Click Here JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, D19203, doi:10.1029/2005JD006802, 2006 for Full Article Cloud-to-ground lightning and surface rainfall in warm-season Florida thunderstorms Bruce Gungle1,2 and E. Philip Krider1 Received 23 October 2005; revised 27 February 2006; accepted 20 April 2006; published 11 October 2006. [1] Relationships between cloud-to-ground (CG) lightning and surface rainfall have been examined in nine isolated, warm-season thunderstorms on the east coast of central Florida. CG flashes and the associated rain volumes were measured as a function of time in storm-centered reference frames that followed each storm over a network of rain gauges. Values of the storm-average rain volume per CG flash ranged from 0.70 104 to 6.4 104 m3/CG flash, with a mean (and standard deviation) of 2.6 104 ± 2.1 104 m3/CG flash. Values of the rain volume concurrent with CG flashes ranged from 0.11 104 to 4.9 104 m3/CG flash with a mean of 2.1 104 ± 2.0 104 m3/CG flash. The lag-time between the peak CG flash rate and the peak rainfall rate (using 5 min bins), and the results of a lag correlation analysis, show that surface rainfall tends to follow the lightning (positive lag) by up to 20 min in six storms. In one storm the rainfall preceded the lightning by 5 min, and two storms had nonsignificant lags. Values of the lagged rain volume concurrent with CG flashes ranged from 0.43 104 to 4.9 104 m3/CG flash, and the mean was 1.9 104 ± 1.7 104 m3/CG flash. For the five storms that produced 12 or more flashes and had significant lags, a plot of the optimum lag time versus the total number of CG flashes shows a linear trend (R2 = 0.56). The number of storms is limited, but the lag results do indicate that large storms tend to have longer lags. A linear fit to the lagged rain volume vs. the number of concurrent CG flashes has a slope of 1.9 104 m3/CG flash (R2 = 0.83). We conclude that warm-season Florida thunderstorms produce a roughly constant rain volume per CG flash and that CG lightning can be used to estimate the location and intensity of convective rainfall in that weather regime. Citation: Gungle, B., and E. P. Krider (2006), Cloud-to-ground lightning and surface rainfall in warm-season Florida thunderstorms, J. Geophys. Res., 111, D19203, doi:10.1029/2005JD006802. 1. Introduction [2] The processes that electrify clouds and produce lightning depend on complex interactions between the cloud microphysics and dynamics, characteristics that in turn depend on the type and location of the storm and the larger meteorological environment [Williams et al., 1989; Zipser, 1994; Baker et al., 1995; Boccippio et al., 2000, 2001; Blyth et al., 2001; McCaul et al., 2002; Lang and Rutledge, 2002; Toracinta et al., 2002; Williams et al., 2002; Brown et al., 2002; Carey and Rutledge, 2003; Seity et al., 2003]. Since precipitation is thought to play a key role in electrifying clouds on both small and large spatial scales (see MacGorman and Rust [1998] for a review of the literature on thunderstorm electrification), and since there are still many questions about how 1 Institute of Atmospheric Physics, University of Arizona, Tucson, Arizona, USA. 2 Now at U. S. Geological Survey, Water Resources Discipline, Tucson, Arizona, USA. Copyright 2006 by the American Geophysical Union. 0148-0227/06/2005JD006802$09.00 lightning relates to rainfall on various scales of space and time [Petersen and Rutledge, 1998; Tapia et al., 1998; Baker et al., 1999; Soula and Chauzy, 2001; Seity et al., 2001], we have undertaken a study of relationships between cloud-to-ground (CG) lightning and surface rainfall in Florida. The motivation for this work has been to determine whether real-time measurements of the location and intensity of CG lightning can assist forecasters in making quantitative estimates of the location and amount of convective rainfall, especially in regions where radar coverage is poor or incomplete. [3] Tables 1 and 2 summarize some recent studies of relationships between lightning and the associated precipitation volume (or mass) that is inferred from rain gauge and weather radar measurements, respectively. The spatial and temporal scales of these studies range from individual thunderstorms, or even the cells within storms, to synoptic and climatic scales where the pattern of lightning and rainfall over broad regions is considered for days, months, and/or years. The studies of individual storms tend to focus on intervals when there is active convection and lightning, whereas the synoptic and climatic studies tend to include all precipitation that is detected within the region of interest, even that which is not associated with lightning. Because of D19203 1 of 15 D19203 GUNGLE AND KRIDER: LIGHTNING AND RAINFALL D19203 Table 1a. Precipitation