Electrical structure in thunderstorm convective regions 3. Synthesis

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. D12, PAGES 14,097-14,108, JUNE 27, 1998 Electrical structure in thunderstorm convective regions 3. Synthesis Madbeth Stolzenburg 1 NOAA National Severe Storms Laboratory, Norman, Oklahoma, and Cooperative Institute of Mesoscale Meteorological Studies, University of Oklahoma, Norman W. David Rust NOAA National Severe Storms Laboratory, Norman, Oklahoma Thomas C. Marshall Department of Physics and Astronomy, University of Mississippi, University Abstract. In this paper, results from nearly 50 electric field soundings through convective regions of mesoscale convective systems (MCSs), isolated supercelis, and isolated New Mexican mountain storms are compared and synthesized. These three types of thunderstorm convection are found to have a common, basic electrical structure. Within convective updrafts the basic charge structure has four charge regions, alternating polarity, and the lowest is positive. Outside updrafts of convection there are typically at least six charge regions, alternating polarity, and the lowest is again positive. Among the three storm types, there are differences in the heights and temperatures which the basic four charge regions are found in updrafts. The height (temperature) of the center of the main negative charge region averages 6.93 km (-16øC) in MCS convective region updrafts, 9.12 km (-22øC) in supercell updrafts, and 6.05 km (-7øC) in New Mexican mountain storm updrafts. In updraft soundings through all three storm types, the center height of the main negative charge region increases with increasing average balloon ascent rate and updraft speed at a rate of about 0.3 km per 1 rn s -1, with a correlation coefficient of A schematic illustrates the basic four- and six-charge structure for thunderstorm convective regions, and it is offered as an improved model for thunderstorm charge structure. 1. Introduction The electrical charge structure of thunderstorms has long been believed to be either a vertical dipole, with positive charge above negative charge, or a tripole, with a lower positive charge region added below the dipole. These charge structure models are ubiquitous both within the atmospheric electricity community and beyond it in more general literature. For example, introductory textbooks in physics and meteorology typically show the "generic" thunderstorm as having either two or three charge regions. As such, the classical dipole and tripole models are the paradigm for thunderstorm charge structure. Dipole models were first proposed by Wilson [ 1916, 1920, 1925] and Simpson [ 1909, 1927] and were based on analyses of surface measurements of electric field and precipitation charge below thunderstorms. The tripole model was originally suggested by Simpson and Scrase [1937] and Simpson and Robinson [1941] as an improvement upon (and compromise between) the dipole INow at Department of Physics and Astronomy, University of Mississippi, University. Copyright 1998 by the American Geophysical Union. Paper number 97JD /98/97 JD $09.00 analyses of Simpson [ 1927] and Wilson [ 1925]. Their tripole model was based on analyses of corona current measurements from 27 of 69 balloon flights made inside thunderstorms. These original in situ data have been supplemented over the years by various remote measurements of lightning-inferred charge regions. However, recent in situ measurements are not in accord with either the dipole or the tripole charge model. Marshall and Rust [ 1991 ] reported on 12 balloon soundings through thunderstorms; all their soundings had at least four charge regions, and some had as many as 10 charge regions. Rust and Marshall [ 1996] have recently reviewed the original 69 soundings presented by Simpson and Scrase [ 1937] and Simpson and Robinson [1941]. Rust and Marshall [1996] found that many of the 69 soundings indicate the presence of more than three charge regions. Based on this finding, on the data shown by Marshall and Rust [ 1991 ], and on other, recent, in situ thunderstorm soundings they cited, Rust and Marshall [ 1996] suggested that the tripole and dipole charge structure models were too simplistic and that a new paradigm of thunderstorm charge structure is needed. Stolzenburg et al. [this issue (a)] (hereinafter referred to as SRSM part 1) describe the typical charge structure that is found to exist in convective updrafts of mesoscale convective systems (MCSs). Stolzenburg et al. [this issue (b)] (hereinaftereferred to as SRM part 2) do the same for two types of isolated storms. This paper is third in our three-part study of the electrical structure in 14,097

2 14,098 STOLZENBURG ET AL.: ELECTRICITY IN CONVECTION, 3 convective regions of thunderstorms. In this part we compare and 3.2. Similarities in Charge Structure synthesize results from the observations of the three types of The charge analyses associated with 27 of the 33 E soundings convection which are the focus of this study: isolated supercelis, isolated New Mexican (multicell air mass) storms, and MCSs. Our convective updrafts of the three types of convection are shown in Figure 1. These 27 charge analyses are reproduced from SRSM part goal in these comparisons is to determine whether there is an 1 (MCSs) and SRM part 2 (supercelis and New Mexican storms). electrical structure typical of thunderstorm convection in general. As discussed in section 5 of SRM part 5, the soundings in or near As discussed above, this goal has long been a fundamental obj tive the center of New Mexican storm convection are not necessarily of atmospheric electricity research. We also examine relationships updraft soundings, but we group them together with the updraft between electrical structure and updraft speed in the three types of soundings of MCSs and supercelis for brevity when referring to all convection studied, and we discuss charging mechanisms relevant three storm types. Within the three groups, the updraft soundings to convective regions of thunderstorms. Finally, we compare the are arranged from left to right as follows: the MCS and supercell classical dipole and tripole models with the charge structures we convective updraft soundings are in order of decreasing average have found in thunderstorm convective regions. balloon ascent rate (with one exception of each type), and the New 2. Instruments and Methods Mexican storm soundings are in order of decreasing height of the uppermost charge region. (The sounding shown rightmost among the supercelis and the sounding shown rightmost among the MCSs are outliers in terms of this presentation, as will be described in section 4.) Soundings shown with asterisks in Figure 1 are from published work of others: [Byrne et al., 1987], [Marshall and Winn, 1982], [Winn et al., 1981], and and [Marshall and Rust, 1991]. The In SRSM part 1 we describe the instrumentation and methods used to acquire the observations and analyses discussed in this paper. Important details and subtleties of the electric field (E) data and charge analysis method are in the Appendix of SRM part 2. Our methods of determining whether a particular balloon sounding is through the convective region of a storm and whether or not it is through the updraft of that storm are described in section 3 of SRSM part 1. The latter determination is based primarily on balloon ascent rate data: since, in most cases, the balloons we use have still-airise rates of about 5 m s -i, the updraft speed in the vicinity of the sounding can be estimated by subtracting the still-air rise rate from the observed balloon ascent rate. Heights are relative to mean sea level (msl). remaining six soundings, which are not shown in Figure 1 but were used in our analysis, were published by Weber and Few [1978], Weber et al. [1982], and Byrne et al. [1983]. Those six were in New Mexican storms and have the same general features as the soundings shown. Their charge analyses are not reproduced here due to the difficulty in accurately transforming the published data into the format used in Figure 1. The repeatability of the electric field profiles among the supercelis, MCSs, and New Mexican storms