Flow-Through Integrating Cavity Optical Absorption Spectrometer for In-Situ Cloud Water Condensed Phase Composition Measurement: Design Constraints and Initial Validation
BENJAMIN LANG, Alexander Bergmann, Graz University of Technology
Abstract Number: 516
Working Group: Instrumentation and Methods
Abstract
Accurate in-situ measurements of cloud microphysical properties are a helpful tool aiding the research of cloud radiative impact, cloud and precipitation formation, and aerosol-cloud interactions.
In mixed-phase clouds, where ice particles coexist with supercooled liquid droplets on different scales, the ratio of cloud water in the liquid phase to water in the ice phase is a parameter of special relevance, as it is important for radar, lidar and satellite retrievals, radiation transfer calculations and climate modelling. Additionally, this property crucially affects aircraft icing during flight through clouds and, therefore, is of importance to the optimization of ice detection systems and the process of representative experimental simulation of clouds in icing wind tunnels for aircraft certification.
Based on previous results demonstrating the possibility of accurately measuring the mass concentration of water droplets of different sizes in an integrating cavity absorption meter [1], we present the design considerations and first validation results of a novel optical instrument for bulk in-situ cloud water phase composition measurement. The instrument features a flow-through type integrating cavity, which largely eliminates scattering contributions to the differential, near-infrared optical absorption measurement and promises to allow direct determination of the relative fractions of both condensed phases via the different absorption coefficients. Mass proportional (linear) measurement is currently limited to particle sizes below approximately 200 µm, as determined from Mie calculations for the chosen optical wavelengths. The lower particle size and water content limit is determined by speckle noise generated in the integrating cavity. Ray tracing simulations were used to optimize the geometry of the integrating cavity based on sensitivity and signal-to-noise ratio. Aspiration efficiencies for relevant cloud particle sizes were determined by CFD simulation combined with Lagrangian particle tracking.
[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.