Volume (PV) Per Lightning Flash Reported by Investigators Who Used Gauges to Measure the Precipitation PV Per Total Flasha (104 m3/flash) PV Per CG Flash (104 m3/CG Flash) Author(s), Location, and Date Battan [1965]: Tucson, AZ 3.0 – Piepgrass et al. [1982]: KSC, FL Holle et al. [1994]: Kansas and Oklahoma Gosz et al. [1995]: New Mexico Sheridan et al. [1997]b: Gulf Coast and Central Plains Soriano et al. [2001]: Iberian Peninsula Ezcurra et al. [2002]: Northeast Spain 1.8 – 2.2 20 0.67 – 0.85 – 3.2 – 4.0 <0.15; 0.15 – 1.0; >1.0 – – 12; 21 – 21, 23, 38 68, 120, 136 – 4.0 – 430 – 0.11 – 6.4 – Kempf and Krider [2003]: Upper Mississippi River Basin Present study [2006]: KSC, Florida Comments Visual CG count, over four summer seasons, 800 – 1000 km2 Two isolated storms Mesoscale Convective System (MCS) Over three summer seasons and 1000 km2 1989 – 1993 warm seasons, six areas 2.58 – 13.1 103 km2, by day 1992 – 1994 warm seasons, 102 km2, by months: semiarid; humid 5 year totals at three locations; daily totals at three locations; CGs in 20 km2 box centered on gauge 1993 ‘‘Great Flood,’’ 1.3 109 km2; daily averages Nine isolated storms a The total flash count is the sum of the intracloud (IC) and cloud-to-ground (CG) lightning flashes. Sheridan et al. [1997] did not include lower and upper values of PV/CG flash; however, thunderstorm days were divided into one of three PV/CG flash categories. b meteorological conditions that produce thunderstorms over the Florida peninsula [Piepgrass et al., 1982; Watson et al., 1987; Watson et al., 1991; Reap, 1994; Laird et al., 1995; Jameson et al., 1996; Bauman et al., 1997; Wilson and this, the synoptic and climatic studies tend to find larger precipitation volumes per flash than the studies that focus on individual storms or the cells within storms. [4] In recent years, there has been much research on the Table 1b. Precipitation Volume (PV) Per Lightning Flash Reported by Investigators Who Used Radar to Measure the Precipitation Author(s), Location, and Date Kinzer [1974]a: Oklahoma Grosh [1978]: St. Louis, Missouri Maier et al. [1978]: South Florida Goodman et al. [1988]: Huntsville, Alabama Goodman and Buechler [1990]: Alabama/Tennessee line Buechler et al. [1990]: Tennessee Valley Nielsen et al. [1990]: Oklahoma PV Per CG Flash (104 m3/CG flash) PV Per Total Flash (IC Plus CG) (104 m3/flash) Comments 1.6* – 1.1 – 9.2 2.0 – 1.4 – 0.1 One squall line storm One isolated storm Over 20,000 km2 and 24 days One microburst-producing storm 1.1 – 9.9 0.12 – 0.26 0.85 – 54.1 – Six isolated storms (PV/CG), and two isolated storms (PV/TF) 21 storms over 6 days 4 – 10 – Lopez et al. [1991]: Cape Canaveral, Florida Williams et al. [1992]: Darwin, Australia Holle et al. [1994]: Kansas and Oklahoma Buechler et al. [1994]: South Florida Senesi et al. [1996]: Southern France Petersen and Rutledge [1998]: several locations Soula et al. [1998]: Spain Tapia et al. [1998]: KSC, Florida Molinie et al. [1999]: Spain 0.1 – 1000 – PV/CG varies over life of 1 Mesoscale Convective System (MCS) Over 14,800 km2; hourly PV/CG ratios 50 (continental), 500 (monsoon) 76.9 – Two wet seasons; about 40,000 km2 – MCS 10.8 – 2 months over 90,000 km2 3.0 – Squall line system 5.7 – 2000 – 11 regimes over months and 100,000 km2 3.1 2.4 – 36.5 – – One storm, possibly supercell 22 storms in August 1992 and 1993 0.32 – 4.7 – Soula and Chauzy [2001]: Paris, France Seity et al. [2001]: Bordeaux, France Zhou et al. [2002]: East Gansu, China 5.10 – 9.94 0.75 – 2.04 Three isolated cells and three cells embedded in an MCS Three summer 1997 and one spring 1999 storm 0.92 – 113.0 1.15 – 106.6 0.204 – 12.89 (land) 0.156 – 31.75 (ocean) – 2.4 a 21 stormy days over 240 km2 of land and ocean Median value observed in thunderstorms in East Gansu Kinzer notes there may have been a systematic 2 dB error in the NSSL radar data; in that case, the PV/CG flash ratio would be 2.6 104 m3. 2 of 15 D19203 GUNGLE AND KRIDER: LIGHTNING AND RAINFALL D19203 Table 2a. Lag Times Between Lightning and Surface Rainfall Reported by Investigators Who Used Gauges to Measure the Rainfall Author(s), Location, and Date Bin Size, min Type of Lightning (CG or IC) Piepgrass et al. [1982]: KSC, Florida Kane [1992]: Northern Virginia Watson et al. [1994]: Arizona 5 and 1 CG and IC 4 – 10 5 60 CG CG 5 and 3 CG 5 – 15 Peak precipitation lags peak CG flash rate (three of six regions) or is concurrent (three of six regions) 0 – 20 Present study [2002]: KSC, Florida Megenhardt, 1997; Bringi et al., 1997; Hodanish et al., 1997; Camp et al., 1998; Gremillion and Orville, 1999; Lericos et al., 2002; Mazany et al., 2002]. Here, we describe a storm-scale, gauge-based study of CG lightning and surface rainfall that was produced by nine warm-season thunderstorms at the NASA Kennedy Space Center (KSC) and Cape Canaveral Air Force Station (CCAFS). Our methods of analysis will be similar to those described by Piepgrass et al. [1982]. [5] For each storm in our data set, we have measured the CG lightning and area-average rainfall as a function of time; we have computed the lag times between the lightning and rainfall rates; and we have estimated the average rain volume per CG flash with and without a correction for the lag time between the lightning and rainfall rates. Originally, 12 thunderstorms were considered for analysis because they occurred over or near the rain gauge network, but as the analysis progressed, three of these storms were found to be unsuitable, either because the storm produced substantial lightning and rainfall outside the perimeter of the network or because the storm was not sufficiently isolated, i.e., it was part of a larger system that produced substantial lightning and rainfall outside the network. We will begin with a description of the sensors that were used to measure the lightning and rainfall, and then we will describe the methods that have been used in the data analysis. Finally, Optimum Lag in Precipitation, min we will describe our results and how they compare with other measurements. 2. Instrumentation 2.1. Cloud-to-Ground Lightning Surveillance System [6 ] The number and locations of CG flashes were obtained from the Cloud-to-Ground Lightning Surveillance System (CGLSS) that is operated by the CCAFS. This system contains five gated, wideband IMPACT sensors similar to those used in the U.S. National Lightning Detection Network (NLDN) [Cummins et al., 1998a, 1998b] but with lower gain. The location accuracy of the CGLSS is approximately 0.6 km over the KSC-CCAFS, and the detection efficiency is estimated to be better than 90% [Maier, 1991; Maier and Wilson, 1996; CSR, 2005; Boyd et al., 2005]. Prior studies have shown that a large fraction of the low-amplitude, positive flashes reported by IMPACT sensors are actually cloud discharges [Cummins et al., 1998a], so positive reports have been included in our counts only if the estimated peak current is greater than 15 kA. 2.2. Rain Gauge Network [7] Figure 1 shows the locations of 31 tipping-bucket rain gauges that were originally installed at KSC as part of the TRMM ground validation program [Habib and Krajewski, Table 2b. Lag Times Between Lightning and Surface Rainfall Reported by Investigators Who Used Radar to Measure the Rainfall Author(s) and Study Location Workman and Reynolds [1949]: New Mexico Shackford [1958, 1960]: Milton, Massachusetts Carte and Kidder [1977]: South Africa Rutledge and MacGorman [1988]: Oklahoma and Kansas Goodman et al. [1988]: Huntsville, Alabama Buechler et al. [1990]: Tennessee Valley Tapia et al. [1998]: KSC, Florida Molinie et al. [1999]: Southern France Soula and Chauzy [2001]: Paris, France Bin Size, min Type of Lightning (CG or IC) – CG and IC – CG and IC First IC flash concurrent with appearance of precipitation at cloud base; CGs follow ‘‘a few minutes later’’ 3 – CG Precipitation both lags and precedes CGs 30 CG 30 to 0 (negative CG-convective rainfall) 45 (positive CG-stratiform rainfall) 1 total 2 10 CG 10 – CG ‘‘significant lag’’ 5 CG ‘‘precipitation occurs after the lightning flashes’’ 5 CG and IC 10 to 0 3 of 15 Optimum Lag in Precipitation, min D19203 GUNGLE AND KRIDER: LIGHTNING AND RAINFALL D19203 Figure 1. Outline of the precipitation area (black lines) in four consecutive 15-min precipitation intervals and the concurrent CG flash locations (black dots) during a thunderstorm on 28 April 1999 (990428). 2002]. The gauges were manufactured by Qualimetrics, Inc. (Model 6011-A), and gauge tips were recorded in real time by the CCAFS. The manufacturer states that the accuracy of each gauge is about ±0.5% at a rainfall rate of 0.5 inches per hour and that the resolution of each gauge is 0.254 mm (0.01 inch) of rainfall per bucket tip. The network covers a total area of about 450 km2, but the sensor spacing is irregular with the distance between gauges ranging from about 2 to 6 km. The average distance between gauges is 3.3 km, and the average network area per gauge is about 14.5 km2. [8] During a storm on 28 April 1999 (990428), data from gauges 10, 15, and 18 were not available, and on 10 June 1999 (990610A), the data from gauge 14 were missing. In the analyses that are discussed below, any gauges that had missing data were treated as if they did not exist, i.e., those sensors were not included in our estimates of the precip- itation areas or the associated precipitation volumes. Sometimes gauges reported two or more bucket tips during a 1 s interval, most commonly two tips but occasionally as many as seven tips per second. The manufacturer did not have information about the maximum number of tips that would be produced by a heavy downpour, but when a gauge was held under an open faucet, the maximum tip rate was two per second, or about 30 mm min1 (Qualimetrics, private communication, 2000). Therefore in our analyses we have limited the maximum tip rate to two tips per second. Multiple tips occurred in fifty-three 1-s intervals in this study. 