Cloud-to-Ground Lightning Characteristics Over Houston, TX: 1989-2000

Cloud-to-Ground (CG) lightning detected by the National Lightning Detection Network (NLDN) indicates a relatively high flash density over Houston, Texas for the twelve-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 based on season and time-of-day, with the greatest increases occurring during the summer (58%), and during the 0900-1800 LST time periods in each season. Observations indicate that large lightning events (defined as days with > 1,000 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 (CCN) concentrations, especially from industrial pollution, are shown 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 measured peak current is examined.