is reflected in the 3. Comparison of MCS, Supercell, and New similarity of their charge structures across Figure 1. The basic charge structure in updrafts of MCS convective regions, in strong Mexican Storm Convection updrafts of supercelis, and in or near the center of convection in 3.1. Similarities in E Profiles New Mexican storms is composed of four charge regions: (1) lower positive charge, above the cloud base and often deeper than 1 km; The observations and analyses presented in SRSM part 1 and (2) main negative charge at midlevels; (3) upper positive charge; SRM part 2 indicate that a typical electric field profile does exist in and (4) uppermost negative charge, sometimes near the upper cloud thunderstorm convection. Soundings of E in updrafts of MCS convective regions, in strong updrafts of supercelis, and in or near the center of the convection of New Mexican storms have the same general features: (1) low magnitude E through cloud base; (2) larger positive E and a positive peak at low to midlevels; (3) a rapid change to large negative E, culminating in a negative E peak at middle to upper levels; (4) a transition to positive E (1-4 km above the negative peak) and a positive E peak at high levels; and (5) in the complete soundings, a rapid decrease to E 0 near the upper cloud boundary. boundary. The uppermost negative charge was not reached in any of the supercell cases nor in nearly half of the MCS cases because these soundings ended prematurely due to lightning strikes to the instruments or balloon burst. Only one of the New Mexican storm soundings (79219, from Marshall and Winn [1982]) ended below cloud top due to a lightning strike at 7.5 km. Although not depicted in Figure 1, we can generalize the charge densities of each of the four charge regions among the updraft soundings. The lower positive charge typically has relatively small In addition to the above features, many large lightning-related charge density: I pl < 1.0 nc m -3. The mai negative charge has the field changes are evident between the negative E peak and the transition to positive E, especially in the MCS and supercell cases. It largest charge density magnitude of any region in most updraft soundings, but the upper positive charge region's density is often is noteworthy that these main features of E soundings through almost as large. (By our definition, the main negative charge has convection are repeatable despite the occurrence of numerous lightning-related field changes in many cases. The features listed above form the basic E structure in updrafts of MCS convective regions, in strong updrafts of supercelis, and the largest I pazl of any negative layer in the sounding.) Maximum charge density magnitudes are less than 3 nc m -3 in all but one updraft sounding. The uppermost negative charge usually has small charge density, -0.5 to nc rn-3; the available data indicate this near the center of convection in New Mexican thunderstorms. region may be a screening layer at the upper cloud boundary, but we Soundings of E through convection but outside these updraft regions are more complex; they typically have two additional I EI peaks (one of each polarity) at low to midlevels. do not know if it is always present at cloud top in updrafts. With a few exceptions, there is apparently no screening layer near cloud base in convective updrafts.

3 STOLZENBURG ET AL.: ELECTRICITY IN CONVECTION, 3 14,099 I :::::::::::::::::::::::::::::::::::::::::::::::::::::::::,...,, ;::::: : :, : : : : : : : : : : : : :::...,, iii:,...,,... ii:.. ß,,,.,,,.., Figure 1. Charge distributions from convective updraft E soundings through three types of thunderstorms. Each vertical dashed line represents an electric field sounding (labeled with year and day along horizontal axis). Numbers below sounding identifiers are average balloon ascent rates between the surface and 12 km (or top of sounding, if below 12 km). With a few exceptions, still-air rise rates of the balloons are 5-6 m s - after 1985 and 3-4 m s - before Negative (positive) charge layers are represented by solid (open) ellipses, with larger ellipses for layers deeper than 1 km. Chargellipses are centered at the center height of each layer as analyzed with the one-dimensional approximation to Gauss's law. Surface heights are indicated by horizontal bars; upper limits of E data are shown by an upward arrowhead (if below cloud top) or small square (if above cloud top). Lower limit of E data, if not at the surface, is shown by a downward arrowhead. Short slanted lines indicate the heights of 0 ø, -10 ø, and -20øC in each sounding. Soundings identified with asterisks are from the published work of others; see SRSM part 1 and SRM part 2 for references. A more complex charge structure than that described above is found outside updrafts of MCS convective regions, outside strong updrafts of supercelis, and away from the center of the convection in New Mexican storms. This complexity is shown in the 14 charge structures in Figure 2. In most of the 16 nonupdraft soundings updrafts means that uncertainties remain about the charge structure at upper levels of supercelis. Figure 3 shows the basic charge structure we have found in convective regions of three types of storms. The dimensionless, simplified, schematic cloud is intended to represent either an considered in this study (including two others previously published isolated thunderstorm or a convective element within an MCS by Winn et al. [1978], Weber et al. [1983]) there are at least two additional charge regions of opposite polarity. In MCSs, where we have the most repeatability, these two additional charges are a positive region over a negative region; they are below the main negative charge, and have large charge density magnitudes ([ 9[ > convective region (or other multicell thunderstorm). The similarities found among the charge analyses of nearly 50 soundings through MCS convective regions, supercelis, and New Mexican storms are depicted in the simplified charge structure in Figure 3. There are four charge regions in the convective updraft, alternating 3.0 nc m-3). Charge regions outside updrafts are typically shallower in polarity, with the lowest in the cloud a positive charge region. than those within updrafts, except for the upper positive charge. The available data indicate that the lowermost and uppermost charge regions outside updrafts may be screening layers at the cloud boundaries. Figure 2 shows that some soundings of this type have more than six charge regions, even when adjacent regions of the same polarity are grouped together as one in the counting. The similarities outlined above between the electrical structures in convective regions of three types of thunderstorms indicate that Within the convection but outside the updraft we show six charge regions, alternating in polarity, with the lowest again a positive charge region. This six-charge structure is best seen in the MCS nonupdraft soundings but is also represented in several of the supercell and New Mexican soundings. However, there are more than six charge regions apparent in some soundings from outside updrafts, and there is more variability among them than among the updraft soundings. with minor modifications in the vertical and horizontal scales, the conceptual model of charge structure in the MCS convective region 3.3. Notable Differences in E and Charge Structures (Figure 8 of SRSM part 1) also applies reasonably well to supercelis and New Mexican storms. This is particularly true for the simple The charge analyses in Figure 1 indicate a tendency for charge charge structure in the updraft regions. It also holds for the more regions to be shallower (fewer regions are deeper than 1 km) in New complex structure outside the updrafts, although there is more Mexican convection than in the other two types of convection. This variability among the supercell and New Mexican nonupdraft is probably related to the fact that the New Mexican storm sampled soundings. The lack of complete soundings through supercell were shallower, with cloud top heights similar to the MCSs but

