Abstract
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 µm tested sizing and counting performance across a 10 µm 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 1–4 µm. Droplets with diameters of 17 and 24 µm were sized to within 2 µm, which is the nominal CDP bin width for droplets of that size. The majority of droplets larger than 17 µm were oversized by 2–4 µm, while a small percentage were severely undersized, by as much as 30 µm. 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.
Highlights
In situ cloud studies often utilize measurements from forward-scattering optical particle counters (OPCs) to provide size and concentration information about cloud hydrometeors up to a few tens of microns in diameter
Dwell time was further reduced such that 200 drops were placed at each location, and the resolution of the grid was reduced to 30 μm by 20 μm, resulting in roughly 450 discrete locations across the qualified Cloud Droplet Probe (CDP) sample area
Experiments reveal that droplet sizing accuracy varies depending on where droplets transit the sample area and the size of the droplets
Summary
In situ cloud studies often utilize measurements from forward-scattering optical particle counters (OPCs) to provide size and concentration information about cloud hydrometeors up to a few tens of microns in diameter. Laser intensity and scattering angles are somewhat variable within the qualified sample area, resulting in counting and sizing error that is dependent on droplet transit location (Brenguier et al, 1998; Wendisch et al, 1996). OPCs are designed to count/measure a single particle at a time so standard coincidence results in undercounting and can lead to oversizing due to the additional light scattered by coincident hydrometeors (Baumgardner et al, 1985; Cooper, 1988). The droplet generator work by Lance et al (2010) tested CDP sizing and counting accuracy throughout the qualified sample area (at a spatial resolution of 200 × 20 μm) using 12 and 22 μm droplets. Droplet exit velocity is limited by turbulence in the vicinity of the flow tube exit, which results in significant deviations of droplet position when sheath flow rate exceeds 13 L min−1
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