Optical Array Probe Research Articles

Measurements of the Ice Water Content in Cirrus Using an Evaporative Technique

Abstract A technique for the measurement of the ice water content (IWC) of cirrus clouds is described. The IWC is obtained by the measurement of the total water content (TWC) and the subtraction of the saturation specific humidity with respect to ice at the ambient pressure and temperature. The method is independent of the measurement of the crystal size spectrum and also of any assumptions about the bulk densities of various crystal habits. Examples of IWC measurements made during the International Cirrus Experiment are presented and compared with conventional measurements from a 2D optical array probe. The prime sources of error are the accuracy of the calibration of the TWC probe and the occurrence of subsaturated air, which invalidates one of the main principles of the technique.

Magnitude of Error Factors in Estimates of Snow-Particle Masses from Images

Abstract Ice particles are captured, photographed, melted, and then photographed again. Mass is estimated from the size of the melted drop. Based on a sample of 640 particles, the standard error in estimating particle mass solely from the maximum dimension of the particle is found to be a factor of 4. The standard error in estimating mass concentration M in a cloud from a sample of n well-characterized particles recorded by an optical array probe is estimated to be approximately a factor of 100.6/n^1/2.

Simultaneous measurements of the charge, size, and shape of hydrometeors in an electrified cloud

With the use of an induction cylinder in conjunction with an optical array probe, simultaneous measurements were obtained of the size, shape, and charge of hydrometeors in a New Mexico thunderstorm, The particles with detectable charges (greater than 1–3 pC) were primarily graupel particles, and their charges are generally consistent with laboratory measurements of ice‐ice collisional charging. The fraction with detectable charge increases with size, as does the magnitude of the mean charge. Also, the positively charged particles occur primarily at higher temperatures (> −22°C), and the degree of negative charging increases with decreasing temperature (down to −35°C).

Multiple Disdrometer Observations of Rainfall

Abstract Raindrop sampling instruments such as disdrometers and optical array probes, have been used by researchers to observe raindrop-size distributions. Estimates of raindrop-size distribution (RSD) can be made from these instruments, which can subsequently be used to study several derived parameters such as liquid-water content, rainfall rate, and radar reflectivity. These instruments have limited sampling volume, which affects the estimates of raindrop-size distribution and the derived integral parameters. Although fluctuations in the derived parameters obtained from the same disdrometer samples are correlated, estimates from independent disdrometers are not. This paper addresses the issues involved in comparing data from disdrometers. Data from four disdrometers sampling the same rain volume are analyzed to study the measurement fluctuations in a single disdrometer with time and between disdrometers at the same time interval. Theoretical analysis and data show that the correlation between deceived p...

Evaluation of Measurements of Particle Size and Sample Area from Optical Array Probes

Abstract The technique of using shadow images of particles, obtained in coherent illumination to measure particle size, is analyzed. The theory of Fresnel diffraction for an opaque disc was used to analyze shadow images of transparent spherical particles. A comparison of theoretical calculations with laboratory results supports the application of this approach to optical array probes. It is shown that the shadow image size of spherical particles is essentially dependent on the distance from the object plane. In particular, for drops with diameters of less than 100 µm, the errors in size measurement from the PMS OAP-200X may reach 65%. These results agree well with laboratory calibrations that use monodisperse water droplets. On the basis of calculated particle diffraction images, the shape of the sample area and its dependence on drop size were calculated. It was found that the sample area has a complicated sawtooth shape. Gaps oriented perpendicular to the axis of a laser beam occur in the sample area in...

Use of Rotating Pinholes and Reticles for Calibration of Cloud Droplet Instrumentation

Calibration devices for the Forward Scattering Spectrometer Probe (FSSP) and the Optical Array Probe (OAP) were developed. The device used with the FSSP is a rotating pinhole calibrator. It utilizes light diffracted by a pinhole of a known diameter to simulate scattered light from a water droplet. This device can be used to calibrate the FSSP, measure the FSSP's optical collection angles and for instrument alignment and troubleshooting. The device used with the OAP is a rotating reticle calibrator. Chrome disks of a known diameter on the reticle are used for calibration of the OAP and for determining the OAP's response to out-of-focus particles in the probe volume.