4 14,100 STOLZENBURG ET AL.: ELECTRICITY IN CONVECTION, 3 Year Day: Ave Asc: Figure 2. Charge distributions from convective region E soundings outside updrafts of three types of thunderstorms. Each vertical dashed line represents an electric field sounding (labeled with year and day along horizontal axis). Numbers below sounding identifiers are average balloon ascent rates between the surface and 12 km (or top of sounding, if below 12 km). With a few exceptions, still- air rise rates of the balloons are 5-6 m s 4 after 1985 and 3-4 m s ' before Negative (positive) charge layers are represented by solid (open) ellipses, with larger ellipses for layers deeper than 1 km. Chargellipses are centered at the center height of each layer as analyzed with the onedimensional approximation to Gauss's law. Surface heights are indicated by horizontal bars; upper limits of E data are shown by an upward arrowhead (if below cloud top) or small square (if above cloud top). Lower limit of E data, if not at the surface, is shown by a downward arrowhead. Slanted lines indicate the heights of 0 ø, -10 ø, and -20øC in each sounding. The sounding (with asterisk) from Marshall et al. [ 1995]. cloud bases about 3 km higher (due at least partly to the surface height of 3.2 km msl for the New Mexican cases rather than about 0.4 km msl in Oklahoma, where many of the other storms were intercepted). There is also slightly more small-scale variability (i.e., additional small charge layers are common within the basic charge structure) in the New Mexican soundings than in the other two types of updraft soundings. This difference may be related to the fact that New Mexican storms cover smaller areas, and hence they vary more over shorter horizontal distances within the convection. Also, each sounding may not have been within the updraft for its entire depth in each case. This is partly why we define the categories of New Mexican soundings more loosely than for the other two storm types. Beyond these minor differences, the charge structure in convection of New Mexican thunderstorms shows the same basic features found in MCSs and supercelis. We note that even though the two categories of New Mexican storm soundings were more loosely defined (see SRM part 2, section 5), this separation of soundings produces a meaningful result. There are general differences between the electrical structures in convective updrafts of the three types of convection. The most obvious differences among the three are in the heights at which the E peaks and basic four charge regions occur. For example, the tables of E characteristics in SRSM part 1 and SRM part 2 show that the average height of the lowermost positive E peak is at 8.0 km in strong updrafts of supercelis, at 5.2 km in MCS convective updrafts, and at 5.1 km in New Mexican convection. The upper positive peak is at 11.7 km in supercelis, 10.9 km in MCSs, and 9.5 km in New Mexican storms. These difference scale reasonably well with the differences in cloud top heights common in each type of convection: 16 km or higher in supercelis, roughly 14 km in MCSs, and 12 km or lower in New Mexican storms. They also support the idea that electrical structure, like thermodynamic structure, can be considered in terms of thunderstorm convective intensity. In SRM part 2 (section 3) we described how the three thunderstom types can represent three scales of convective intensity: small (New Mexican storms), medium (MCS, with organized multicell convection), and large (supercell). On the basis of characteristics listed in the tables of SRSM part 1 and SRM part 2, and on the features depicted in Figure 1, we can now state that in more convectively intense storms the electrical structure is elevated more. They are not more electrically intense in the sense that they have larger E magnitudes: they do not. The more convectively intense storms do tend to have larger and more frequent lightning-related field changes, especially in and above the region of negative E at middle to upper levels in the updraft soundings. Since updraft speed is one of the basic parameters used to outline our convective intensity scale, we examine its importance in the electrical structure of convection in section Relationship Between Electrical Structure and Updraft Speed in Convection We remarked in SRSM part 1 that the main negative charge region in MCS convective regions is apparently higher in soundings with larger ascent rates. This is most evident when looking at the 11 MCS updraft soundings together. Charge analyses of the 27 updraft soundings in the three storm types (Figure 1) also indicate

5 STOLZENBURG ET AL.: ELECTRICITY IN CONVECTION, 3 14,101 upper _.:.: -_ negative'* - ---_---_ --_---: charge... _., positive charge _- -:.'-=-.-'"-25 øc. _-.. main -25øC == _.---- negative - ==.. :-=..= -.:: - charge OøC Outside Updraft positive charge), Figure 3. Schematic of the basic charge structure in the convective region of a thunderstorm. Four charge regions are shown near the updraft, and six charge regions are shown outside the updraft within the convective precipitation region. Density of the plus (minus) signs in the cloud corresponds roughly to the typical charge density in each positive (negative) charge region, as estimated from balloon soundings of electric field and the one-dimensional approximation to Gauss's law. Charge structure shown applies to convective elements of mesoscale convective systems (MCS) convective regions, isolated supercell storms, and New Mexican air mass storms. Horizontal and vertical scales and heights of featureshown vary with the type of convection, as do the size, strength, and relative positions of the updraft and downdraft. The height and temperature of the main negative charge region in or near the updraft also vary with type of convection and are related to the updraft speed in the vicinity of the balloon sounding (estimated from balloon ascent rates). Charge structure outside the convective region is not shown. This schematic of convective region charge structure is based on 33 E soundings in convective updrafts and 16 E soundings outside convective updrafts in MCS convection, isolated supercell convection, and isolated multicell convection. The charge regions shown are the most common features indicated by those 49 soundings. Note that there is variability about this basic structure, especially outside the updrafts. that a relationship may exist between updraft speed and charge is rejected at the 95% confidence level). Comparing instead the height, and it may extend across differen types of convection. In mean height of the center of the main negative charge region, 6.93 this section we include statistical analyses to test the strength of km in the updrafts and 5.07 km in the nonupdrafts, we find that these relationships. Definitions of the statistical parameters used these are statistically different at the 99% confidence level. The throughout this section, including confidence intervals, null- idea that charge regions in updrafts are elevated with respect to their hypothesis testing, and least squares regression analysis, can be nonupdraft counterparts has been suggested by MacGorman et al. found in the work of Mendenhall and Sincich [ 1992]. All statistical [ 1989]. From our direct comparisons of charge region heights in hypotheses are examined with one-sided tests and the Student's t MCS convective regions, we can conclude that their idea is distribution appropriate to the small sample sizes available. supported by our observations. We can also perform a linear regression analysis to look for a 4.1. Charge Height Inside and Outside MCS Convective significant relationship between ascent rate and height of the main Region Updrafts negative charge region. We use the main negative charge because One straightforward test is to compare the height of charge it is clearly defined in both the updraft and the nonupdraft regions found in the MCS convective region updraft and nonupdraft soundings. (The lowest charge in the nonupdraft cases may be a soundings. The mean height of the center of the lowest charge screening layer at the cloud boundary, so it does not seem inforregion in the updrafts is 4.05 km, while in the nonupdrafts, it is 3.12 mative to compare this with the lowest charge in the updraft cases, km (Table 2 of SRSM part 1). These mean values are statistically which do not appear to have lower screening layers.) As stated different at the 95% confidence level (that is, the null hypothesis above, we use balloon ascent rate as a measure of the updraft speed that these two samples are from populations with equivalent means in the soundings. True vertical air velocity can be estimated by sub-

