The Relationships between Network Lightning Surface and Hourly Observations of Thunderstorms

Ronald M. Reap Techniques Development Laboratory, National Weather Service, NOAA, Silver Spring, Maryland

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Richard E. Orville Department of Atmospheric Science, State University of New York at Albany, Albany, New York

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Abstract

Relationships were established between lightning location data and surface hourly observations of thunderstorms for 132 stations in the northeastern United States. The relationships are based on statistics derived from 2 × 2 contingency tables that were constructed for each station from a two-year sample of data for the 1985–86 warm seasons. To construct the tables, ground strike totals for the valid period of the hourly observations were accumulated for circular regions of varying radius (8–80 km) centered at each station. Separate contingency tables were constructed for each radius.

For the entire sample, the fraction of observer reported thunderstorms that were also recorded by the lightning network was found to increase rapidly from 0.44 within a 16 km radius to 0.82 at 50 km, beyond which the rate of increase was much smaller with a maximum of 0.89 at 80 km. Values over 0.9 at 50 km were, however, typical for stations located well within the interior of the network. The continuing improvement in the relationships beyond the audible range of thunder (20 km) was most likely related to the errors biases, and uncertainty inherent in both sets of observations. The agreement between the network reports and station observations was slightly degraded by the possible reporting of intracloud flashes or weak ground strikes that were detected by the observer but not the network, especially at night. A significant increase was found in the ratio of the number of thunderstorms reported by observers at night to the number detected by the network, reflecting the improved ability of the observer to see lightning at night. In general, the lightning network was able to detect more thunderstorms than the observer beyond 17 and 26 km, respectively, for the daytime and nighttime samples. The improved detection capability for expanded regions about the station should have a significant impact on operational forecasts and warnings, especially with regard to aviation-related activities both en route and near airport terminals. Finally, improvements in the observer's ability to see lightning and hear thunder were found to be related to corresponding increases in flash density and peak signal strength.

Abstract

Relationships were established between lightning location data and surface hourly observations of thunderstorms for 132 stations in the northeastern United States. The relationships are based on statistics derived from 2 × 2 contingency tables that were constructed for each station from a two-year sample of data for the 1985–86 warm seasons. To construct the tables, ground strike totals for the valid period of the hourly observations were accumulated for circular regions of varying radius (8–80 km) centered at each station. Separate contingency tables were constructed for each radius.

For the entire sample, the fraction of observer reported thunderstorms that were also recorded by the lightning network was found to increase rapidly from 0.44 within a 16 km radius to 0.82 at 50 km, beyond which the rate of increase was much smaller with a maximum of 0.89 at 80 km. Values over 0.9 at 50 km were, however, typical for stations located well within the interior of the network. The continuing improvement in the relationships beyond the audible range of thunder (20 km) was most likely related to the errors biases, and uncertainty inherent in both sets of observations. The agreement between the network reports and station observations was slightly degraded by the possible reporting of intracloud flashes or weak ground strikes that were detected by the observer but not the network, especially at night. A significant increase was found in the ratio of the number of thunderstorms reported by observers at night to the number detected by the network, reflecting the improved ability of the observer to see lightning at night. In general, the lightning network was able to detect more thunderstorms than the observer beyond 17 and 26 km, respectively, for the daytime and nighttime samples. The improved detection capability for expanded regions about the station should have a significant impact on operational forecasts and warnings, especially with regard to aviation-related activities both en route and near airport terminals. Finally, improvements in the observer's ability to see lightning and hear thunder were found to be related to corresponding increases in flash density and peak signal strength.

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