different altitudes. The PPI 1.5 degree elevation scan (Figure 5) shows a large area of 55 dBZ at 2153 UTC. The classification algorithm depicts a small hail/rain core region with a larger area of heavy and moderate rain. Also seen in the figure is the presence of a strong inflow region, depicted by the existence of biological scatters in the southern part of the storm. The majority of the lightning located at the lower levels of about 1 km, are located to the east of the hail/rain region seen in Figure 6. An interesting observation is that the VHF radiation seems to stay away from the region to the west of the rain/hail region. Looking at vertical and horizontal cross sections, radiation activity occurs in the frozen part of the storm and not in the rain regions. Both

SOARS®2003, Maribel Martinez, 5 Figure 7 and 8 illustrate this point. Note the concentration of events in the lower values of ZDR. At 6 km ZDR values greater than 2 reveal areas of supercooled water. No VHF radiation occurs in these areas but they do encompass the core area. At 8 km, VHF radiation are located in areas of low ZDR and KDP (<0.1). There also appears to be a lightning free region at this elevation.

Figure 3: VHF radiation rates for May 8, 2003

Figure 4: VHF Radiation Height Distributions from 2150-2200 UTC.

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Figure 5: May 8, 2003 PPI 1.5 degree radar scan.

Figure 6: May 8, 2003 1 km CAPPI image with VHF radiation data overlaid.

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Figure 7: May 8, 2003 6 km CAPPI image with VHF radiation data overlaid.

Figure 8: May 8, 2003 8 km CAPPI image with VHF radiation data overlaid.

SOARS®2003, Maribel Martinez, 8 2210-2220 UTC No radar data was available for analysis during the time period from 2210 to 2220 UTC. A tornado was reported on the ground from 2210 to 2242, and struck the community of Moore, OK at 2215. Radiation rates in Figure 3, show that the number of events are less than those that occurred from 2150-2200. During this time period VHF events varied from 18000-25000. A contour of the VHF radiation events (Figure 9) show that the radiation events also occurred at lower altitudes of 6-8 km. The figure also shows that at a time period from 2216-2218, there was a decrease in the number of VHF radiation events in the mid-levels but an increase at higher altitudes.

Figure 9: VHF radiation event height distribution for 2210-2220 UTC.

May 10, 2003 On this day the Oklahoma community gets pounded yet again by a series of tornadoes. Below in Figure 10 is a timeline of events for May 10, 2003 (National Weather Service Forecast Office in Norman, OK, 2003). LMA data were available for analysis from 0200-0210 UTC, 0230-0235, and 0300-0310 UTC, depicted by the blue boxes in Figure 10. The radar data analysis includes the sequences from 0204-0209, 0230-0235, 0305-0310. Tornadoes occurred in two of the time periods, the first at 0235 and the second at 0303. A tornado warning was issued during the first period at 0209.

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Figure 10: May 10, 2003 evolution of events.

0200-0210 UTC The radiation rates from 0200-0210 (Figure 11) reveal gradually increase of radiation rates. The number of events range from 27000-34000. A tornado warning was issued at 0209. The contour of radiation events, Figure 12, show that the majority of events during this time period occurred at an altitude of 10 km, higher altitudes than the previous day. There is a decrease in events from 0200-0204 at the altitudes of 8-9 km. The 1.5 degree PPI radar image shows a large area of 55 dBZ coinciding with a low value of ZDR. The hail classification algorithm shows this area as a large hail/rain region. Horizontal and vertical sections of radar images show that the supercell is increasing in intensity. In fact, the RHI image (Figure 15) through the core of the storm shows that the updraft draft region (larger values of Z) extend up to 10 km. The freezing level is at about 3 km (shown by the ZDR plot in Figure 15). At the altitude of 10 km is where the majority of VHF radiation are occurring. So it seems to be that the most VHF radiation occur at the altitude of the vertical extent of the updraft. At 10 km, Figure 14, the concentrations of event coincide in areas of low ZDR, low KDP, and in the convective region of the storm. A linear feature exists and crosses the reflectivity contours but stays south of a region of extreme negative KDP (<-0.2).

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Figure 11: May 10, 2003 VHF radiation events from 0200-0210.

Figure 12: VHF radiation height distributions for May 10, 2003 from 0200-0210.

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Figure 13: May 10, 2003 PPI 1.5 degree radar scan.

Figure 14: May 10, 2003 10 km CAPPI image with VHF radiation data overlaid.

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Figure 15: May 10, 2003 RHI radar image.