3. Data Analysis and Results [9] Because thunderstorms are rarely stationary, their evolution and movement must be taken into account when 4 of 15 GUNGLE AND KRIDER: LIGHTNING AND RAINFALL D19203 analyzing relationships between lightning and surface rainfall. In the following, we have used quasi-Lagrangian (i.e., moving, storm-centered) reference frames for the data analysis. 3.1. CG Lightning [10] CG lightning flashes that were located within or just outside the perimeter of a storm precipitation area (see section 3.2) were counted over consecutive 5 min intervals. Whether a flash was included or excluded from this count depended on whether it was located within or near the precipitation area and how that area evolved with time. The times and locations of the lightning were used to determine which precipitation area the flashes belonged to, and if the storm was completely isolated, flashes up to 10 km from the perimeter of the area were included. If two storms occurred over the network at the same time, flashes were assigned to the closest precipitation area. 3.2. Precipitation [11] The precipitation area, Ap, was estimated for each storm in consecutive 15-min ‘‘precipitation intervals,’’ and within each of these intervals, the rainfall was grouped into consecutive 5-min bins, the smallest interval that could be used for averaging without producing significant sampling error [Habib et al., 2001b]. If any gauge produced one or more bucket tips (0.254 mm or more) during a precipitation interval, that gauge was deemed ‘‘active,’’ and it was included in the precipitation area. The precipitation area included all gauges that detected rainfall in each 15-min precipitation interval, or the sum of the areas of two or more groups of gauges that were separated by a short distance (usually less than 5 km). In cases where a second storm was active over the network, the decision to include or exclude precipitation was made using factors such as proximity to a cluster of CG lightning, how the shape of the precipitation area evolved with time, and the movement of the storm. In order to compensate for edge effects, we assumed the perimeter of the precipitation area extended 2 km beyond the locations of all gauges that detected rainfall in that precipitation interval. Figure 1 shows an example of how the spatial pattern of CG lightning and the associated precipitation area moved over the network during a storm on 28 April 1999 (storm 990428). [12] The average depth of rainfall over the precipitation area, Dp, was computed for each 5-min sampling interval by averaging the rain reported by each active gauge in the precipitation area, Dp ¼ 1X di n i ð1Þ where di is the rain depth [di = (0.254 mm) (no. tipsi)] that was reported by gauge i, and n is the number of active gauges. The precipitation volume, Vp, was computed by multiplying the value of Dp in each 5-min interval by the precipitation area, Ap, i.e., Vp ¼ Dp Ap ð2Þ D19203 The total precipitation volume, PV, was obtained by summing the values of Vp from the time of the first CG flash to the time of the last gauge tip. [13] In four of the nine storms that were analyzed (see Table 4 to follow), a small amount of precipitation was detected at one or more gauges before the first CG flash was detected, and there were no cases where the precipitation ended before the last CG flash. In storms 990428, 990509, 990610A, and 990611A, the time of the last gauge tip could not be determined precisely because the storm had moved outside the network or merged with other areas of precipitation. In these cases, the stop times were estimated from the movement of the lightning and precipitation relative to the perimeter of the network (and relative to other areas of lightning and precipitation). This procedure may have underestimated the true precipitation volume, but the rainfall rate peaked well before the rain volume became indeterminate in all cases, and the CG lightning stopped at least 10 min before the last gauge tip in all storms. [14] Sampling biases can be significant in rainfall measurements, especially when the spacing between the gauges is larger than the area of intense rainfall; indeed, it is rare for the maximum or even near-maximum rain depth to be recorded by a network of gauges [Weather Bureau, 1947; Huff and Shipp, 1969; Huff, 1970]. The spatial correlations and sampling errors to be expected in rainfall measurements in central Florida have been discussed by Woodley et