Laboratory and in-flight evaluation of measurement uncertainties from a commercial Cloud Droplet Probe (CDP)

Laboratory and in-flight evaluations of uncertainties of measurements from a Cloud Droplet Probe (CDP) are presented. A description of a water-droplet-generating device, similar to those used in previous studies, is provided along with validation of droplet sizing and positioning. Seven experiments with droplet diameters of 9, 17, 24, 29, 34, 38, and 46μm tested sizing and counting performance across a 10μm resolution grid throughout the sample area of a CDP. Results indicate errors in sizing that depend on both droplet diameter and position within the sample area through which a droplet transited. The CDP undersized 9μm droplets by 1-4μm. Droplets with diameters of 17 and 24μm were sized to within 2μm, which is the nominal CDP bin width for droplets of that size. The majority of droplets larger than 17μm were oversized by 2-4μm, while a small percentage were severely undersized, by as much as 30μm. This combination led to an artificial broadening and skewing of the spectra such that mean diameters from a near-monodisperse distribution compared well (within a few percent), while the median diameters were oversized by 5-15%. This has implications on how users should calibrate their probes. Errors in higher-order moments were generally less than 10%. Comparisons of liquid water content (LWC) calculated from the CDP and that measured from a Nevzorov hot-wire probe were conducted for 17917 1Hz in-cloud points. Although some differences were noted based on volume-weighted mean diameter and total droplet concentration, the CDP-estimated LWC exceeded that measured by the Nevzorov by approximately 20%, more than twice the expected difference based on results of the laboratory tests and considerations of Nevzorov collection efficiency.