Optical backscattering from near-spherical water, ice, and mixed phase drops

Area of Science:

• Atmospheric optics
• Cloud physics
• Laser scattering

Background:

• Understanding hydrometeor scattering is crucial for remote sensing.
• Previous studies have limitations in characterizing phase-dependent scattering.

Purpose of the Study:

• To experimentally assess the scattering behavior of artificial raindrops in ice and mixed phases.
• To investigate the influence of drop shape and phase transitions on light depolarization.
• To relate scattering properties to lidar measurements of atmospheric particles.

Main Methods:

• Simultaneous measurement of parallel and cross-polarized backscattered laser light (6328 Å).
• Experimental setup using freely falling and mechanically suspended artificial drops.
• Analysis of linear depolarization ratios (delta) across different drop states.

Main Results:

• Depolarization ratios (delta) were <0.01 for liquid drops, ~0.5 for regular ice, and 0.35-1.0 for irregular ice.
• Anomalous scattering (delta > 1.0) observed during liquid-to-solid phase transition.
• Significant parallel polarized energy returned from larger raindrops (>~4 mm).
• Melting ice drops maintained ice-like depolarization until most ice had transformed.

Conclusions:

• Scattering behavior varies significantly with hydrometeor phase and shape.
• Observed phenomena provide insights into scattering mechanisms of large, near-spherical particles.
• Findings have implications for interpreting lidar data of atmospheric hydrometeors.