0230-0235 UTC Twenty minutes later, the number of VHF radiation events increased to 41000 (Figure 16). However, from 0234 to 0235 there was a sharp decrease in events to 15000. This is perhaps the most interesting feature. A continuous data set is needed to see if this is consistent. The contours (Figure 17) show that although the number of events have increased, the majority are now located at lower altitudes of 6-7 km unlike previously where they were located at 10-11 km. Also evident is the lack of events from 0232-0234 at an altitude of 9 km. The PPI 1.5 degree image (Figure 18) shows the hail/rain core region decreased from that seen from 0204-0209. A region of 55 dBZ is still present. The majority of the VHF events occurred at 6 km (Figure 19). At this level the events encompass the core (higher Z region). Concentrations exist in the lower values of ZDR and once again not in areas of supercooled water. They were also not in areas of KDP greater than 0.3. An RHI section through the core region (Figure 20) shows a bounded weak echo region (BWER). The strong inflow and updraft of the storm produced this notch of weak reflectivity intruding stronger reflectivities. The updraft extended to 9 km, lower than the previous time-period analyzed. This may account for the decrease in altitude of the majority of VHF radiation events.

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Figure 16: VHF radiation rates from 0230-0235.

Figure 17: May 10, 2003 VHF radiation height distribution for 0230-0235 UTC.

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Figure 18: May 10, 2003 PPI 1.5 degree radar image.

Figure 19: May 10, 2003 6 km CAPPI image with VHF radiation data overlaid.

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Figure 20: May 10, 2003 core RHI of supercell.

0300-0310 UTC VHF radiation events were at the most intense stage at this time period. The supercell had produced two reported tornadoes and put out another during this time period. Figure 21 shows that events ranged from 50000-60000. The tornado was reported on the ground at 0303. The majority of events were located at an altitude of 9-10 km before and even during the tornado (Figure 22). Beginning at about 0306, the majority of events appeared at lower altitudes from 7- 8 km. The radar data was analyzed during this time. The 1.5 degree PPI image still depicted a hail/rain region with an extensive area of heavy rain. What is most interesting about this time period than from the previous two is what appears to be going on at the higher levels. From 4 to 12 km, concentrations of VHF radiation are visible along the convective region in lower values of ZDR and KDP. Again no radiation exists in the supercooled region. A linear feature exists once again, in reaching higher altitudes, a V feature and then a sort of W feature appear. These follow the convective regions as well. At 10 km, Figure 24, the VHF radiation split into two regions. The region where they do not occur are in areas of very negative KDP. This is seen also in Figures 25 and 26 at altitudes of 11 and 12 km. These very negative regions of KDP are due to a presence of an electric field, as described by Ryzhkov et al. (1999). Although it would be expected that VHF radiation would occur in these areas, personnal communication with Don MacGorman suggested that these regions are drawing charge from the lower levels thus there is more charge. At lower altitudes these regions are filled.

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Figure 21: Radiation rates for May 10, 2003 from 0300-0310 UTC.

Figure 22: VHF radiation height distributions for May 8, 2003 from 0300-0310 UTC.

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Figure 23: May 10, 2003 1.5 degree radar scan.

Figure 24: May 10, 2003 10 km CAPPI radar image.

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Figure 25: May 10, 2003 11 km CAPPI radar image.

Figure 26: May 10, 2003 12 km CAPPI radar image.

SOARS®2003, Maribel Martinez, 19 SUMMARY AND CONCLUDING REMARKS A combination of radar and VHF radiation data reveal new information about the hydrometeor classifications and thunderstorm electrical activities. Analysis of the May 8 and May 9, 2003 tornadic supercells shows that: 1. VHF radiation occurs in the frozen part of the storm. 2. The majority of the VHF radiation occur at the height of the vertical extent of the updraft. Definitive results on the relationship between lightning and tornadoes cannot yet be determined due to the lack of continuous lightning data. A continuous look at the lightning evolution is needed to better investigate this relationship. The combination of detailed radar and lightning networks will give a better understanding of the microphysics and electrification of tornadic storms. Flash rates, the type of lightning (whether positive or negative), and percentage of each type will need to be determined from the NLDN data. Since previous research has found that storms with higher percentage of +CG lightning are more likely to produce severe weather, NLDN data analysis will determine if this was the case for the two supercells presented in this research. Electric field data for each of these days would also provide a greater understanding of the electrical structure of these storms. This research will be an on-going effort with other researchers at the University of Oklahoma.

ACKNOWLEDGEMENTS This work would not be possible without the help of numerous people. Special thanks to my mentors Alexander Ryzhkov and Thomas Windham for their instruction and guidance. Thanks also to Don MacGorman and Terry Schuur for their help with questions and data interpretation, and to Daphne Zaras whose help in many areas made my experience at the National Severe Storms Laboratory a wonderful learning opportunity.

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SOARS®2003, Maribel Martinez, 20 MacGorman, D. and D. Rust, 1998: The Electrical Nature of Storms. Oxford University Press, 422 pp.

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Schuur, T. J., D. S. Zrnic, and S. E. Saffle, 2001: The Joint Polarization Experiment: An Operational Test of Weather Radar Polarimetry. Proceedings, 30th International Conference on Radar Meteorology, Munich, Germany, Amer. Meteor. Soc., 722-723.

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