al. [1975], Habib et al. [2001a], and Habib and Krajewski [2002]. If we ignore the irregular spacing of the gauges and apply the factor-ofdifference method of Woodley et al. [1975], assuming that the average area per gauge is about 14.5 km2, then we expect our measurements of area-average rainfall should be accurate to within a factor of 2 about 98% of the time, and within a factor of 1.5 about 92% of the time. [15] Figure 2 shows two examples of how the CG lightning and surface rainfall rates evolved with time during a small (Figure 2a) and a large (Figure 2b) thunderstorm. Our estimates of the total PV in these storms were 6.7 105 m3 and 3.8 106 m3, respectively, and the average PV per CG flash was 1.1 104 m3/CG flash and 2.3 104 m3/CG flash, respectively. The average rain mass per CG flash or ‘‘rain yield’’ was 1.1 107 kg/CG flash and 2.3 107 kg/CG flash in Figures 2a and 2b, respectively. 3.3. Lag Times Between Lightning and Rainfall [16] Prior studies at the KSC-CCAFS have shown that the frequency of CG flashes tends to peak a few minutes before the surface rainfall [Piepgrass et al., 1982; Tapia et al., 1998], and a similar behavior is evident in Figures 2a and 2b. The third column in Table 3 summarizes the lag times between the peak CG flashing rate (tCG,MAX) and the peak rainfall rate (tp,MAX) Dtpeak ¼ tCG;max tp;max ð3Þ (using 5-min bins) for all storms in our data set. In storms where Dtpeak is positive, the lightning rate peaked before the rainfall rate, and in the storm where Dtpeak is negative, the rainfall rate peaked before the lightning rate. 5 of 15 D19203 GUNGLE AND KRIDER: LIGHTNING AND RAINFALL D19203 Figure 2. Surface rainfall (shaded) and the number of CG flashes (black) in 5-min bins for (top) a small storm on 5 August 1998 (980810B) and (bottom) a large storm on 10 August 1998 (980805B). The times on the x-axis show the beginning of each bin. [17] We also performed a lag-correlation analysis on each storm to find the optimum lag time and to determine whether this optimum was significant. We began the lag analysis by computing a linear regression between the lightning and rainfall rates (using 5 min bins), and then we shifted the lightning rate forward and backward in 5-min steps and recomputed the regression until the best fit was obtained. Plots of the optimum fit between the lightning and rainfall rate 6 of 15 GUNGLE AND KRIDER: LIGHTNING AND RAINFALL D19203 D19203 Table 3. Lag Times Between the CG Lightning and Surface Rainfall Rates Storm Number of CG Flashes Optimum Lag Time, min Lag in Peak Rates, min PV/CG Flasha (104 m3/CG) R2 P-Value 980805A 980805B 980810B 990428 990509b 990521 990610A 990611Ab 990611B 3 167 61 42 13 12 6 81 54 10 20 10 5 (0) 5 15 (5) 5 10 20 10 5 – 5 0 – 5 0.32 1.39 0.73 2.94 (1.18) 0.69 0.80 (0.30) 0.48 1.0 0.66 0.87 0.63 0.06 0.87 0.80 0.17 0.53 – <0.0001 <0.0001 0.01 0.41 0.0021 0.041 0.13 0.0009 a PV/CGF ratios are taken from the regressions shown in Appendix A. P > 0.05; regression lags and lag in peak rates are indeterminate. b for each storm in our data set are given in Appendix A. The values of the optimum lag-time and the associated R2 and Pvalues are summarized in Table 3. The P-value indicates the likelihood that the correlation coefficient will occur at random given a normal distribution of random samples from the parent population, but because consecutive samples of both the lightning and rainfall rates are not necessarily independent, the R2 and P-values in Table 3 will tend to overestimate the true significance of the regressions. Seven of the nine storms in Table 3 had acceptable regressions, i.e., P-values less than 0.05. It should be noted, however, that the lagged correlation for storm 980805A is based on just two points and thus R2 = 1.0 and P is undefined. Because the lag in the peak rates for this storm agreed with the optimum (statistical) lag, this storm has been retained in Table 3 and included in the ‘‘All Storms’’ plot in Figure 3. [18] Figure 3 summarizes the optimum lag times as a function of the total number of CG flashes for the seven storms that had acceptable regressions. Although the dashed fit in Figure 3 is clearly positive, the fit is not conclusive (R2 = 0.16, P = 0.37). If the two storms that produced less than 12 CG flashes are omitted from the analysis, because it is likely that there were sampling biases in the rainfall measurements, then the fit in Figure 3 improves (solid line), but it is still not statistically significant (R2 = 0.56, P = 0.15). It should be noted that storm 980805B is the only storm in our dataset that produced a large number of flashes (167 versus 61 in the next largest storm). Given the small sample, this storm clearly has a strong influence on the statistics. 