Characteristics. Lightning Locations Observed in 1988 Thunderstorms. Bulletin American Meteorological Society. Maribe,h. Abstract. 1.

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1 Characteristics ofr the r^.. Bipolar Pattern of i Lightning Locations Observed in 1988 Thunderstorms Abstract Locations of positive and negative cloud-to-ground lightning flashes can be distinctly regionalized within some mesoscale storm systems. This phenomenon, observed as a bipolar pattern, was documented and analyzed for the year 1988 using archived lightning data of the Eastern United States. The feature was found in all seasons and in all geographical areas studied, although it is more common in summer than in winter and has somewhat different characteristics. The region of negative flashes was determined to be between south and west of the positive flash region in nearly all of the 91 bipolar cases found. The ratio of positive to negative flash density for the pattern ranged from to 0.30, and the feature is most common in the middle half of the storm's lightning activity. Detailed analysis using synoptic meteorological data has shown that cloud top height and radar echo intensity are generally lower over the positive flash region. It is hypothesized that the source charge of the positive flashes is generated above the main convective area and the area of negative lightning. It appears that positive charge could be advected downshear by the upper-level winds to the location of the positive lightning in 60 to 75 minutes. This result agrees with many of the characteristics of the bipolar pattern, within current understanding of the electrical structure of thunderstorms, and with recent studies of anvils and stratiform clouds. Evaluation of satellite imagery and more precise radar data should reveal whether this is a valid explanation. 1. Introduction Lightning between a cloud and the earth can be categorized in terms of the sign of the charge, positive or negative, that is effectively lowered to ground by the flash. By convention, polarity is considered an effective lowering of charge, although it is actually the motion of electrons upward or downward that results in the overall electric field change. Measurements of the polarity of lightning flashes, along with data on location, time, peak magnetic field, and total number of individual strokes in each flash (multiplicity) are available for the eastern half of the U.S. from the State University of New York at Albany (SUNYA) Lightning Detection Network (LDN) (Orville et al. 1987). This network uses gated, wideband, magnetic direction finders, as *First-place winner of 1990 Father James B. Macelwane Annual Award American Meteorological Society Bulletin American Meteorological Society Maribe,h s,4ur esyof M New Yorktat Albany Atmospheric Science A,bany,NY12222 explained by Krider et al. (1976). Observations of a bipolar pattern of positive and negative cloud-to-ground (CG) lightning flashes have been reported by Orville et al. (1988). This feature is characterized by a spatial separation of ground-strike locations of lightning centroids within a mesoscale thunderstorm system. Preliminary analyses of a limited number of bipolar storms indicate a characteristic length of about 100 km between the negative and positive flash regions, with the positive flashes typically fewer in number and more spatially dispersed than the negative flashes. These conclusions are based upon data for the Eastern and Gulf Coast states recorded by the SUNYA Network. By reviewing National Weather Service reports, it was found by Orville et al. (1988) that the bipolar pattern is aligned with the geostrophic winds, and furthermore, that the vertical speed shear of the horizontal wind may be important in its formation. This study is a secondary, more detailed analysis of the bipolar phenomenon. Many of the questions raised by Orville et al. (1988) are addressed, specifically those relating to the frequency of occurrence of this feature, over both temporal and spatial scales, and the conditions which may be responsible for its formation. The preliminary conclusions made about the pattern's characteristics are also tested for a larger sample of thunderstorms. Finally, evidence supporting a hypothesis of downshear displacement of the positive charge region by upper-level environmental winds, perhaps of the type seen in anvil clouds (Marshall et al. 1989), is introduced. 2. Data sources and method of analysis Archived lightning data from the SUNYA LDN was examined to find cases of the bipolar pattern which occurred during 1988 in the eastern part of the network's coverage area. This geographical region extends roughly from Wisconsin, southeastern Canada and Maine southward to the Florida Keys and westward to Oklahoma and Nebraska, and includes the Atlantic and Gulf of Mexico coastal waters. The western portion of the network's coverage was not included in the present study due to data gaps in some regions. This is not considered to be a severe limitation in the study, 1331

