2DC Hydrometeor Imaging Probe

The 2DC probe records images of hydrometeors that pass through its sample volume, and so provides measurements of ice or water drop concentration, their size distribution, and their shapes. It obtains these images by recording the status (illuminated or shadowed) of a 64-element photodiode array as the shadow of the hydrometeor passes over the array. Probes with 25 µm and 10 µm resolution are available; at 25 µm, the 64-element array provides a sample of about 8 L per 100 m of flight. Images of individual particles are recorded, usually with no loss except at very high concentrations. Special records containing these images in digital form are recorded as needed, so they will be interspersed with the standard periodically sampled records. The 2DC probe was originally manufactured by Particle Measuring Systems, Inc., but the electronics have been replaced with high-speed circuitry matched to the flight speed of the G-V, data transmission has been changed to USB-2, the photodiode array was replaced with one having twice as many elements and supporting faster response, and other changes were made to the optics and electronics of the G-V 2DCs.

Because the depth of field reduces to less than the distance between the arms that define the sample aperture for particle sizes less than about 125 µm, and because diffraction makes the sizes of such small particles hard to determine, the probe has limited ability to measure concentrations at sizes less than about 100 µm, even though it has resolution smaller than this. The array size and optics limit the largest size that can be imaged fully to 1600 µm for the 25-µm-resolution probe. The probe also has been shown to measure falsely high concentrations as a result of shattering (Korolev et al., 2011), so new tips have been installed that reduce but do not eliminate the effects of shattering.

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Cloud Droplet Probe

The Cloud Droplet Probe (CDP), manufactured by Droplet Measurement Technologies, measures the concentration and size distribution of cloud droplets in the size range from 2-50 µm. The instrument counts and sizes individual droplets by detecting pulses of light scattered from a laser beam in the near-forward direction, using a sample area of 0.24 mm2 or a sample rate of 48 cm3 at a flight speed of 200 m/s. The probe is mounted in an underwing canister and is designed to operate at up to 200 m/s; the G-V often exceeds this flight speed, but usually not in penetrations of clouds containing cloud droplets. Droplet sizes are accumulated in 30 bins with variable sizes, as specied in the header of the netCDF data files. Measurements are usually provided at a rate of 1 Hz in the standard data files but can be made available at 10 Hz in special high-rate processing. The instrument is similar to, and might be considered a high-speed replacement for, the Forward Scattering Spectrometer Probe. At high droplet concentration (> 500 cm-3), coincidence losses have been observed with this probe, and these are especially serious at G-V flight speeds. The probe is designed for cloud droplets, and its response to ice crystals is not intended to be quantitative; measurements in ice clouds should not be used except as qualitative indications of cloud.

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Ultrafine CN - LaRC

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SPP-100

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Multiple-Angle Aerosol Spectrometer Probe

The Multiple-Angle Aerosol Spectrometer Probe (MASP) determines the size and concentration of particles from about 0.3 to 20 microns in diameter and the index of refraction for selected sizes. Size is determined by measuring the light intensity scattered by individual particles as they transit a laser beam of 0.780µm wavelength. Light scattered from particles into a cone from 30 to 60 degrees forward and 120 to 150 degrees backwards is reflected by a mangin mirror through a condensing lens to the detectors. A comparison of the signals from the open aperture detector and the masked aperture detector is used to accept only those particles passing through the center of the laser beam. The size of the particle is determined from the total scattered light. The index of refraction of particles can be estimated from the ratio of the forward to back scatter signals. A calibration diode laser is pulsed periodically during flight to ensure proper operation of the electronics. The shrouded inlet minimizes angle of attack effects and maintains isokinetic flow through the sensing volume so that volatilization of particles is eliminated.

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2D-S Stereo Probe

The 2D-S Stereo Probe is an optical imaging instrument that obtains stereo cloud particle images and concentrations using linear array shadowing. Two diode laser beams cross at right angles and illuminate two linear 128-photodiode arrays. The lasers are single-mode, temperature-stabilized, fiber-coupled diode lasers operating at 45 mW. The optical paths are arbitrarily labeled the “vertical” and “horizontal” probe channels, but the verticality of each channel actually depends on how the probe is oriented on an aircraft. The imaging optical system is based on a Keplerian telescope design having a (theoretical) primary system magnification of 5X, which results in a theoretical effective size of (42.5 µm + 15 µm)/5 = 11.5 µm. However, actual lenses and arrays have tolerances, so it is preferable to measure the actual effective pixel size by dropping several thousands of glass beads with known diameters through the object plane of the optics system.

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