Evaluation of Cloud-to-Ground Lightning Data from the Western United States for the 1983–84 Summer Seasons

Over two million cloud-to-ground lightning strike locations for the period from mid-June through mid-September for the 1983–84 summer seasons were evaluated to determine the large-scale climatological characteristics of summertime lightning activity over mountainous terrain in the western United States. During the evaluation, manually digitized radar data and GOES satellite observations from the Techniques Development Laboratory's data archives were included for comparison. Generalized statistical relationships were established among the lightning data, radar data, and minimum cloud-top infrared temperature and maximum visible brightness values from the satellite VISSR data. In the climatological analysis, a high correlation between terrain elevation and the hour of maximum frequency of lightning was found along with a pronounced increase in the magnitude of lightning activity over high terrain. Maps of daily lightning frequency were in general agreement with conventional thunderstorm climatologies but contained considerably more detail, especially in the higher elevations. Due to inadequate radar coverage in the West, approximately 41% of the lightning strikes occurred when no radar echoes were reported. In addition, 87% of the strikes occurred with radar intensity levels less than VIP3, the threshold normally used for delineating thunderstorms in the eastern United States. During daylight hours, lightning frequencies were 10–20% higher for the GOES infrared observations compared to frequencies associated with the visible brightness values, reflecting a tendency for the lightning strikes to be clustered under or near the coldest cloud tops. The effect of thunderstorm cells imbedded in large cloud masses on the relationship between lightning frequency and GOES infrared data was found to be significant. For a given infrared cloud-top temperature, it was not unusual to find a 40–50% increase in lightning frequency for subsets of grid blocks with VIP4 or greater echoes compared to blocks where no echoes were reported. A fairly stable seasonal and geographical distribution of lightning activity was found in the West, resulting from the strong control exerted by the underlying topography on the timing, location, and magnitude of the lightning activity. This is especially significant because it will allow the lightning frequencies to be used as climatic predictors in the development of new objective thunderstorm probability forecasts for this region, thereby improving the temporal and spatial resolution of the forecasts by the implicit introduction of small-scale topographic effects into the forecasts.