Measurement of drop spectra from rotary cage aerial atomizers

The drop spectra produced by various examples of Micronair AU3000 rotary cage atomizer were measured using a laser-based Particle Measuring Systems optical array probe. To simulate aerial application by fixed-wing aircraft, the atomizers were mounted in an open jet wind tunnel operating in the range 130–225 km h −1. An empirical model which predicted the volume median diameters (VMDs) and volume quartiles of the spectra was developed. The model showed a high degree of correlation with the experimental results. The model took into account not only the speed of rotation of the cage, but its diameter, the liquid flow rate, and the air speed of the aircraft. The model showed general agreement with research into rotary disc atomization. Although a water-based liquid was used in the tests, it was concluded that the model would be suitable for use as a performance guide for most pesticide formulations.

Use of Holography for Airborne Cloud Physics Measurements

Abstract The use of the holographic cloud particle imaging system developed by the Cloud Physics Branch of the Meteorological Office and carried on the C-130 Hercules aircraft of the Meteorological Research Flight (MRF) has hitherto been limited by the extremely labor intensive data extraction process. A new image reconstruction system has now been developed that enables numerous holograms from a single flight to be analyzed. A brief description of this system is given, and some of its uses and limitations are demonstrated by examples of both droplet and ice-crystal data. In each case, the holographic data are compared with those from what are now conventional cloud microphysical probes, principally the ASSP and 2-D Optical Array Probe. Results show that the holographic system can measure gross features of the droplet size spectrum in conditions when the ASSP data may be unreliable. Ice crystal measurements confirm the ability of the holographic technique to produce data down to sizes of about 60 μm, well...

Three-Dimensional Trajectory Analysis of Two Drop Sizing instruments: PMS* OAP and PMS* FSSP

Flow-induced distortions of water drop flux and speed are predicted by three-dimensional calculations. The instruments are studied in isolation and mounted under the wing of a DeHavilland Twin Otter airplane. Several free stream air speeds and angles of attack of 0° and 4° are studied for drop diameters ranging from 2 to 1000 μm. For the PMS Optical Array Probe (OAP) in isolation and under the Twin Otter wing with the airplane at 0° angle of attack, distortions of practical consequence are not found. At 4° airplane angle of attack, 17% under-measurement of both flux and speed is predicted for cloud-size droplets. The PMS Forward Scattering Spectrometer Probe (FSSP) presents greater flow obstruction than the OAP, and requires in addition that air and drops traverse the measurement tube. As expected, larger flow-induced effects are predicted under all circumstances than for the OAP. For the FSSP in isolation and mounted on the Twin Otter at 0° angle of attack, both speed and flux are predicted to be undermeasured by about 10% for cloud-size droplets. At 4° airplane angle of attack, 24% undermeasurement of both flux and speed is predicted for cloud-size droplets. For the wing-mounted instruments we find that a large part of the flow-induced effects (approximately half) is caused by the instruments themselves. This shows that it is not always justified to assume that instrument-induced flow effects are insignificant compared with aircraft-induced effects.

The Influence of Cloud Droplets on the Measurement of Ice Particle Concentrations with a Particle Measuring System's 2DC Optical Array Probe

The Particle Measuring System's 2DC optical array probe has been used as a primary instrument to measure ice particle concentrations since its introduction in 1976. In certain mixed-phase clouds containing cloud droplets in size ranges 30–100 μm, the 2DC has been found to respond to both liquid and ice particles. This is of real concern, since ice crystals occur naturally in tens per liter, while supercooled droplets in this size range can occur in tens to hundreds per liter. In this note, the effect of cloud droplets on the measurement of ice particle concentrations with the 2DC probe is discussed. An example is presented where supercooled droplets overwhelm the probe, making accurate measurements of ice particle concentrations difficult to obtain.

Simulation of Radar Reflectivity and Surface Measurements of Rainfall

Raindrop size distributions (RSDs) are often estimated using surface raindrop sampling devices (e.g., disdrometers) or optical array (2D-PMS) probes. A number of authors have used these measured distributions to compute certain higher-order RSD moments that correspond to radar reflectivity, attenuation, optical extinction, etc. Scatter plots of these RSD moments versus disdrometer-measured rainrates are then used to deduce physical relationships between radar reflectivity, attenuation, etc., which are measured by independent instruments (e.g., radar), and rainrate. In this paper we simulate RSDs of the gamma form as well as radar reflectivity (via time series simulation) to study the correlation structure of radar estimates versus rainrate as opposed to RSD moment estimates versus rainrate. Simulations offer a powerful method of studying the statistics of radar and surface RSD measurements since the “natural” RSD fluctuations can be introduced separately. In our simulations we vary the parameter No, Do and m of a gamma distribution over the range normally found in rainfall, as well as varying the device sampling volume. We apply our simulations to explain some possible features related to discrepancies which can arise when radar rainfall measurements are compared with surface or aircraft-based sampling devices.