3.4. Precipitation Volume Per CG Flash [19] In section 3.2, we defined the total precipitation volume to be the sum of all rainfall from the time of the Figure 3. The optimum lag time versus total the number of CG flashes for the seven storms that had P < 0.05 (dashed) and for the five storms that had P < 0.05 and 12 or more CG flashes (solid). 7 of 15 GUNGLE AND KRIDER: LIGHTNING AND RAINFALL D19203 D19203 Table 4. Number of Cloud-to-Ground Flashes (NCG), Precipitation Volume (PV) Calculated Three Ways (See Text), and the PV/CG Flash Ratios Calculated for Each Method Storm 980805A 980805B 980810B 990428 990509b 990521 990610A 990611Ab 990611B Total Median Mean (PV) Mean composite PV/CG (SPV/SNCG) Mean storm PV/CG [S(PV/NCG)]/n Standard deviation PV Concurrent Lagged PV Total Total PV/CG, PV/CG Concurrent, Lagged PV/CG NCG PV, 104 m3 104 m3/CG With CGs, 104 m3 Concurrent, 104 m3 Concurrent, 104 m3/CG 104 m3/CG 3 167 61 42 13 12 6 81 54 439 42 49 2.90 384 66.9 167a 66.8a 11.2 38.1a 140a 37.8 915 66.8 102 0.97 2.3 1.1 4.0 5.1 0.93 6.4 1.7 0.70 0.32 302 55 158 67 6.5 29 101 27 746 0.11 1.8 0.9 3.8 5.1 0.54 4.9 1.2 0.51 1.7 1.3 337 61.3 151 – 9.1 29.1 – 22.5 611 1.2 83 0.43 2.0 1.0 3.6 – 0.76 4.9 – 0.42 1.0 87 2.1 1.7 1.8 2.6 2.1 1.9 2.1 2.0 1.7 a Final gauge tip indeterminate; total storm precipitation volume may be an underestimate. b P > 0.05; regression lag indeterminate. first CG flash to the time of the last gauge tip in the precipitation area. When Piepgrass et al. [1982] computed their precipitation volumes, however, they summed the rainfall between the starting and stopping times of the lightning (either intracloud or cloud-to-ground), and other investigators have integrated between the starting and stopping times of the precipitation whether lightning was present or not [e.g., Kinzer, 1974; Grosh, 1978; Goodman et al., 1988]. In order to facilitate a comparison of our results with other investigators, we have integrated the rainfall over three different intervals: (1) the entire storm as defined in section 3.2; (2) an interval that is concurrent with CG flashes, i.e., starting with the first CG flash and ending with the last CG flash; and (3) over a lagged interval that is concurrent with the CG flashes, i.e., for storms that had a significant lag-correlation, the precipitation was shifted forward (or backward) by the optimum Figure 4. Linear regression of the precipitation volume computed using methods 1, 2, and 3 (see text) versus the total number of CG flashes in nine Florida thunderstorms. 8 of 15 GUNGLE AND KRIDER: LIGHTNING AND RAINFALL D19203 D19203 Table 5. Precipitation Volume (PV) Per CG Flash and the Optimum Lag Times Between Lightning and Surface Rainfall in Florida Type of Precipitation Measurement PV Per CG Flash, 104 m3 Lag in Precipitation, min Storm-Scale Studies 5 – 10 (5 min bins) 4 – 9 (1 min bins) Comments Piepgrass et al. [1982]: KSC, FL Tapia et al. [1998]: KSC, FL Present study [2005]: KSC, FL 25 rain gauges, 250 km2 WSR-88D radar 1.8 – 2.2a 2.4 – 37b ‘‘significant lag’’ 22 storms in August 1992 and 1993 31 rain gauges, 450 km2 0.70 – 6.4b 0.11 – 5.1a 5 – 20 (5 min lag) Nine isolated storms Maier et al. [1978]: South Florida Lopez et al. [1991]: Cape Canaveral, FL Buechler et al. [1994]: South Florida WSR-57M radar 1.1 – 9.2c WSR-74 S-band radar; 0.5 angle WSI commercial radar product 0.1 – 1000d – Over 14,800 km2 and 11.75 hours 11c – Over 90,000 km2 and 2 months Large-Scale Studies – Two isolated storms Over 20,000 km2 and 24 days a Precipitation during CG flashes only. Total storm precipitation. c Multiple hour PV/CG. d Hourly PV/CG ratio. b lag time and integrated between the starting and stopping times of the lightning. [20] Table 4 summarizes the results. The values of ‘‘PV/ CG flash’’ in Table 4 do depend on which method is used to integrate the rainfall, and it should be noted that the large storms tend to have similar values of PV/CG flash and that the small storms are more variable, probably because of sampling biases in the rainfall measurements. The smallest PV/CG flash in Table 4 is 0.11 104 m3/CG flash (storm 980805A, precipitation concurrent with CG flashes), and the largest is 6.4 104 m3/CG flash (storm 990610A, total PV). (Note that the final gauge tip was indeterminate in storm 990610A; therefore this PV is likely to be an underestimate.) The values of the precipitation volume and the PV/CG flash ratios are largest when they are computed using the total PV without corrections for being concurrent with the lightning or the lag times. [21] Figure 4 shows plots of the precipitation volume, computed using each of the three ways discussed above, versus the total number of CG flashes in the storm. Although the number of storms is limited, all regressions appear to be significant with R2 values ranging from 0.75 to 0.83. The best fit in Figure 4 (solid line) is for lagged precipitation concurrent with CG flashes, and in this case the slope corresponds to 1.9 104 m3 of (lagged) precipitation per CG flash. 