Laboratory and in-flight evaluation of measurement uncertainties from a commercial Cloud Droplet Probe (CDP)
Abstract. 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
46  µ m tested sizing and counting performance across a
10  µ m 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
9 µ m droplets by 1–4  µ m. Droplets with diameters of 17 and
24  µ m were sized to within 2  µ m, which is the nominal CDP
bin width for droplets of that size. The majority of droplets larger than
17  µ m were oversized by 2–4  µ m, while a small percentage
were severely undersized, by as much as 30  µ m. 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 17 917 1 Hz 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.