A Framework for the Statistical Analysis of Large Radar and Lightning Datasets: Results from STEPS 2000

A framework for the statistical analysis of large radar and lightning datasets is described and implemented in order to analyze two research questions in atmospheric electricity: storms dominated by positive cloud-to-ground (+CG) lightning and estimating the probability of lightning in convection. The framework-a collection of computer programs running in series-is fully modular, allowing the analysis of a variety of datasets based on a study's objectives, including radar observations, lightning data, observations of meteorological environments, and other data. The framework has been applied to over 2 months of observations of 28 463 cells. The results suggest that +CG-dominated cells contain midlevel positive charge (-10 degrees to -30 degrees C), in contrast to cells dominated by -CG lightning, which typically had positive charge at upper (near -40 degrees C) and lower levels (0 degrees to -10 degrees C). The +CG cells also were larger and more intense, and were associated with environments that were more convectively favorable-in terms of increased moisture, shear, and especially instability-when compared to -CG cells. The framework was also used to examine the probability of lightning occurrence for a spectrum of radar structures. The existence of 30-dBZ echo above the freezing altitude is a necessary'' condition (in similar to 90% of cases) for lightning occurrence. A sufficient'' condition (in similar to 90% of cases) is 40-dBZ echo breaching the freezing altitude. Altitude or volume of 40-dBZ echo was the superior estimator for the occurrence of lightning, while 30 dBZ was better for inferring the lack of lightning.