4. Discussion 4.1. Previous Studies in Florida [22] Table 5 summarizes the results of prior studies of lightning and rainfall in Florida and our results. Maier et al. [1978] examined the natural variability of CG flashes over an area of about 2 104 km2 in south Florida using a network of gated, wideband magnetic direction finders [Krider et al., 1976, 1980] to locate and count the lightning. Rainfall was measured using the National Weather Service (NWS) WSR-57M radar in Miami. Maier et al. found that storms that produced the highest lightning frequency also produced the lowest PV/CG flash (1.1 104 m3/CG flash), and such storms occurred on days that had normal or slightly above normal convection. The average PV/CG flash was 2.6 104 m3/CG flash, and Maier et al. noted that as the number of flashes increased, so did the rain volume (i.e., the PV/CG flash remained fairly constant) up to a total volume of 4 107 m3 to 8 107 m3. When storms produced a larger rain volume, the number of flashes tended to be lower relative to the rainfall (and the PV/CG flash ratios were larger). Lang and Rutledge [2002] have noted a similar behavior in large storms in the midwestern United States. [23] Piepgrass et al. [1982] studied one small and one large thunderstorm at the KSC-CCAFS using lightningcaused changes in the surface electric field and tippingbucket rain gauges. The storms were located directly over the rain gauge network, and the precipitation areas were about 100 km2 and 350 km2. The peak rainfall rate lagged the peak lightning rate by 4 and 10 min in the small and large storms, respectively, and the authors noted that lag times of this order are consistent with the time it takes elements of precipitation to fall from 7 to 8 km altitudes (the 10 to 20oC temperature level), where the electrification is strong, to the surface. The rain volume per total flash was 6.7 103 m3/flash and 8.5 103 m3/flash, and the estimated PV/CG flash ratios were 1.8 104 m3/CG flash and 2.2 104 m3/CG flash in the small and large storms, respectively. [24] Lopez et al. [1991] located and counted CG lightning near the KSC-CCAFS using three gated, wideband direction finders similar to those described by Krider et al. [1976, 1980] and they measured precipitation using a NWS WSR-74 weather radar at Daytona Beach. The study area was 14.8 103 km2, and the PV/CG flash ratios were computed at hourly intervals under various prevailing wind regimes. The ratios varied by 4 orders of magnitude, from 103 to 107 m3/CG flash, depending on the wind direction. The highest ratios (106 m3/CG flash) occurred on days when winds were onshore from the northeast, and 9 of 15 D19203 GUNGLE AND KRIDER: LIGHTNING AND RAINFALL the lowest (104 to 105 m3/CG flash) occurred on days when the winds were calm or there was southwesterly flow. [25] Buechler et al. [1994] measured CG lightning over south Florida using the NLDN and rainfall using a WSI composite radar product on storms that produced convective rain rates greater than 10 mm h1. The counts of CG flashes and inferred rain volumes were grouped into 15-min bins, and the best linear fit to the data (R2 = 0.86) corresponded to a PV/CG flash of 1.1 105 m3/CG flash. Buechler et al. concluded that CG lightning provided a better estimate of the location and intensity of convective rainfall than a convective stratiform technique that is based on satellite measurements of the cloud-top temperature [Adler and Negri, 1988]. [26] Tapia et al. [1998] studied 22 warm-season thunderstorms near the KSC-CCAFS using the NLDN and the WSR-88D (NEXRAD) weather radar at Melbourne, Fla. These authors found a ‘‘significant lag’’ between CG lightning and the rainfall rate in the late afternoon which they attribute to the formation of appreciable stratiform rain (without lightning) late in the storm. Values of PV/CG flash ranged from 2.4 104 to 3.6 105 m3/CG flash, with a mean of 7.1 104 m3/CG flash and the composite mean and median were 5.6 104 and 4.3 104 m3/CG flash, respectively. Storms that produced the highest lightning and rainfall rates had the lowest PV/CG flash, and the minimum PV/CG flash occurred near the maximum CG flashing rate in all storms. These authors also noted that storms with low cloud tops tend to produce a higher PV/CG flash than storms with high tops. 4.2. Precipitation Volume Per CG Flash [27] As can be seen in Table 4, our values of the PV/CG flash depend on which method is used to compute the precipitation volume, and for each method, the results vary by about a factor of 2 from storm to storm. Our mean ratios are in excellent agreement with Piepgrass et al. [1982] (2.0 104 m3/CG flash), and they are a factor