2 because many of the mesoscale storm systems which form west of the region of interest later move into the coverage area and exhibit similar characteristics over sufficiently long periods of time to be studied there. For each of the bipolar cases found in this study, statistics on the feature length, orientation from positive to negative flash region, and duration of the pattern were recorded, as well as date, time, size, and location of the storm. Eight representative cases were selected for further analysis based on the above characteristics. National Weather Service (NWS) synoptic reports at 12-h intervals, surface analysis maps at 3-h intervals, and radar summaries at 1-h intervals were obtained for these cases. A videotape recording of the time sequence of lightning in these storms was made to aid in viewing the patterns and to conceptualize their formation and evolution more clearly. A thorough study of the hourly lightning activity was completed. This includes data on the number, areal extent, and density (number of flashes per unit area), as well as mean peak current and multiplicity for each hour of the storm. FIG. 1. Lightning activity in Michigan-Wisconsin area, UTC September total flashes: 1011 negative (dots), 39 positive (crosses). Large crosshairs are at a bearing of, 170 km apart. Contours show negative flash density. 3. Observations and results A typical example of the bipolar pattern is illustrated in figure 1. This storm's activity was recorded between 0900 UTC 22 September and 1200 UTC 23 September, 1988, as it moved from eastern Wisconsin to Pennsylvania. The bipolar pattern is evident during two separate intervals within the storm (i.e., lightning) period as it progressed eastward. Part of the first bipolar interval is shown in figure 1, with negative flashes plotted as dots and positive flashes as small crosses. For the period shown, 2200 UTC through 0000 UTC September, the pattern length is about 170 km. This is the distance between the two large crosshairs on the map. There are 1050 flashes; 3.7% (39) of these are positive. The orientation of the negative region of the feature from the positive region is about (measured relative to north). The second bipolar interval, shown in figure 2, exhibits similar length, orientation, number of flashes, and percent positive as the first, but the flash density is lower (i.e., the flashes are dispersed over a larger area). The bipolar part of this storm cell's movement also occurs in the same geographical location (of Michigan), and follows the one shown in figure 1 by about 90 minutes. Before describing this storm in more detail, it is necessary to determine on what basis it is defined as a representative case of the bipolar pattern. To do this, a summary of all the bipolar cases found in 1988 is needed. Table 1 shows monthly values of number 1332 FIG. 2 Lightning activity in Michigan-Wisconsin, September total flashes: 1004 negatives (dots), 37 positives (crosses). Large crosshairs are at a bearing of, 170 km apart. Contours show negative flash density. of cases, average length and average angle between positive and negative regions. A total of 91 cases was found in these data, with the greatest number seen in spring and summer (Mar - Aug). This is not surprising as these are the seasons of maximum thunderstorm activity for most of the U.S. (Changnon 1988b). However, a much larger percentage of the winter storms exhibit bipolar patterns during their lifetime. The pattern is also much more distinct in winter storms than in summer storms. This is evidenced by the longer average distance between positive and negative regions found in winter cases. Bipolar storms were found in all regions of the Network. They are seen in the Southeast states, Gulf of Mexico and Atlantic coastal regions throughout the entire year, and in the more northward and westward areas in the summer months. Again, this corresponds closely to thunderstorm activity in general (Changnon 1988b). Orientation of the bipolar pattern varied little throughout the year or in the different geographical areas. The table shows the monthly averages ranging from 230 to, with little seasonal variation evident. This Volume 71, No. 9,N o v e m b e r19

3 TABLE 1. Summary of 1988 bipolar cases by month. Length is the average distance between approximate centers of opposite polarity flashes. Orientation is location of negatives with respect to positives (360 is due North). Month Dec. Jan. Feb. Mar. Apr. May Number Length Orientation (degrees) Month June July Aug. Sept. Oct. Nov. Number Total Average characteristic is in general agreement with the observation by Orville et al. (1988) that the feature is aligned with the ambient wind direction. Only six of the 91 cases do not have orientations at any angle between 180 and 270. Four of these are oriented NW-SE, and could be aligned with upperlevel winds around a high- or low-pressure system. (This was determined to be the case in one of these storms for which the wind data were obtained.) The remaining two abnormal orientations