6 14,102 STOLZENBURG ET AL.: ELECTRICITY IN CONVECTION, 3 tracting the balloon's still-airise rate of 5-6 m s ' (in most cases) The MCS updraft soundingshow the strongest linear relationfrom the ascent rate. Our threshold for soundings in the updraft ship between charge height and average ascent rate, with r = category is an average ascent rate > 6 m s - between the surface and In other words, the linear relationship to average ascent rate 12 km. explains r 2 = 60% of the variance in charge height. This linear Figure 4 shows center height of the main negative charge versus relationship is statistically significant at 95% confidence (that is, the average ascent rate for 13 MCS convective region soundings. (Our null hypothesis that there is no dependence of main negative charge other two soundings ended below the height of the main negative height on ascent rate in the population, based on the characteristics charge and could not be included in this analysis.) Also shown are of the sample, is rejected at the 95% confidence level). The three lines fit by least squares regression to the three samples: standard deviation s about the fitted line is 0.42 km. There is one updrafts alone, nonupdrafts alone, and all soundings together. outlier, the updraft sounding, in which the main negative Linear regression analysis gives us a quantitativestimate of the charge height is more than three standardeviations from the value amount of variation in the height of the main negative charge that predicted by the least squares regression line. Among the noncan be accounted for by its linear relationship to ascent rate. This updraft soundings, there is no significant relationship between estimate is r 2, the coefficient of determination. The correlation average ascent rate and main negative charge height evident in our coefficient r is a measure of the strength of the linear relationship. data. Least squares, linear regression analysis does not tell us there is When both the updraft and the nonupdraft soundings are necessarily a physical cause-and-effect relationship between the two included in the analysis, the slope of the regression line (Figure 4) parameters. increases from 0.22 to 0.31 km per m s -1, and r decreases to This is still significant at the 95% confidence level, but the variation about the fitted line is larger (s = 0.97 km) than for the updraft All soundings: soundings alone. The nonupdraft soundings are at the low-height z = 0.31(asc) end of the linear regression, and the updraft soundings are at the lo S :0.97 km, ra:0i377 high-height end. On the basis of this analysis, we again conclude that our data in MCS convection support the idea that charge Updrafts: } regions in updrafts are elevated with respect to their nonupdraft z = 0.22(asc) + 5.,46 counterparts. Physically, the relationship between ascent rate and s =0.42 km, r2=0i599//r = 0'77! main negative charge height in convective updrafts may indicate that vertical advection of charge is important within the MCS convective region Charge Location Versus Updraft Speed in Three Types of Convection. U MCS updrafts We next compare the height and temperature of the main / / U/ ' x updraft outlier negative charge region in updrafts of supercelis, MCS convective / / o non-updrafts regions, and New Mexican thunderstorms. The mean height of the / fit to updrafts o _ / fit to non-updrafts center of the main negative charge region is 6.05 km in the New Mexican convection, 6.93 km in MCS updrafts, and 9.12 km in -Vo--- fito all points supercell updrafts. The corresponding measured mean temperatures / a x are - 7 ø, - 16 ø, and - 22øC, respectively. These mean values of height and temperature for each sample are statistically different N. o o.- u_ p.d_r.a_f_t.s.;... from the mean values for the other two samples at 2 90% confiz = O.01(asc) + 5.',43 dence level. From this we can conclude that our data supporthe r2=6.81 x 10 '5 idea, first suggested by Vonnegut and Moore [ 1958], that thunderstorms with very large updraft speeds ("giant" storms in their terminology) are different from those with smaller updraft speeds: the main negative charge region in supercell storms is at higher Ave Ascent Rate (asc, in rn s'j), surface to 12 km height and colder temperature. Statistical comparison of our data indicates that the main negative charge in supercell storms is Figure 4. Relationship between updraft speed and main negative elevated relative to that in the other storm types. (In terms of the charge height in MCS convective regions. Height of the center of number, polarity, and order of charge regions, we have already the main negative charge region is plotted against average balloon shown that supercell storms have updraft charge structures that are ascent rate below 12 km for 13 MCS convective region E soundings not different from those of other thunderstorms.) through updrafts (U) and nonupdrafts (O). Updraft speeds can be To quantify further the relationship between updraft speed and estimated by subtracting 5-6 m s '] from the balloon ascent rates. charge structure among storms with different types of convection, Least squares linear regression curves fit to each sample and to the updrafts and nonupdrafts together are also shown. Coefficients of Figure 5 shows center height of the main negative charge region determination (r2), correlation coefficients (r), and standard versus average ascent rate between the surface and 12 km for all the deviations (s) of the curves are given. The updraft outlier is more updraft soundings. Least squares regression lines are shown for than 3 standardeviations away from the predicted, least squares each convective type individually and for all the updraft soundings regression height for the updraft sample. taken together. The regression analysis for the MCS convective

7 STOLZENBURG ET AL.' ELECTRICITY IN CONVECTION, 3 14, All'uPdraftS:....-n' a '",//' ' ' z = 0.31(asc) 'i - ' s=0.51 km, r =0.8',,,79 S f. "r=0.86 SupercelIs: :: /' z = o(asc) ß... <---i... M... :... ß - ::. v. ):: -X- MCS outlier r=o.7z ' 'i - fit to MCSs.,4(, ' S supercell //Y " :i ß supercell outlier /. ;,' '.- fitto supercelis /'/AA A NM stoa'ms / ',,,,., fi t to N M storms r=0.86 x New Mexican ston ns: '...,...,... i Fito all points the main negative charge among the soundings J differentypes of thunderstorms. As shown height increases by 0.31 km for every 1 m s - increase average... / 0.37 km per m s 'l. The standard deviation about the least squares Ave Ascent Rate (asc, in m S'1), surface to 12 km rates for these. For reasons described in SRM part 2, we have added 2 m s 'l to the average ascent rates for the and soundings (acquired with polyethylene balloons) to compare them with the recent soundings (acquired with rubber balloons). The resulting linear relationship for the New Mexican storms is significant at the 95% confidence level despite the small sample size. Comparison of the three regression lines indicates that the three types of convection have similar relationships between charge height and ascent rate. Slope values for the three lines are not different (at 80% confidence level). For example, the 95% confidence interval for the slope of the line fit to the MCS updrafts is km per m s -1, which encompasses the slopes of the other two lines. The intercept values are different at 80% confidence (but not at higher confidence), as are the predicted height values at an average ascent rate of 5 m s -1(i.e., roughly zero updraft). Regression analysis for all the updraft soundings together indicates a strong relationship between center height of the main negative charge region and average ascent rate. The linear relationship accounts for 88% of the variation in the height of the center of through three in Figure 5, the charge