Water Droplet Calibration of the DMT Cloud Droplet Probe (CDP) and In-Flight Performance in Liquid, Ice and Mixed-Phase Clouds during ARCPAC

Laboratory calibrations of the CDP sample area and droplet sizing were performed using water droplets of known size and concentration. 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 with one droplet outside and one inside the instrument sample area, is evaluated in terms of a newly defined “extended sample area” (SAE), the area in which individual droplets can affect the sizing detector without registering on the qualifier. SAE is calibrated using standardized water droplets, and a Monte-Carlo simulation is developed to estimate the effect of coincidence on the measured droplet size distributions. Coincidence was found to be important for the CDP at droplet concentrations even as low as 100 cm-3, and is 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 suggest up to 30% oversizing and 25% undercounting errors at droplet concentrations of 500 cm-3. Modifications of the optical design of the CDP are currently being explored, in an effort to reduce this coincidence bias.