are opposite cases (SW-NE) which both occur at the edge of the network coverage. These may be explained by failure of the network to detect the entire storm at too great a distance from any two direction finders. Positive flashes have a higher mean peak current than negative flashes (Cooray and Lundquist 1982) and may be detected further outside the nominal range of the network, which could lead to the opposite appearance as the storm moves into range. If this is the case, these opposite bipolar patterns should be eliminated by completion of network expansion into all areas of the United States in Analysis of the meteorological characteristics of eight representative bipolar cases revealed qualities that can be assumed to apply to at least a large percentage of all bipolar storms. However, these qualities are not significantly different from the general thunderstorm morphology as described in Kessler (1985). Thus, the obvious question of why every storm with both positive and negative lightning is not bipolar must be subsequently addressed. All of the chosen cases are associated with a surface frontal system, although evidence of the observation made by Orville et al. (1988) that the surface front divides the positive and negative regions of the feature was not found in this study. Four of the sample bipolar patterns appear in storms associated with cold fronts. All of these are aligned parallel to the cold fronts, and generally the lightning activity is ahead of the frontal boundary (i.e., further down stream in the direction of movement). One is found to coincide with the location of an intense Bulletin American Meteorological Society Length Orientation (degrees) squall line ahead of a surface cold front. The remaining three are observed in the vicinity of warm fronts. Location of the positive and negative flash regions in relation to the front orientation varies considerably in these cases, although generally the pattern's orientation is not parallel to the warm front as it is discerned to be with cold fronts. Cloud top heights, as deduced from radar echo tops, range from 6 km to 15 km. These are observed to be lower over the positive flash region than over the negative region of the bipolar feature in all cases. In addition, cloud top heights close to the storm center often peak near the onset of a bipolar pattern development. Radar echo intensity of the two areas is sometimes the same during the bipolar period, but more often is higher in the negative region (Video Integrator Processor level 5 vs. 3). Only limited generalizations can be made about the upper level (500 to 200 mb) winds in these storms because they differ in many ways from one case to the next and from one report to the next. Typically, the winds are directed with the mean flow from west to east, at angles between 230 and 270, although one case shows wind directions between 140 and 180. The average vertical shear of the horizontal wind between these two levels for all the cases is 2.7 m s 1 per km. Although an average value of this parameter is somewhat meaningless, it is significant to note that this is above the minimum threshold shear value of 1.5 m s-1 per km suggested by Brook et al. (1982) to produce positive CG lightning. Detailed study of the hourly lightning activity of the eight bipolar cases reveals interesting characteristics. Storm duration ranges from 13 to 36 hours, and the bipolar pattern within these storms lasts between 2 to 15 hours. In five of the cases, the bipolar period is in the middle of the storm, while in the remainder it occurs near the end. Generally, the hourly rate of positive flashes peaks before and during the bipolar period. The percentage of total flashes lowering positive charge to ground usually does not reach its highest value during this time, however, due to the 1333

4 high rate of negative flashes taking place concurrently. Typically, the percent positive is highest at the beginning and end of the storm, with a relative maximum appearing during the bipolar interval. This is illustrated in figure 3, which shows the number of negative and positive flashes per hour and percent positive for the September storm. The figure shows relatively low percent positive during the first bipolar interval and relatively high percent positive during the second. This difference is due almost entirely to the rate of negative flashes, because the rate of positive flashes is nearly the same in both. The ratio of positive to negative maximum flash density for the bipolar patterns ranges from to 0.3 in these eight cases. In addition, an interesting inverse relationship between rate of negative CG lightning and mean peak current appears in two of the cases (i.e., negative lightning has higher peak current when there are fewer flashes per hour), although this is not considered to be an exclusive characteristic of storms exhibiting bipolarity because it has been found in other thunderstorms as well (R.E. Orville, pers. comm. 1989). Based on all of the above observations, the sample case of