ascent rate; the 95% confidence interval of the line's slope is line is 0.51 km, and the two outliers have main negative charge heights that are more than 3 standard deviations away from their predicted values. In general, the regression line for all three storm types together does not fit the New Mexican storms as well as it does the MCSs and supercelis. However, the three storm types clearly take up Figure 5. Relationship between updraft speed and main negative charge height in convective updrafts. Height of the center of the different positions along the line. The supercelis are at the high end, the New Mexican storms are at the low end, and the MCSs are main negative charge region is plotted against average balloon intermediate. This distribution leads us to conclude that our data ascent rate below 12 km for eight MCS convective region updraft supporthe idea [Vonnegut and Moore, 1958] that supercell (or soundings, seven supercell strong updraft soundings, and five "giant") storms are different from other ("typical") storms in terms soundings near or through convective updrafts of New Mexican of the height of the center of the main negative charge region. This storms. Updraft speeds can be estimated by subtracting 5-6 m s 4 from the balloon ascent rates. Least squares linear regression curves difference is linearly related to the difference in updraft speed, as fit to each sample and to all the updrafts together are also shown. Coefficients of determination (r2), correlation coefficients (r), and standard deviations (s) of the curves are given. The MCS and supercell outliers are more than three standardeviations away from estimated by the average balloon ascent rate in our soundings. A similar but weaker relationship is evident in Figure 6, where we show the temperature at the center of the main negative charge versus average ascent rate for all the updraft soundings. Least the predicted, least squares regression height for their respective squares linear regression results in r = 0.64 and slope = - 1.0øC per samples. m s -1, which are significant at the 95% confidence level. However, there is considerable scatter about the fitted line (s = 6.2 øc), even though no clear outliers exist. In contrast to the height analysis updrafts, with r = 0.77 and slope = 0.22, is the same as shown in Figure 4. The supercell updrafts indicate a strongerelationship, with r = 0.86, between charge height and updraft speed. This relationship is statistically significant at 95% confidence. There is one outlier among the supercelis, the sounding, which has a charge height more than three standardeviations (s = 0.43 km) away from the predicted value. This result may be partly due to the fact that the sounding ends within the main negative charge region; the center height is probably biased downward because we have used the center of the portion of this region seen in the incomplete sounding. The New Mexican storm soundings also indicate a strong relationship between charge height and updraft speed, with r = We include only five of the New Mexican soundings from Figure 1 (where r 2 = 88%), only 41% of the variance in the center temperature of the main negative charge can be accounted for by the linear relationship to average ascent rate. This weaker relationship between temperature of the main negative charge and average ascent rate suggests that the thermodynamic profiles, like the charge, are affected by the updraft. The finding that there is, nonetheless, a significant linear relationship between temperature of the negative charge and average ascent rate suggests that the temperature profile is not elevated as much as the main negative charge within the updraft. Figure 6, like Figure 5, shows that the New Mexican convection soundings are generally at the low end (warmer temperature and slower average ascent rates) of the relationship, most of the MCS updrafts are near the middle, and a few of the supercell soundings because we have the best information about balloon still-air rise are at the high end. If we look beyond the linear relationship,

8 ,, 14,104 STOLZENBURG ET AL.: ELECTRICITY IN CONVECTION, 3 E 'B -20 o ) -10 E All UPd;a;ts:... T =- 1. O(asc)- 5.2 s =6.2øC, r2= '" "- i '... []...! e... ß i.64 ß ß ß ß supercelis ß ß NM storms a fit to all points ß Ave Ascent Rate (asc, in m S'1), surface to 12 km Figure 6. Relationship between updraft speed and main negative charge temperature in convective updrafts. Temperature at the center of the main negative charge region is plotted against average balloon ascent rate below 12 km for the same 20 convective updraft soundings as shown in Figure 5. (Note that temperature decreases upward on the vertical axis.) Updraft speeds can be estimated by subtracting 5-6 m s - from the balloon ascent rates. Least squares linear regression curve fit to all the updraft soundings together is also shown. Coefficient of determination (r2), correlation coefficient (r), and standardeviation (s) of the curve is given. There are no outliers in any of the individual samples. ated in all storms at -20øC by a mechanism that puts negative charge on the larger particles. In regions of weak updrafts in the cloud, this charge would precipitate through the updraft and would thus be located at warmer temperatures, as we find in the New Mexican storms. In stronger updraft regions, the negative charge would levitare near -20 øc or even rise to colder temperatures. Thus we cannot conclude that the main negative charge is not generated a particular temperature simply because it is not always centered at a particular temperature in the different updraft soundings. In section 5 we discuss some specific charging processes that may play a role in convective regions. 5. Charging Mechanisms in Convection Our observations provide information and constraints about the typical net result of charging in thunderstorm convective regions. We do not have specific data regarding charging processes or the histories or makeup of the charge carriers. In this section we discuss some of the possible charging mechanisms that seem especially applicable, based on our analyses and interpretations. This is not an exhaustive discussion. Rather, it is intended to provide focus for future laboratory investigations of charge transfer and for modeling studies of charge separation in convection. In updraft regions, the noninductive collisional charging of ice crystals and graupel [e.g., Reynolds et al., 1957; Takahashi, 1978; Jayaratne et al., 1983; Saunders et al., 1991] may be able to account for the lower three of the four basic charge regions (Figure 3). (Noninductive mechanisms are those which do not require or depend on an electric field.) For much of the past decade this 0 mean =-6.6øC Figure 6 also shows a tendency for the main negative charge in 0 s=1'3ø[3' var=l'7 [ l updrafts of MCS convective regions and supercelis to be found between - 11ø and -22øC, while in New Mexican storms, the main negative charge is found between -4 ø and -8øC.(Recall thathe mean temperatures of the main negative charge in MCSs, supercelis, E z and New Mexican storms are - 16 ø, -22 ø, and -7 øc, respectively, and that these mean values are statistically different for at least the 90% confidence level.) Figure 7 is a histogram, with 2øC bins, of the temperature the center of the main negative charge region in updraft soundings. The T (øc) at center of main negative charge region mode is -17 ø to -19' C, but a secondary mode corresponding to New Mexican storms is also apparent at -5 ø to -7øC. The data in Figure 7. Histogram of temperature at the center of the main negative charge region for soundings through convective updrafts Figure 7 may indicate that the charging mechanism responsible for of MCSs, supercelis, and New Mexican storms. Data are grouped the main negative charge is different in some storms or is not in 2øC bins centered at the temperature of the bar. (For example, associated with a particular temperature. However, it is also mode is shown at-18øc but includesoundings with temperatures plausible that the charging mechanism responsible for the main at the center of the main negative charge between -17 ø and -19øC.) negative charge is tied to a particular temperature or range of temperatures. For example, suppose the negative charge is gener- Mean, standardeviation, and variance in each sample and in the three samples combined are given along the left. 'ID All Upd!afts: ' mean ', -15.7øC s=7.9ø'c, var=62.18 Superc 11s: o mean :.=-22.2 C s=6'8ø ' Mar=45'67... i... mean s=5.2ø C, Mar=27.14 NM storms: L

