A Radar Study of the Plasma and Geometry of Lightning

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  • 1 Weather Radar laboratory, Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts
  • | 2 Department of Physics, Serampore College, West Bengal, India
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Abstract

Radar measurements and model studies are combined to investigate the plasma condition and the physical structure of lightning in thunderclouds. The lightning radar target is inferred to be an arclike plasma whose temperature exceeds 5000 K, thereby implying overdense plasma at all meteorological wavelengths. Lightning echoes are treated as volume targets and are modeled as treelike assemblages of conductive channels which are each long and thin compared to the radar wavelength. The channel lengths per unit volume deduced from more than one thousand reflectivity measurements at 11 cm wavelength range from 10−3 to 102 km km−3. Comparisons with more than 200 measurements at 5 cm wavelength show that the wavelength dependence is highly variable. On the average, the apparent dependence is λ−2 but this is unreliable because of the masking effects of precipitation. The infrequent detection of lightning at short wavelengths (λ ≥ 5 cm) is also attributed to masking rather than to an underdense plasma condition.

Abstract

Radar measurements and model studies are combined to investigate the plasma condition and the physical structure of lightning in thunderclouds. The lightning radar target is inferred to be an arclike plasma whose temperature exceeds 5000 K, thereby implying overdense plasma at all meteorological wavelengths. Lightning echoes are treated as volume targets and are modeled as treelike assemblages of conductive channels which are each long and thin compared to the radar wavelength. The channel lengths per unit volume deduced from more than one thousand reflectivity measurements at 11 cm wavelength range from 10−3 to 102 km km−3. Comparisons with more than 200 measurements at 5 cm wavelength show that the wavelength dependence is highly variable. On the average, the apparent dependence is λ−2 but this is unreliable because of the masking effects of precipitation. The infrequent detection of lightning at short wavelengths (λ ≥ 5 cm) is also attributed to masking rather than to an underdense plasma condition.

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