September, 1988 (figures 1 and 2) is considered a typical example of the bipolar phenomenon. This storm lasted for a total of 27 hours ( UTC) and exhibited the bipolar pattern twice within this period in two different cells. The first of these patterns lasted about 3 hours in the middle of the storm ( UTC), when the main forcing for the lightning activity is deducted to be a warm front just behind the pattern. The ratio of positive to negative maximum flash density averages for these 3 hours. The second bipolar pattern lasted about 8 hours ( UTC) near the end of the storm's lightning activity. By this time an occluded front is approaching the area of CG lightning, and the bipolar pattern is between this front and the warm front. The positive to negative flash density ratio averages 0.15 for this 8-h period. In total, this storm is very representative of the bipolar phenomenon. 4. Discussion An understanding of the existence of the bipolar pattern requires an initial overview of the electrical structure of thunderstorms, which is described in Krehbiel (1986). The dominant accumulations of charge in the storm cell consist of positive charge in the upper part of the cloud and negative charge below, and thus it is said to have a dipolar structure. A secondary, smaller region of positive charge is also found beneath and inside the base of the cloud. Heights of these regions are determined to vary with 1334 FIG. 3. Number of negative and positive flashes per hour (left axis) and percent positive (right axis) for each hour of the September 1988 thunderstorm. season and location (speculated to be temperature dependent), but generally the upper positive region is between 8 and 15 km above sea level and the main negative charge is at a height of between 4 and 9 km. Studies of the electrification processes contributing to this charge buildup and separation have been presented extensively throughout the literature (see lllingworth 1985; Williams 1985; and Moore and Vonnegut 1977 for summaries). Two major theories exist. One involves the transfer of charge associated with cloud particle interactions, wherein large precipitation particles attain a net negative charge and are brought to the bottom part of the cloud while small cloud particles acquire a net positive charge (thought to take place in some temperature-related cloud physics process), and move to the top part of the cloud following the vertical air motions. The other major theory of thunderstorm electrification is concerned with convective motions (i.e., those driven by differential temperatures, usually associated with strong vertical motions) within the thundercloud. This mechanism relies on the convection of positive space charge (charges found in the air at any time) into a growing cloud. These positive charges are carried to the upper portions of the cloud where they attract negative ions. The negative ions subsequently become attached to cloud particles at the cloud surface, are carried down by convective motions at the outside, and become incorporated into the cloud near the base to become the lower negative charge region. Both these theories have difficulties with some experimental results, but currently the dipole structure of thunderstorms is believed to be the consequence of one or both of these mechanisms. In suggesting an explanation of the bipolar pattern, the electrical structure of the thunderstorm and the location of positive CG flashes play crucial roles. Rust et al. (1981), in a study of severe storms in the Great Plains, suggest that downshear anvils are often the Volume 71, No. 9,N o v e m b e r199

5 TABLE 2. Cloud top parameters and bipolar pattern characteristics for time intervals of the September 1988 storm. Last column shows calculated minutes to travel given distance at given wind speed. Time Interval (UTC) Cloud height Wind direction (degree) Wind speed (m s- 1 ) Bearing pos-> neg (degree) Distance Advection time (min) source region for positive lightning. These anvil clouds form when rising air in a convective storm reaches the stratosphere, loses its buoyancy and extends horizontally downwind tens to hundreds of kilometers (Marshall et al. 1989).Their electrical character is generally assumed to be positive in the interior, originating from the upper positive charge region, surrounded by a negative screening layer.another believed source of positive CG lightning is in regions of stratiform clouds behind squall lines (Rutledge and MacGorman 1988). These authors suggest that the occurrence of positive flashes is due to the advection by upper level winds of positive charge on small ice particles from the upper positive region of the main convective cell. Both these findings provide important clues to the formation of the bipolar phenomenon. Evidence that the source region for the positive flashes of the bipolar feature is being advected from above the negative flash locations is equally FIG. 4. Lightning activity in central Michigan during bipolar pattern. Diamonds are negative flash locations and small