9 STOLZENBURG ET AL.: ELECTRICITY IN CONVECTION, 3 14,105 particular mechanism has been used to explain the classical tripole are continuously being swept away from the updraft center in the charge model of thunderstorms. The reasoning is as follows: At divergent outflow and are being replaced by new (uncharged) temperatures colder than the charge reversal temperature (varying particles from below. This idea comes from aircraft measurements from about -10 ø to -20 ø C, depending primarily on liquid water of nonzero E values above convective turrets [e.g., Gish and Wait, mixing ratio), the smaller ice crystals obtain a net positive charge; 1950; Stergis et al., 1957; Vonnegut et al., 1966; Blakesleet al., these may compose the upper positive charge region. The graupel 1989]. However, we can easily imagine (as Grenet [1947] and charges negatively and may compose the main negative charge Vonnegut [1955] have suggested) that this inefficient screening region. The lower positive charge may be the result of the same process could yield prodigious amounts of negative charge at upper process occurring below (warmer than) the charge reversal temp- levels in and adjacento active convection. erature, whereby the larger particle is charged positively. The The same process of screening layer formation may also be uppermost negative charge in the convective region cannot be occurring at the lower cloud boundary if the updraft is strong explained by using this noninductive collisional charging mech- enough to continually move the particles (to which the attracted anism, hence our appeal to screening layer formation for this region ions attach) upward into the cloud. In this case, E below the cloud near the upper cloud boundary. boundary would need to be positive (as it often is) to attract positive Other noninductive mechanisms, such as depositional-growth ions upward and (in turn) to account for the lower positive charge charging of ice crystals [e.g., Dong and Hallett, 1992] and melting region. This idea of screening layer formation followed by charged charging [Dinger and Gunn, 1946; Drake, 1968], may also be at cloud particle motion is intimately connected with the so-called work in some parts of convective clouds. convective charging mechanism, which we discuss next. There are also inductive charging mechanisms that can account Convective charging of thunderstorms [e.g., Wilson, 1920, 1956; for charge in convective updrafts. For example, rebounding Vonnegut, 1955; Phillips, 1967; Chiu and Klett, 1976; Wagner and collisions between graupel and cloud water droplets in the presence Telford, 1981 ] is another candidate for the charge in at least part of of a nonzero E can result in charge transfer [Aufdermauer and the convective region. With the convective mechanism, charge Johnson, 1972]. When E is positive, as it is at low to midlevels in from the surrounding clear air moves into and is collected by the the updraft soundings, the falling graupel retains positive charge, cloud. These ions quickly attach to cloud particles, then convective while the ascending or levitating droplets carry away negative motions (e.g., updrafts, downdrafts, outflows) distribute the charged charge. This mechanism can at least partially explain the lowest particles through the cloud. The previous paragraph discusses one two charge regions (though perhaps only partially, because an initial way in which the convective mechanism, in conjunction with E is needed before inductive mechanisms can proceed). Alterna- screening layer formation, can contribute to positive charge at low tively, this inductive charging mechanism can be used to explain the levels in convective updrafts. In addition, this mechanis may main negative and upper positive charge layers if the rebounding account for the lower positive charge region in the updrafts if collisions between graupel and droplets occur in the region of predominantly positive ions are swept up by the storm's inflow negative E at midlevels. In this scenario, the main negative charge (instead of being attracted by the electric field as described in the is composed of the negatively charged graupel, while the upper previous paragraph) and become attached to hydrometeors in the positive charge region is composed of the droplets. Although this updraft. Positive ions may become available below the cloud in the mechanism does not work at heights above which all the cloud following way: when E is positive at the ground, as it is in most of particles are frozen, it can still affecthe charge distribution at upper our updraft soundings, positive ions are produced by corona from levels if the charged particles (droplets or frozen droplets) continue grounded objects [e.g., Standler and Winn, 1978; Chauzy and to move upward in the updraft after becoming charged. Another Raizonville, 1982]. In New Mexican convection soundings (e.g., inductive charging process that may be important in parts of Figure 4 of SRM part 2) this process forms a distinct corona charge convective clouds is the breaking drop mechanism [Simpson, 1909]. layer just above the surface. We do not observe corona layers near A common criticism of inductive charging mechanisms is that the surface in most strong updraft soundings, perhaps because the they cannot yield the initial electrification of thunderstorms. corona ions are being swept into the storm by the stronger surface- However, it seems reasonable that inductive mechanisms can play level winds and updrafts in these cases. In the few updraft an important role in developing the observed complex charge soundings where E at the ground and at the lower cloud boundary structures after some other mechanism has resulted in sufficient is zero or negative, the convective charging mechanism does not charging for E to build up inside the cloud. It is our opinion that readily explain the formation of the lowermost positive charge more laboratory and modeling research is needed to improve the region. understanding of inductive charging mechanisms. Near the top of the cloud, the convective mechanism probably Screening layer formation [Grenet, 1947; Vonnegut, 1955] may moves upper screening layer charge downward via upper level explain the uppermost negative charge layer in convective regions. downdrafts and turbulent mixing, thus contributing to the apparent By this process, negative ions in the clear air are attracte down- increased depth of the upper negative charge region outside the ward to the cloud when E at the cloud boundary is positive. These updrafts (e.g., see Table 2 of SRSM part 1). ions attach to particles at the cloud boundary and form a charge As mentioned in SRM part 2, the four charge regions we find in layer that is typically shallow due to reduced conductivity [Brown convective updrafts are similar to those modeled in the updraft of a et al., 1971; Klett, 1972]. If the screening particles remain at the supercell storm by Ziegler and MacGorman [ 1994]. Their simulacloud boundary, E above the cloud boundary is rapidly decreased tion accounted for noninductive graupel-ice collisional charging, to zero. inductive collisional charging, screening layer formation, and intra- Screening layers at the extreme top of convective updrafts are cloud lightning discharging in a three-dimensional kinematic cloud thoughto have low efficiency (i.e., they cannot keep E outside the model. Their results point toward the need for more than a single cloud boundary zero) because the particles to which the ions attach charging mechanism to explain even the simplest of observed

