Data from the National Lightning Detection Network (NLDN) indicate that there are about 50% more cloud-to-ground lightning strikes in and around Houston than over the surrounding rural areas. Several explanations of this Houston Lightning Anomaly (HLA) have been suggested. One is that the higher daytime surface temperature over the city (the "urban heat island") may account for increased convection, storm development, and hence lightning. Another is that the sea breeze circulation in the Texas coastal region may tend to organize low-level convergence in a way to favor convection over the city. Still another is that the increased loading of pollution aerosols over the city may affect cloud microphysical processes in a way that modifies the generation and distribution of electrical charge within clouds, promoting the occurrence of lightning discharges. This award supports field observations in and around Houston that will help to explain the HLA by providing new and more complete information on the characteristics of the lightning. A network of twelve VHF receiving stations called the Lightning Detection and Ranging system (LDAR) will be operated during the storm seasons of 2005, 06, and 07 to measure the time and location of lightning flashes. Whereas the NLDN provides only the approximate locations of cloud-to-ground flashes, the LDAR system detects essentially all flashes - those within cloud as well as those to the ground - with great accuracy and precision. Moreover, the LDAR provides information on the location of lightning initiation, the configuration of lightning discharge channels, the altitude from which electrical charge is lowered, the polarity of cloud-to-ground strokes, estimates of peak currents, and other lightning characteristics. The observations will be carried out in parallel with an extensive study of air quality and the dispersion of pollution in the Houston area under the auspices of the Texas Commission on Air Quality, called the Texas Air Quality Field Study. That study includes the operation of a pair of Doppler radars for measuring air motions and storm structure as well as surface and airborne measurements of aerosols and trace gases. Together the LDAR observations and the supporting data from the Air Quality Field Study will provide new insight on the relations between pollution, air motions, storm structure, and lightning, which will help to explain the Houston Lightning Anomaly. Broader impacts of the study include the potential for improved forecasts of severe weather using LDAR data and progress towards resolving long-standing questions about the microphysical and electrical structure of thunderstorms.