crosses are positive flashes during UTC. Contours are flash density of negative flashes that occurred UTC (flash locations not shown). Bearing between large crosshairs is and distance is 149 km. speculative in this study. Indications that this is so result from a closer look at upper-level wind direction and speed in relation to the displacement of the positives from the negatives. Simply stated, the positive flash region should be located downwind of the negative region at a distance proportional to its speed of travel. Table 2 shows values for various time intervals in the September thunderstorm when the bipolar pattern was evident. The cloud top height given is actually the radar echo top from the NWS radar summary closest in time to the beginning of the interval. The next column lists the wind direction and speed as obtained from the NWS radiosonde observations for the nearest station and closest report time to the bipolar pattern interval. The values listed are for the height nearest the radar echo top for which wind data were reported. The fifth and sixth columns show the bearing and distance from the approximate center of the positive lightning region of the pattern during the last 15 minutes of the interval to the location of the center of the negative region in the first 15 minutes of the interval. The final column shows the calculated amount of time it would take to move the given distance at the given wind speed. This should correspond to the time lapse between the occurrence of the negative flashes at the beginning of the interval and the downwind positive flashes at the end of the interval. More precisely, this time lapse should approximate the duration of the interval if the negative flashes considered occur near the beginning of this interval and the positive flashes occur near the end. This movement is illustrated in figure 4. This figure shows 15 minutes of lightning activity of the storm shown in figure 1 in southern Michigan. Positive flashes are plotted as small crosses and negative flashes are shown as diamonds. The negative flash density Bulletin American Meteorological Society 1335

6 contours are shown for previous flashes that occurred in the area between 2230 and 2245 UTC. These help to illustrate the storm's movement in the hour between 2230 and The bearing from the large crosshair embedded in the positive region to the one in the (hour- old) negative region is. These are 149 km apart on the map, which can be seen from table 2 (row 3) to correspond to 65 minutes displacement time. This supports the idea that the source region of the positive flashes could be blown downshear by the winds at 12 km. Positive charge, being generated above the main convective area and above the negative flash region, is transported downshear at m s 1 for roughly 60 minutes and subsequently discharges 149 km away. Table 2 illustrates an additional aspect of this hypothesis. The first four rows include data for the first bipolar period of this storm; the remaining eight rows contain data for the second. Comparison of the length of the intervals in the first column between the two parts of this storm reveals an interesting difference. In the first bipolar period, 60-min intervals are used, while in the second 75-min and later 60-min intervals are used. This variation may seem arbitrary, but actually it is dependent upon the speed of advancement of the storm and the cloud top height at the beginning of the interval. Although it is difficult to quantify precisely, detailed study of several of these storms shows that for faster moving storms (those that move through an area in a shorter amount of time) the interval is longer. This indicates that the distance downwind the positive charge travels (displacement) should be fairly constant, while the time it takes to get this distance is dependent upon the wind speed at the particular height. This important detail requires further study before it can be accepted as part of the explanation of the bipolar pattern. However, evidence for this may exist in the annual summary of pattern length. Table 1 indicates that the average length in the summer is shorter than that in the winter. If summer storms are more convectively intense and move through an area more slowly than winter storms, then the result should be that although the positive region is displaced equally in both, the negative region moves less in the given amount of time in the summer than it does in the winter. Because the negative and positive regions are usually displaced in different directions, the net result of all this would be that the two regions would be closer when the negative region moves less. A distinction must be made here between bipolar pattern length and positive charge displacement. Bipolar pattern length, as defined by Orville et al. (1988), is the instantaneous distance between the flash density contours of different polarity flashes, while displacement of the positive charge is understood to 1336 be the distance between the present positive flash region and a