10 14,106 STOLZENBURG ET AL.: ELECTRICITY IN CONVECTION, 3 charge structures. Outside the updrafts, the charge structure is more complex, and charging mechanisms must be able to account for at least two additional charge regions, one of each polarity, at low to midlevels. In the nonupdraft soundings at least six charge layers are typically present. Perhaps the two additional charge regions often found between the main negative and the lower positive charge regions are not evident in the updraft soundings because the oppositely charged particles have not had sufficient time to undergo gravitational separation. The particles comprising the two lowest negative charge layers and the positive layer between them (outside the updraft) may remain mixed together in the updraft, resulting in smaller charge densities within the main negative charge region there. This speculation implies the particles comprising the additional (lower) negative charge layer in the nonupdraft soundings are larger than those comprising the additional positive charge layer just above it. It also implies that additional charging mechanisms may be working in the updrafts, but their effects are hidden in the analysis of net charge density from E soundings. Effects of additional charging mechanisms may be discernible from in situ measurements of the charge carded by individual cloud and precipitation particles. It is also plausible that the two additional charge regions are missing in the updraft soundings because the charging process(es) responsible for them are not so efficienthere as they are outside the updrafts. Inductive mechanismseem particularly applicable here, since E is much larger at midlevels outside the updrafts than within updrafts. Also, screening layer formation at the lower cloud boundary may be responsible for the lower positive charge region outside updrafts. The presence of melting precipitation and breaking droplets may be important microphysical differences that yield more complex charge structures outside updrafts of convective regions. As described by Stolzenburg et al. [ 1994] and Shepherd et al. [1996] in the context of MCSs, the importance of these microphysical differences to electrical charging may extend outside the convective region to the stratiform region. Lightning deposition of charge on hydrometeors is another charging mechanism that may create additional charge regions in thunderstorms. 6. Comparison of Observed Charge Structures and Classical Models In this section we compare the charge structures derived from our 49 thunderstorm soundings to the classical tripole model of thunderstorm charge structure [Simpson and Scrase, 1937; Simpson and Robinson, 1941]. The tripole model is composed of the classical dipole of a main negative charge below a main (upper) positive charge [Wilson, 1925] plus a lower positive charge. The tripole model originated from 27 balloon soundings in storms, but 44% of the soundings used did not sample an entire storm's depth, and all provided only a rough indication of field strength and height [Simpson and Scrase, 1937; Simpson and Robinson, 1941; Rust and Marshall, 1996]. Many of the subsequent "confirmations" of the tripole model are based on lightning field-change data [e.g., Krehbiel, 1986; Koshak and Krider, 1989]. However, analyses of these remote measurements assume that only one or two point charges are involved in a lightning flash [e.g., Marshall and Rust, 1991 ], so they are inherently biased toward the dipole or tripole model. Our modem balloon soundings, 60% of which sampled the entire storm depth, should provide good means of investigating the completeness and appropriateness of the tripole model. Our observations in updrafts of convective regions, represented by Figure 1, indicate a basic charge structure composed of four charge regions over a wide range of updraft speeds in all three types of convection studied. It is likely that the lower three charge regions we observe correspond to the tripole. However, none of our 17 charge analyses of updraft soundings that sampled the entire cloud depth show only the three charges of the classical tripole. Furthermore, eight of the incomplete soundings through updrafts indicate at least four charge regions. Thus in updrafts, the tripole model gives most of the picture, missing only the upper negative charge region. Our observations outside updrafts in convective regions, represented by Figure 2, indicate a basic charge structure composed of at least six charge regions. Probably the tripole is embedded within these six regions (in this study, we use nomenclature as if it is), but the tripole model seems a poor description of what is observed. It seems possible, from the discussion by Simpson and Robinson [ 1941 ], that the tripole model was not originally intended to be applicable to all parts of a thunderstorm. Earlier results have shown that it is clearly not applicable to anvil clouds [e.g., Byrne et al., 1989; Marshall et al., 1989] and stratiform regions of MCSs [e.g., Marshall and Rust, 1993; Stolzenburg et al., 1994]. The results in this paper indicate that the tripole model is also not applicable outside the updraft of convective regions. As discussed in section 5, there is evidence to suggesthat the noninductive graupel-ice collisional mechanism could account for the three charge regions in the tripole model. If the charge structure is simply a tripole, then perhaps only one mechanism needed. Our in situ soundingshow four charge regions in the updraft, and at least two mechanisms are needed to explain those four charge regions. Outside the updraft, with at least six charge regions to explain, even more charging mechanisms will be needed. Thus in the search for a more complete understanding of the mechanisms important in electrifying thunderstorms, it seems that an accurate description of the charge structure is a necessary prerequisite. This paper's anonymous reviewers have stated that our observations supporthe tripole model because we showed that there is a "tripole plus one" charge structure. It seems to us that the observations outside updrafts are equally important, where the description must be a "tripole plus three" charge structure. To us, it does not make sense to stick with the "tripole plus however-manymore-are-really-found" description. Our data support earlier findings [Marshall and Rust, 1991; Rust and Marshall, 1996] that the tripole model is incomplete. 7. Summary and Conclusions This paper is the culmination of a three-part study of electrical structure in convective regions of thunderstorms. In the preceding two parts of this study [Stolzenburg et al., this issue (a) and (b)] we presented electric field soundings and charge analyses for three different types of convection: organized multicell, isolated supercell, and multicell air mass. In this paper, we compare and synthesize those results to develop a more general understanding of electricity in convection. From this examination of 49 electric field soundings through convective regions of mesoscale convective systems, supercelis, and New Mexican mountain storms, we conclude that a typical electrical structure does exist in convection. Within convective updrafts the basic charge structure has four charge regions, alternating in polarity, and the lowest is a positive charge region. Outside updrafts of convection there are typically at

