Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 1990
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990phdt........88d&link_type=abstract
Thesis (PH.D.)--CLEMSON UNIVERSITY, 1990.Source: Dissertation Abstracts International, Volume: 51-07, Section: B, page: 3421.
Astronomy and Astrophysics
Astronomy
Scientific paper
Radar echoes from lightning and precipitation were observed during a thunderstorm in the summer of 1985 with the 430 MHz Doppler radar at the National Astronomy and Ionosphere Observatory in Arecibo, Puerto Rico. All measurements were made with the radar pointing vertically. In 8 cases lightning discharges were observed to precede the development of spatially large regions of power enhancement that involved growth rates of 13 to 60 dB/min. Coincident with these lightning events, vertically adjacent regions of long-term velocity enhancements were observed with changes nearly equal in magnitude but opposite in sense. Five of these "polar" velocity events involved convergence, where increases in velocity (toward the radar) occurred in the upper region and decreases occurred in the lower region, while three events were divergent with the sense of the velocity changes reversed. Three types of change in the precipitation particle size distribution accompanying lightning flashes were identified: (1) diameters larger than 2 to 3 mm growing at an enhanced rate at the expense of smaller diameters with a consequent mean Doppler velocity increase; (2) a narrow range of intermediate diameters grow at a faster rate than larger diameters and at the expense of smaller diameters with a resultant mean Doppler velocity decrease and (3) an abrupt decrease in the growth rate of particles greater than approximately 3 mm coincident with an abrupt increase in the growth rate of small diameter particles (possibly due to particle break -up) which produces a mean Doppler velocity decrease. From statistical analysis of data from 174 gates for 22 lightning events, it was determined that (1) the temporal duration of the flashes ranged from 48 to 923 ms with a mean of 478 ms; (2) the average updraft velocity was 4.1 m/s (no downdraft velocities were detected); (3) transient increases in the noise level were greater than 10 dB in 83% of the gates; (4) the coherent spectral peak associated with the lightning plasma achieved peak power levels greater than 10 dB above the noise in over 90% of the gates; (5) spectrum widths of the lightning echoes were less than 1 m/s in 90% of the gates, and (6) evidence of acceleration of the lightning channel was observed in 80% of the gates with only 10 cases of acceleration downward.
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