previous negative flash region (above which the positive flash source charge is speculated to originate). Thus the length is dependent on a storm's rate of progression, while positive charge displacement is hypothesized to be independent of this complex parameter. Returning to the problem of determining the appropriate interval of time to use for each bipolar pattern, it is found that the upper level wind direction relative to the direction of storm progression provides a useful clue. Admittedly, this seems like a backward approach in that the lightning data is searched for a time when the negative flashes were at a particular angle upwind from the present positive flashes, but at this level of understanding it seems to be a good technique to formulate and test hypotheses. Additional insight may be gained when a bipolar pattern persists in a storm long enough for the storm's progression rate to change. This is thought to be the case in the second bipolar interval in the sample storm. As mentioned previously, table 2 indicates the displacement interval changes at the end of the occurrence of the pattern, which coincides nearly with the end of the lightning activity of the storm. From this evidence it is speculated that the storm may be slowing down. The positive region requires a shorter interval to travel downwind because the negative region is moving less in any given amount of time. To show that the sample case is not the only storm for which this hypothesis can be documented, figure 5 is included. This figure is interpreted in a similar way to figure 4. It shows a bipolar pattern near Tampa Bay, Florida, that was observed in a storm that moved through the Gulf of Mexico and along the Gulf Coast on December, All flashes that were recorded between 2000 and 2100 UTC are plotted on this figure, with positives again small crosses and negatives this FIG. 5. Lightning activity in Tampa Bay area during bipolar pattern. Dots are negative flash locations and small crosses are positive flashes during UTC 11 December Contours of flash density are shown for both negative and positive regions. Bearing between large crosshairs 256 ; distance is 137 km. Volume 71, No. 9,N o v e m b e r19

7 TABLE 3. Cloud top parameters and bipolar pattern characteristics for time intervals of the December 1988 storm. Time in last column is again minutes to travel distance at given wind speed. Time Interval (UTC) ^ Cloud height Wind direction (degree) Wind speed (m s 1 ) Bearing pos- >neg (degree) Distance Advection time (min) time dots. Contours of flash density are also shown. The large crosshair west of the negative contours shows the approximate center of the negative flash activity between 2000 and Bearing from positive to negative (crosshair to crosshair) is 256 and distance is 137 km. Data on cloud height and winds for this storm are found in table 3, which is read in the same manner as table 2. This bipolar pattern is also in very good agreement with the hypothesis that the source region of the positive flashes originates in the vicinity of the negative flashes. In total, the hypothesis was tested for four different storms; all were in general accordance with these results. Various other issues need to be addressed in an explanation of these results. The bipolar pattern is thought to be more distinct in winter storms mainly due to the fact that winter storms generally have much fewer total flashes, are smaller in areal extent, and usually last for shorter periods of time than summer storms (Changnon 1988a). Thus there is less overlap of positive and negative flashes and the bipolar pattern is better defined. The question raised earlier as to why every storm does not exhibit bipolar characteristics is crucial in the understanding of the phenomenon; it is also not easily answered. Probably the best approximation at this time is embedded in the findings of Brook et al. (1982), which give a minimum vertical shear value needed for positive discharges to occur. Likewise, a minimum wind speed may be necessary to tilt the dipole and displace the positive charge region. Electrical considerations may be equally important. Marshall et al. (1989) emphasize that the strength of the electric field in the core is influenced by the amount of positive charge moving into the anvil region, so it is conceivable that electrical limitations could be placed on the large scale separation of charges of the type hypothesized to be occurring. Development of the charge structure of the thunderstorm, as yet only partially understood, is most likely the key to understanding the bipolar phenomenon. Alternative explanations for the bipolar pattern should not be excluded. In particular, charge generation in the source region of the positive flashes can not be ruled out, especially due to the lack of cloud physics Bulletin American Meteorological Society data. Ice crystal interactions in the presence of small amounts of liquid water have been shown in the laboratory to transfer charge, and the sign of this transferral is determined to