11 STOLZENBURG ET AL.: ELECTRICITY IN CONVECTION, 3 14,107 least six charge regions, alternating in polarity, and the lowest is Byrne, G. J., A. A. Few, and M. F. Stewart, Electric field measurements within a severe thunderstorm anvil, J. Geophys. Res., 94, , again a positive charge region. We have developed a schematic (Figure 3) to illustrate the charge structure that we have found to be Chauzy, S., and P. Raizonville, Space charge layers created by coronae at typical of the thunderstorm convective region. ground level below thunderclouds: Measurements and modelling, J. Among the three storm type studied, there are differences in the Geophys. Res., 87, , Chiu, C. S., and J. N. Klett, Convectiv electrification of clouds, J. Geophys. heights and temperatures which the charge regions are found in Res, 81, , updrafts. Compared to MCS convective region updrafts, the center Dinger, J. E., and R. Gunn, Electrical effects associated with a change of state of the main negative charge is higher in supercelis and lower in of water, Terr. Magn. Atmos. Electr., 51, , New Mexican storms. Similarly, the main negative charge region Dong, Y., and J. Hallett, Charge separation by ice and water drops during growth and evaporation, J. Geophys. Res., 97, 20,361-20,372, tends to be at colder temperatures in supercelis and warmer Drake, J. C., Electrification accompanying the melting of ice particles, Q. J. temperatures in New Mexican storms than it is in MCS convective R. Meteorol. Soc., 94, , region updrafts. Gish, O. H., and G. R. Wait, Thunderstorms and the Earth's general The differences in charge locations among the three thunder- electrification, J. Geophys. Res., 55, , Grenet, G., Essai d'explication de la charge 61ectrique des nuages d'orages, storm types can be partly accounted for by the different ranges of Ann. Geophys., 3, , updraft speeds that are typically present in each type. Our statistical Jayaratne, E. R., C. P. R. Saunders, and J. Hallett, Laboratory studies of the investigation of the relationship between charge structure and charging of soft hail during ice crystal interactions, Q. J. R. Meteorol. Soc., 109, , updraft speed shows that the center height of the main negative Klett, J. D., Charge screening layers around electrified clouds, J. Geophys. charge region increases with increasing average balloon ascent rate Res., 77, , and updraft speed (at a rate of about 0.3 km per 1 m s 4) in Koshak, W. J., and E. P. Krider, Analysis of lightning field changes during soundings through New Mexican storms, MCS convection, and active Florida thunderstorms, J. Geophys. Res., 94, , Krehbiel, P. R., The electrical structure of thunderstorms, in The Earth's supercelis. The center temperature of the main negative charge Electrical Environment, edited by E. P. Krider and R. G. Roble, pp. 90- region decreases with increasing average balloon ascent rate and 113, Natl. Acad. Press, Washington, D.C., updraft speed, although this relationship is less pronounced than MacGorman, D. R., D. W. Burgess, V. Mazur, W. D. Rust, W. L. Taylor, and that for charge height. The correlation coefficient for the B. C. Johnson, Lightning rates relative to tornadic storm evolution on 22 temperature-ascent rate relationship is 0.64 and is 0.94 for the May 1981, J. Atmos. Sci., 46, , Marshall, T. C., and W. D. Rust, Electric field soundings through thunderheight-ascent rate relationship. storms, J. Geophys. Res., 96, 22,297-22,306, The classical tripole model of thunderstorm electrical structure Marshall, T. C., and W. D. Rust, Two types of vertical electrical structures is clearly represented as the lowesthree of the four charge regions in stratiform precipitation regions of mesoscale convective systems, Bull. Am. Meteorol. Soc., 74, , we find in convective updrafts. The tfipole charges are not so easily Marshall, T. C., and W. P. Winn, Measurements of charged precipitation in recognizable in the six-charge structure we find outside updrafts in a New Mexico thunderstorm: Lower positive charge centers, J. Geophys. convective regions, but they are probably contained within it. Res., 87, , Although the tripole charges are important in the thunderstorm Marshall, T. C., W. D. Rust, W. P. Winn, and K. E. Gilbert, The electrical electrical structure, we believe it is obvious that the tripole model structure in two thunderstorm anvil clouds, J. Geophys. Res., 94, , is an incomplete description of the charge structure in convection. Marshall, T. C., W. Rison, W. D. Rust, M. Stolzenburg, J. C. Willett, and W. We offer our four- and six-charge conceptual model as an improved P. Winn, Rocket and balloon observations of electric field in two thunmodel for thunderstorm charge structure. derstorms, J. Geophys. Res., 100, 20,815-20,828, Mendenhall, W., and T. Sincich, Statistics for Engineering and the Sciences, 963 pp., Macmillan, Indianapolis, Indiana, Acknowledgments. This paper is based on the Ph.D. dissertation of the Phillips, B. B., Convected charge in thunderstorms, Mon Weather Rev., 95, lead author. We gratefully acknowledge the other members of her doctoral , committee, William Beasley, Frederick Carr, Stewart Ryan, and Bradley Reynolds, S. E., M. Brook, and M. F. Gourley, Thunderstorm charge Smull, for their time and beneficial suggestions. Brad Smull's efforts separation, J. Meteorol., 12, 1-12, especially have improved the quality of this work. Many people have Rust, W. D., and T. C. Marshall, On abandoning the thunderstorm tripolecomprised the balloon launch crews and otherwise helped collecthe data charge paradigm, J. Geophys. Res., 101, 23,499-23,504, used in this study; their contributions of time and hard work are greatly Saunders, C. P. R., W. D. Keith, and R. P. Mitzeva, The effect of liquid water appreciated. 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Smull, Horizontal concentrations of charged regions of four thunderstorms during TRIP distribution of electrical and meteorological conditions across the 1981 based upon in situ balloon electric field measurements, Geophys. stratiform region of a mesoscale convective system, Mon. Weather Rev., Res. Lett., 10, 39-42, , , Byrne, G. J., A. A. Few, M. F. Stewart, A. C. Conrad, and R. L. Torczon, In Stolzenburg, M., W. D. Rust, B. F. Smull, and T. C. Marshall, Electrical situ measurements and radar observations of a severe storm: Electricity, structure in thunderstorm convective regions, 1, Mesoscale convective kinematics, and precipitation, J. Geophys. Res., 92, , systems, J. Geophys. Res., this issue (a).

12 14,108 STOLZENBURG ET AL.: ELECTRICITY IN CONVECTION, 3 Stolzenburg, M., W. D. Rust, and T. C. Marshall, Electrical structure in thunderstorm convective regions, 2, Isolated storms, J. Geophys. Res., this issue (b). Takahashi, T., Riming electrification as a charge generation mechanism in thunderstorms, J. Atrnos. Sci., 35, , Vonnegut, B., Possible mechanism for formation of thunderstorm electricity, Geophys. Res. Pap. 42, AFCRC-TR , pp , Air Force Cambridge Research Center, Bedford, Mass., Vonnegut, B., and C. B. Moore, Giant electrical storms, in Recent Advances in Atmospheric Electricity, edited by L.G. Smith, pp , Pergamon, Tarrytown, N.Y., Vonnegut, B., C. B. Moore, R. P. Espinola, and H. H. Blau Jr., Electrical potential gradients above thunderstorms, J. Atmos. Sci., 23, , Wagner, P. B., and J. W. Telford, Cloud dynamics and an electric charge separation mechanism in convective clouds, J. Rech. Atmos., 15, , Weber, M. E., and A. A. Few, A balloon-borne instrument to induce corona currents as a measure of electric fields in thunderclouds, Geophys. Res. Lett., 5, , Weber, M. E., H. J. Christian, A. A. Few, and M. F. Stewart, A thundercloud electric field sounding: Charge distribution and lightning, J. Geophys. Res., 87, , Weber M. E., M. F. Stewart, and A. A. Few, Corona point measurements in a thundercloud at Langmuir Laboratory, J. Geophys. Res., 88, , Wilson, C. T. R., On some determinations of the sign and magnitude of electric discharges in lightning flashes, Proc. R. $oc. London, $er. A, 92, , Wilson, C. T. R., Investigations on lightning discharges and on the electric field of thunderstorms, Phil. Trans. R. Soc. London, Ser. A, 221, , Wilson, C. T. R., The electric field of a thundercloud and some of its effects, Proc. R. Soc. London, 37, 32D-37D, Wilson, C. T. R., A theory of thundercloud electricity, Proc. R. Soc. London, Ser. A, 236, , Winn, W. P., C. B. Moore, C. R. Holmes, and L. G. Byerley III, Thunderstorm on July 16, 1975, over Langmuir Laboratory: A case study, J. Geophys. Res., 83, , Winn, W. P., C. B. Moore, and C. R. Holmes, Electric field structure in an active part of a small, isolated thundercloud, J. Geophys. Res., 86, , Ziegler, C. L., and D. R. MacGorman, Observed lightning morphology relative to modeled space charge and electric field distributions in a tornadic storm, J. Atrnos. Sci., 51, , T. C. Marshall and M. Stolzenburg, Department of Physics and Astronomy, University of Mississippi, University, MS ( marshall@ beauty 1.phy.olemiss.edu; beauty 1.phy.olemiss.edu) W. D. Rust, NOAA National Severe Storms Laboratory, 1313 Halley Circle, Norman, OK ( drust@nsslgate.nssl.noaa.gov) (Received January 6, 1997; revised August 25, 1997; accepted December 3, 1997)