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

Published

Author(s)

Sara Lance, Charles Brock, Dave Rogers, Josh Gordon

Abstract

Laboratory calibrations of the CDP sample area and droplet sizing are performed using water droplets of known size, generated at a known rate. Following calibration, the sizing error is shown to be less than 10% and the sample area is determined to within 10% for 12 um and 22 um diameter droplets. However, comparison with an independent measure of liquid water content (LWC) during in-flight operation suggests additional biases in the droplet size and/or droplet concentration measured by the CDP. 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 using 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 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. The simulations also show that 50% oversizing error and 47% undercounting error can occur at droplet concentrations of 550 cm-3. Modification of the optical design of the CDP is currently being explored in an effort to reduce this coincidence bias.
Citation
Atmospheric Measurement Techniques
Volume
3
Issue
6

Keywords

clouds, aerosol, droplet, climate

Citation

Lance, S. , Brock, C. , Rogers, D. and Gordon, J. (2010), Water droplet calibration of the Cloud Droplet Probe (CDP) and in-flight performance in liquid, ice and mixed-phase clouds during ARCPAC, Atmospheric Measurement Techniques, [online], https://doi.org/10.5194/amtd-3-3133-2010, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=905401 (Accessed May 24, 2024)

Issues

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Created December 13, 2010, Updated October 12, 2021