be dependent on temperature, liquid water content and crystal impurities (Jayaratne et al. 1983). Preferential generation of positive charge to the northeast of a thunderstorm dipole could be taking place due to some microphysical property of the clouds involved. Another possibility is that the main thunderstorm cell is somehow inducing an inverted dipole (a main positive charge region below an upper negative region) in some instances, and positive lightning results from discharge of this region. This conjecture of an inverted dipole could explain the presence of many negative flashes that often occur in the positive region of the bipolar pattern as the equivalent (opposite) of positive flashes that are observed in a normal thunderstorm. This hypothesis also requires much more detailed cloud data than are presently available. 5. Conclusions These observations of a bipolar pattern of thunderstorms demonstrate a significant correlation between the direction and speed of the upper level winds and the relative location of the positive and negative cloud-to-ground lightning regions. The data support a hypothesis that the source region of these positive flashes is being blown downshear from above the main negative charge region of the thunderstorm cell by cloud-top level winds. Although this result is considered preliminary, it is in agreement with many of the characteristics of the bipolar phenomenon and with current understanding of the electrical structure of thunderstorms. Further evaluation of additional sources of data must be completed before a better understanding of this feature can be attained. The essentials for even a rudimentary explanation of the type undertaken in this study are more frequent radiosonde observations at the specific time and location of a bipolar thunderstorm and satellite imagery of cloud type, extent and temperature. These would 1337

8 be especially useful as a thorough test of the hypothesis formulated in this study. More precise radar data and supplemental balloon experiments of the motions within clouds would be ideal for a comprehensive determination of how and why this pattern develops. Acknowledgments. I would like to thank Jon Scott for initially motivating my research, and Richard E. Orville for making this project possible. I also appreciate the assistance and support of everyone involved with the SUNYA Lightning Network, especially Cynthia Erdt for tolerating my interruptions. Finally, I am very grateful to Vincent Idone for his original ideas, thought-provoking discussions, and valuable encouragement throughout this project. It would not have been conceivable without his efforts. This research was performed within the SUNYA lightning project that is supported in part by the National Science Foundation (ATM ) and by the Electric Power Research Institute (RP2431-1). References Brook, M., M. Nakano, P. Krehbiel and T. Takeuti The electrical structure of the Hokuriku winter thunderstorms. J. Geophys. Res., 87, Changnon, S.A., Jr. 1988a. Climatography of thunder events in the conterminous United States. Parti: temporal aspects. J. Climate, 1: Changnon, S.A., Jr. 1988b. Climatography of thunder events in the conterminous United States. Part II: spatial aspects. J. Climate, 1: Cooray, V.,andS. Lundquist On the characteristics of some radiation fields from lightning and their possible origin in positive ground flashes. J. Geophys. Res., 87: 11,203-11,214. Illingworth, A.J Charge separation in thunderstorms: small scale processes. J. Geophys. Res., 90: Jayaratne, E.R., C.P.R. Saunders and J. Hallett Laboratory studies of the charging of soft-hail during ice-crystal interactions. Quart. J. R. Meteorol. Soc. 109: Kessler, E. (ed.) Thunderstorm Morphology and Dynamics. Norman, Oklahoma: The University of Oklahoma Press. Krehbiel, P.R The Electrical Structure ofthunderstorms. In Studies in Geophysics--The Earth's Electrical Environment. Washington, D.C.: National Academy Press. Krider, E.P., R.C. Noggle, and M.A. Uman A gated, wideband, magnetic direction finder for lightning return strokes. J. Appl. Met., 15: Marshall, T.C., W.D. Rust, W.P. Winn and K.E. Gilbert Electrical structure in two thunderstorm anvil clouds. J. Geophys. Res., 94: Moore, C.B. and B. Vonnegut The Thundercloud. In Lightning, Vol. 1: Orlando, Academic Press, Inc. Orville, R.E., R.W. Henderson and L.F. Bosart Bipole patterns revealed by lightning locations in mesoscale storm systems. Geophys. Res. Lett., 15: , R.A. Weisman, R.B. Pyle, R.W. Henderson, and R.E. Orville, Jr Cloud-to-ground lightning flash characteristics from June 1984 through May J. Geophys. Res., 92: Rust, W.D., D.R. MacGorman and R.T. Arnold Positive cloud-to-ground lightning flashes in severe storms. Geophys. Res. Lett., 8: Rutledge, S.A., and D.R. MacGorman Cloud-to-ground lightning activity in the June mesoscale convective system observed during the Oklahoma-Kansas PRE-STORM project. Mon. Wea. Rev., 116: Williams, E.R Large-scale charge separation in thunderclouds. J. Geophys. Res., 90: Volume 71, No. 9, November19