Progress on a Flow-Through Integrating Cavity Optical Absorption Spectrometer for In-Situ Cloud Water Condensed Phase Ratio Measurement

Accurate in-situ measurement of cloud microphysical properties, such as the water content, is essential in the research and modelling of cloud and precipitation formation, or the prediction of macrophysical properties, e.g., cloud radiative properties.

In mixed-phase clouds, where ice crystals coexist with supercooled liquid droplets on different scales and with varying mixing ratio, accurate knowledge of this mixing ratio of cloud liquid water content to ice water content is important, in particular for advancing radar, lidar and satellite retrievals.

Based on previously presented results demonstrating the possibility of measuring the mass concentration of liquid water droplet streams in an integrating sphere absorption meter [1][2], we present the progress made and first validation results of a novel optical instrument for bulk in-situ cloud water condensed phase ratio measurement. The proposed instrument features a flow-through type integrating cavity for differential, near-infrared optical absorption measurement. Such cavities, by nature of their light homogenizing property, largely eliminate particle scattering contributions, which typically prohibit simple optical absorption measurement of particles. This promises to allow direct and in-situ determination of the fractions of both condensed phases in mixed-phase clouds via their optical absorption. The current design is limited to drop and particle sizes below 200 µm due to absorption saturation, as determined from Mie calculations for the chosen optical wavelengths. The lower water content detection limit is determined by speckle noise generated by the integrating cavity, which is subject to presented optimization efforts. We also present numerical Monte-Carlo based ray tracing simulations of the integrating cavity geometry for sensitivity and signal-to-noise ratio optimization.

[1] Grafl, M., Bergmann, A., & Lang, B. (2021). Validation of Integrating Cavity Absorption Spectroscopy for Cloud and Aerosol Mass Concentration Measurement: 39th Annual Meeting of the American Association for Aerosol Research. 153.

[2] Lang, B.,, Bergmann, A. (2024). Flow-Through Integrating Cavity Optical Absorption Spectrometer for In-Situ Cloud Water Condensed Phase Composition Measurement: Design Constraints and Initial Validation. 42nd Annual Meeting of the American Association for Aerosol Research.