of 2 to 6 lower than those of Buechler et al. [1994] and Tapia et al. [1998], probably because these authors used radar to measure rainfall, and radar estimates of surface precipitation are often different from gauge measurements [Brandes and Wilson, 1988; Joss, 1990; Brandes et al., 1999; Habib and Krajewski, 2002]. [28] Lopez et al. [1991] found a much larger variation in the values of the PV/CG flash than this or any other Florida study (a factor of 10,000 versus a factor of 10), and they attribute this to differences in the flow regime, with storms originating under a NE flow producing the highest PV/CG flash and those under a SW flow producing the lowest. The discrete 1-hour sampling intervals may also have exaggerated the extremes. 4.3. Lag Times [29] Although Tapia et al. [1998] did report ‘‘significant’’ lags between lightning and rainfall in warm-season Florida storms, Piepgrass et al. [1982] are the only other authors who have examined individual storms with enough time resolution to resolve the lag in the precipitation rate, and the lag times in Table 3 are in good agreement with D19203 Piepgrass et al. In our discussion of Table 3 and Figure 3, we have noted that storms that produce larger numbers of CG flashes tend to produce longer lag times. This was also the case in the two storms that were studied by Piepgrass et al. [1982]; their small storm had a lag time of 4 min and the large storm had a lag time of 10 min. [30] Williams et al. [1989], Rutledge et al. [1992], Zipser [1994], and Petersen and Rutledge [1998] have noted that storms that have high updraft speeds also have high CG flashing rates; therefore it is reasonable to suppose that the longer lag times in large storms are caused by higher updrafts carrying precipitation particles to higher altitudes where they are suspended longer and have a larger distance to fall to the surface. 5. Summary [31] We have examined relationships between CG lightning and surface rainfall in nine relatively isolated, warmseason thunderstorms in Florida, and we have found that there is a linear relationship between the precipitation volume and the number of CG flashes. Values of the lagged rain volume per concurrent CG flash for individual storms range from about 0.43 104 to 4.9 104 m3/CG flash, with a mean and standard deviation of 1.9 104 ± 1.7 104 m3/CG flash. When the total rain volume per CG flash is considered without a lag, the values range from about 0.70 104 to 6.4 104 m3/CG flash with a mean of 2.6 104 ± 2.1 104 m3 per CG flash. For the (unlagged) rain volume that is concurrent with CG flashes, the values range from 0.11 104 to 4.9 104 m3/CG flash with a mean of 2.1 104 ± 2.0 104 m3 per CG flash. A linear fit to the lagged rain volume vs. the concurrent CG flashes (Figure 4) suggests that the optimum PV/CG flash in Florida storms is about 1.9 104 m3/CG flash. [32] These results clearly support the suggestions of Gosz et al. [1995], Chèze and Sauvageot [1997], Holle and Bennett [1997], Soula et al. [1998], and Tapia et al. [1998] that CG lightning can be a useful tool for estimating the location and intensity of convective rainfall and for generating near real-time warnings of flash floods, especially when weather radar signals are attenuated by heavy precipitation or blocked by intervening topography. Our data also support the suggestions of Buechler et al. [1994], Tapia et al. [1998], and Alexander et al. [1999] that lightning-inferred rainfall rates and the associated latent heat transport might provide useful information for the initialization of numerical forecast models. Since relationships between lightning and rainfall in other seasons and in other geographic locations could well be different from those in warm-season storms in Florida, we believe such relationships merit further study. Appendix A [33] Figures A1– A9 show the regressions between the precipitation volume at the optimum lag time, Dt, versus the number of CG flashes in consecutive 5-min bins for each storm in our data set. 10 of 15 D19203 GUNGLE AND KRIDER: LIGHTNING AND RAINFALL Figure A1. Linear regression showing the volume of surface precipitation (5-min bins) at the optimum lag time, Dt, versus the number of CG flashes for storm 980805A. Figure A2. Same as Figure A1 except for storm 980805B. Figure A3. Same as Figure A1 except for storm 980810B. 11 of 15 D19203 D19203 GUNGLE AND KRIDER: LIGHTNING AND RAINFALL Figure A4. Same as Figure A1 except for storm 990428. Figure A5. Same as Figure A1 except for storm 990509. Figure A6. Same as Figure A1 except for storm 990521. 12 of 15 D19203 D19203 GUNGLE AND KRIDER: LIGHTNING AND RAINFALL Figure A7. Same as Figure A1 except for storm 990610A. Figure A8. Same as Figure A1 except for storm 990611A. Figure A9. 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