Water droplet calibration of the Cloud Droplet Probe (CDP) and in-flight performance in liquid, ice and mixed-phase clouds during ARCPAC

By S. Lance, C. Brock, D. Rogers, Joshua A. Gordon. 2010

378 citations recorded by the source corpus.

Abstract. Laboratory calibrations of the Cloud Droplet Probe (CDP) sample area and droplet sizing are performed using water droplets of known size, generated at a known rate. Although calibrations with PSL and glass beads were consistent with theoretical instrument response, liquid water droplet calibrations were not, and necessitated a 2 μm shift in the manufacturer's calibration. We show that much of this response shift may be attributable to a misalignment of the optics relative to the axis of the laser beam. Comparison with an independent measure of liquid water content (LWC) during in-flight operation suggests much greater biases in the droplet size and/or droplet concentration measured by the CDP than would be expected based on the laboratory calibrations. Since the bias in CDP-LWC is strongly concentration dependent, we hypothesize that this discrepancy is a result of coincidence, when two or more droplets pass through the CDP laser beam within a very short time. The coincidence error, most frequently resulting from the passage of one droplet outside and one inside the instrument sample area at the same time, is evaluated in terms of an "extended sample area" (SAE), the area in which individual droplets can affect the sizing detector without necessarily registering on the qualifier. SAE is calibrated with standardized water droplets, and used in a Monte-Carlo simulation to estimate the effect of coincidence on the measured droplet size distributions. The simulations show that extended coincidence errors are important for the CDP at droplet concentrations even as low as 200 cm−3, and these errors are necessary to explain the trend between calculated and measured LWC observed in liquid and mixed-phase clouds during the Aerosol, Radiation and Cloud Processes Affecting Arctic Climate (ARCPAC) study. We estimate from the simulations that 60% oversizing error and 50% undercounting error can occur at droplet concentrations exceeding 400 cm−3. Modification of the optical design of the CDP is currently being explored in an effort to reduce this coincidence bias.

Subfields: Atmospheric Science, Global and Planetary Change

View Paper: https://www.semanticscholar.org/paper/7c04c607ed4c6af31545011ab05dfbe49d419f7c

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Citations in the corpus

Particle sizing calibration with refractive index correction for light scattering optical particle counters and impacts upon PCASP and CDP data collected during the Fennec campaign (2012, methodology): “This concentration is used to avoid coincidence which can cause problems above ∼500 s−1 (Lance et al., 2010) and yet provide a useful particle distribution in a short time period.”
The Microphysics of Ice and Precipitation Development in Tropical Cumulus Clouds (2015, methodology): “The C-130 was equipped with a suite of microphysics instruments that included cloud droplet probe (CDP; Lance et al. 2010), CPI, and 2D-S probes, along with inertial navigation system (INS)-based air motion, remote sensing (W-band Doppler radar and lidar), and aerosol instrumentation.”
Mixed-Phase Clouds: Progress and Challenges (2017, background): “Lance et al. (2010) and Jackson et al. (2012) have examined the importance of these effects in Arctic mixed-phase clouds but have not categorically determined the relative importance of each effect because of the need to acquire data in a large variety of surface, meteorological, and cloud…”
Cloud Ice Properties: In Situ Measurement Challenges (2017, background): “…sample volumes of the OAPs mean that there is a higher probability that the shattered Unauthenticated | Downloaded 07/24/22 08:33 PM UTC fragments will be detected (Korolev and Isaac 2005; Korolev et al. 2011, 2013a; Field et al. 2006; Lawson 2011; Jackson et al. 2014; Korolev and Field 2015).”