Measurements of the complex dielectric constant of volcanic ash from 4 to 19 GHz

Dielectric data in volcanic ash at weather radar wavelengths (centimeter range) are extremely sparse and are crucial for radar sensing of ash clouds and for imaging of volcanic terrains. This study extends previous data to include a wavelength range of 1.5–7.5 cm and volcanic ash compositions of 50–75% silica. The real part of the complex permittivity, ε′, of volcanic ash is 6 ± 0.5 (1σ) for all wavelengths. The imaginary part, ε″, ranges from 0.08 to 0.27. Both ε′ and ε″ show higher values at lower SiO2 concentration. It is safe to assume in any weather radar applications that the reflectivity factor is K = |(ε − 1)/(ε + 2)|2 = 0.39 ± 0.02 (1σ), regardless of composition or wavelength. The results will help quantify radar observations of volcanic clouds.