Cloud-to-ground lightning characteristics over Houston, Texas: 1989–2000 Scott M. Steiger and Richard E. Orville Department of Atmospheric Sciences, Cooperative Institute for Applied Meteorological Studies, Texas A&M University, College Station, Texas, USA Gary Huffines Department of Engineering Physics, Air Force Institute of Technology, Wright-Patterson Air Force Base, Ohio, USA Received 17 July 2001; revised 26 November 2001; accepted 28 November 2001; published 7 June 2002. [1] Cloud-to-ground (CG) lightning detected by the National Lightning Detection Network (NLDN) indicates a relatively high flash density over Houston, Texas, for the 12-year period 1989–2000. A significant enhancement of 45% in the flash density is observed compared to the nearby surrounding areas. The strength of the enhancement varies on the basis of season and time of day, with the greatest increases occurring during the summer (58%) and during the 0900–1800 LT time periods in each season. Observations indicate that large lightning events (defined as days with >100 flashes in a geographic region that includes Houston and nearby rural areas) were responsible for the climatological lightning anomaly and that increased thunderstorm initiation was not the most significant cause of the enhancement. A decrease (12%) in the percentage of positive flashes is observed over the city. Higher negative median peak currents along the coast and well into the Gulf of Mexico were also discovered. Several explanations for our observations are suggested. The urban heat island and increased cloud condensation nuclei concentrations, especially from industrial pollution, are speculated to be significant factors in creating lightning enhancement. Pollution effects are speculated to cause a change in a thunderstorm’s charge distribution, which can affect the polarity of CG flashes. The potential effect of the nearby coastal Gulf salt water on the calculated peak current is examined. Variations in multiplicity values across the region are observed but not explained. INDEX TERMS: 3324 Meteorology and Atmospheric Dynamics: Lightning; 3304 Meteorology and Atmospheric Dynamics: Atmospheric electricity; 3314 Meteorology and Atmospheric Dynamics: Convective processes; 3329 Meteorology and Atmospheric Dynamics: Mesoscale meteorology 1. Introduction [2] A 12-year climatological analysis (1989 – 2000) of National Lightning Detection Network (NLDN) data has indicated a sig- nificant enhancement of lightning activity over the Houston, Texas, region as compared to nearby rural areas [Orville et al., 2001]. The Westcott [1995] study was the first to use NLDN data to reveal the effect of several cities on enhancing cloud-to-ground (CG) light- ning activity over and downwind of them, but Houston was not in that study. Orville et al. are the first to document the effect that Houston has on CG lightning. They showed the enhancement was evident during both the winter and the summer seasons throughout the 1989 – 2000 period. In this paper we extend the study of Orville et al. Please note that hereinafter, ‘‘lightning’’ and ‘‘flash’’ refer only to cloud-to-ground lightning. [3] Westcott [1995] and Orville et al. [2001] discussed several mechanisms for explaining the enhanced lightning activity. These include the urban heat island circulation, addition of thermal energy, frictional lift, and air pollution. Orville et al. also examined the sea breeze influence on lightning activity over and near Houston. The main problem that remains to be solved is to determine the significance of each contributing factor listed above to the climatological lightning enhancement over Houston. Assess- ing the impact of these urban effects on climate has been a significant research problem for meteorologists over the past several decades [e.g., Landsberg, 1981]. [4] The NLDN, in addition to measuring the flash location, also measures the polarity, peak current, and multiplicity of each detected flash. An analysis of these characteristics during the 1989 – 2000 period over the Houston region reveals some inter- esting results. The percent of flashes lowering positive charge to ground is observed to be less over and downwind of the Houston urban area than nearby upwind areas. In contrast, Lyons et al. [1998b] and Murray et al. [2000] correlated the ingestion of forest fire smoke from Mexico in 1998 with a higher percentage of positive CG lightning, but the microphysical reasons of how pollution might affect the polarity of lightning were not sug- gested. Large negative peak current flashes were observed imme- diately off the Gulf of Mexico coastline during the 1989 – 2000 period. Lyons et al. [1998a] suggested surface conductivity as a possible cause because the conductivity of seawater is greater than that for land. 2. Previous Studies of Urban Weather Modification [5] In the work of Westcott [1995], 16 Midwestern cities (during the months of June, July, and August 1989– 1992) were shown to have an enhancement of CG lightning frequency by as much as 40 – 85% over and downwind of the urban areas as compared to rural areas upwind. Westcott speculated the following causal JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. D11, 4117, 10.1029/2001JD001142, 2002 Copyright 2002 by the American Geophysical Union. 0148-0227/02/2001JD001142 ACL 2 - 1