[1] Three-dimensional lightning mapping observations are compared to cloud charge structures and electric potential profiles inferred from balloon soundings of electric field in New Mexico mountain thunderstorms. For six individual intracloud and cloud-to-ground flashes and for a sequence of 36 flashes in one storm, the comparisons consistently show good agreement between the altitudes of horizontal lightning channels and the altitudes of electric potential extrema or wells. Lightning flashes appear to deposit charge of opposite polarity in relatively localized volumes within the preexisting lower positive, midlevel negative, and upper positive charge regions associated with the potential wells. The net effect of recurring lightning charge deposition at the approximate levels of potential extrema is to increase the complexity in the observed storm charge structure. The midlevel breakdown of both normal intracloud flashes and negative cloud-to-ground flashes is observed to be segregated by flash type into the upper and lower parts of the deep potential well associated with the midlevel negative charge. The segregation is consistent with perturbations observed in the bottom of the negative potential well due to embedded positive charge that was probably deposited by earlier flashes. It is also consistent with an expected tendency for vertical breakdown to begin branching horizontally before reaching the local potential minimum. The joint observations reconcile the apparent dichotomy between the complex charge structures often inferred from balloon soundings through storms and the simpler structures often inferred from lightning measurements.

[1] Simulations of relativistic runaway breakdown (RRB) are performed as functions of both time and space, resulting in explicit calculations of e-folding lengths (l) and times (t). The ratio of l to t agrees well with the speed of the avalanche, which ranges from 2.61 Â 10 8 m s -1 to 2.72 Â 10 8 m s -1 . Thus, using the speed of light, c, for the ratio of l to t can cause a 10% error when estimating l from t. A 10% error in l will cause a factor of three error in the predicted number of runaway electrons for every ten estimated e-foldings. In addition, previous models that predict peak radiated electric fields from RRB have used avalanche speeds of 0.987c and higher. Using a propagation speed of 0.89c causes a dramatic change in the predicted beaming pattern of electromagnetic radiation caused by RRB in these models. Citation: Coleman, L. M., and J. R.Dwyer (2006), Propagation speed of runaway electron avalanches,

[1] In situ electric field (E) measurements and inferred lightning initiation locations of three cloud-to-ground flashes are used to identify a thunderstorm region in which the preflash E exceeded the threshold for runaway breakdown. The maximum measured E in the region was 186 kV m À1 at 5.77 km altitude, which for runaway electrons is equivalent to 370 kV m À1 at sea level; this E value is $130% of the estimated threshold for an avalanche of runaway electrons. In addition, the volume where E exceeded the runaway threshold was estimated to be 1 -4 km 3 , with a vertical depth of about 1000 m. At least within part of this volume the characteristic scale height for exponential growth of runaway electrons was 100 m or less. Thus for these three flashes the electric field conditions necessary for runaway breakdown existed, and runaway breakdown could have initiated the flashes.

Characteristics of lightning flashes in three storms are compared to simultaneous electric field (E) measurements at various altitudes to examine three hypotheses. The first is the idea that horizontal lightning branches propagate at altitudes near potential extrema (or wells). The analyses show that horizontal lightning activity and potential extrema are coincident in time and altitude, and so are consistent with the idea that lightning moves charges into potential wells as a means of using a storm's electrostatic energy to drive a lightning flash's dielectric breakdown processes. Second, these data are used to verify the usual interpretation of breakdown polarity of lightning radiation sources detected by the Lightning Mapping Array. The third hypothesis investigated is that normal cloud‐to‐ground flashes have a period of preliminary breakdown if and only if a potential well for negative charge exists between the altitudes of flash initiation and ground. The analyses show that in 14 flashes when a low‐level well was indicated, the period of preliminary breakdown before the first return stroke lasted an average of 117 ms, considerably longer than the average lifetime of a stepped leader. In 15 flashes in which no low‐level well was indicated, the time between initiation and first return stroke averaged 15 ms.

The lightning data that are recorded with a three-dimensional lightning mapping array (LMA) are compared with data from an electric field change sensor (in this case a flat-plate antenna operated both as a “slow” and a “fast” antenna). The goal of these comparisons is to quantify any time difference that may exist between the initial responses of the two instruments to a lightning flash. The data consist of 136 flashes from two New Mexico thunderstorms. It is found that the initial radiation source detected by the LMA usually precedes the initial response of both the slow and fast antennas. In a small number of cases, the flat-plate antenna response precedes the initial LMA source, but by no more than 2 ms. The observations of such a close time coincidence suggest that the first LMA radiation source of each flash was located at or very near the flash-initiation point. Thus, the first LMA radiation source and the initial sequence of sources from a lightning flash can be used as remote sensing tools to give information about the magnitude of the electric field (relative to lightning-initiation thresholds) and the direction of the electric field at the initiation location.