Cloud-to-ground lightning characteristics over Houston, Texas

Abstract

The U. S. National Lightning Detection Network (NLDN) has recorded cloud-to-ground (CG) lightning observations regularly for more than a decade (1989-2000). The main research focus using the recorded NLDN data has been the creation of lightning climatologies (e.g. Orville and Huffines 1999). These climatologies have revealed important results that indicated certain patterns of lightning behavior across the U. S. One of the most significant findings from such work has been the observance of lightning "hot spots" over and near cities (Westcott 1995). Houston, TX is one such "hot spot," with significant enhancement (45%, all 12 years) of lightning activity observed over and downwind of the urban area. Enhancement varied based on season and time of day, with the greatest enhancements occurring during the summer (58%) and the late morning/afternoon time periods in each season. Two other interesting features discovered during this study included a decrease (-12%) in the percentage of flashes lowering positive charge to ground over the city, and significantly larger values of negative median peak current measured along the coast and well into the Gulf of Mexico. One hypothesis proposed for explaining the Houston enhancement includes the complex sea breeze and associated low-level convergence that result because of the proximity of Galveston Bay to the southeast of Houston. Also, there are a multitude of factors associated with a large city such as Houston that can modify the local climate. Some of these urban factors include: increased cloud condensation nuclei (CCN) concentrations, especially from industrial pollution, the urban heat island, and frictional lift. It has been proposed by Rosenfeld and Lensky (1998) that the large quantity of CCN in polluted areas can enhance a storm's electrical state because they act to decrease the coalescence and rainout cloud processes, therefore allowing more supercooled liquid water to exist within the storm. The urban heat island can enhance thunderstorm initiation through increasing the low-level mesoscale convergence and upward motion directly over the city. The observations showed support for each one of these aspects to be a factor in enhanced lightning activity over Houston.