Some Techniques and Uses of 2D-C Habit Classification Software for Snow Particles

Abstract A technique has been designed that uses observable properties of images from a 2D-C optical array probe (size, linearity, area, perimeter, and image density) to classify unsymmetrical ice particles into nine habit classes. Concentrations are calculated by requiring that the center of each accepted particle appear to be within the field of view of the probe. Once the size and habit are estimated, a generic mass and terminal velocity can be assigned to each particle to calculate its contribution to ice water content and to precipitation rate. Examples are given to indicate the value of a habit classifier in analyzing the structure of storms, showers, orographic clouds, and seeded clouds. Though the techniques work well for most natural snowfalls, some examples of imperfections are given to remind the analyst to look at the images and to understand how the classifer will treat them.

Aircraft observations of large raindrops in warm, shallow, convective clouds

Raindrop size distributions have been obtained using airborne optical array probes during the 1985 Joint Hawaiian Warm Rain Project near Hilo. Drops often extended to 4 or 5 mm diameter, and on one occasion even reached 8 mm—much larger than had been previously reported. Large raindrops were detected frequently enough to indicate that they may be a regular feature of tropic clouds which are rather shallow but convectively active. The preliminary analysis of the data, involving some simple cloud physics calculations, indicates that the conditions in these clouds are well suited for the growth and survival of large raindrops. The presence of large raindrops suggests that a number of wide‐spread views on the formation and evolution of warm rain should be altered, and that earlier data obtained in the orographic clouds of Hawaii should not be extrapolated uncritically to all types of warm rain.

Visualization of gas voids behavior above the distributor in a three-dimensional gas fluidized bed.

A new system, consisting of a two-dimensional optical fiber array probe, video system and microcomputer, was used to visualize the behavior of jets and bubbles in a three-dimensional fluidized bed. By this system a series of successive pictures with time difference 1/30 s was obtained. The bed was 28 cm in diameter, provided with a nozzle of 0.325 cm i.d. fixed at the center of the gas distributor. The behavior of jets and initial bubbles in the grid region was visualized. The effect of bed material, distributor configuration, superficial gas velocity and nozzle gas velocity on this behavior was examined. The results showed that the jet height was affected by the properties of the bed material, but that the initial bubble diameter was scarcely affected thereby. Gas distributor configuration had an important effect on jet height and initial bubble properties other than frequency. Initial bubble shape and frequency were strongly dependent on superficial gas velocity rather than on nozzle gas velocity. A correlation to evaluate the jet height is proposed.

Application of an Airborne Optical Array Probe for Ground-Based Microphysical Observations

Abstract A mobile ground-based vehicle has been equipped with an aspirated 2D-C optical array cloud droplet probe from Particle Measuring Systems, Inc. for measuring the size and concentration of precipitation particles at ground level. This automated equipment continuously records images of precipitating particles that can be processed in the same manner as aircraft measurements. Probe aspiration, necessary to operate the 2D-C in a stationary mode, introduces new uncertainties that require detailed laboratory calibrations before confidence limits can be placed on output parameters. However, preliminary results show that the ground application of the 2D-C probe provides an improved method of observing precipitation changes continuously and with high time resolution.

Explosion dust particle size measurements

In situ measurements of the sizes and concentrations of dust particles generated by the detonation of high explosives in clay soil near Leesville, La., sandy clay soil near Huntsville, Ala., and sandy soils near Orogrande, N.M. are reported. Measurements were generally made within 1 m of the surface (in one case 10 m) at distances ranging from 10 to approximately 50 m from the detonation point with a combination of Knollenberg lightscattering counters (for particles with equivalent radius in the submicron to 15-microm range) and a Knollenberg optical array probe (for particles of 10-150 microm). Measurements were made for periods of several tens of seconds following detonation. All dust size distributions, irrespective of soil or explosive type, exhibit a bimodal character with mass mean radii of approximately 7 and 70 microm. Peak aerosol mass loadings inferred from the distributions have values ranging from 0.05 to 10 g gm-3 with the larger mode of particles contributing most to the mass loading. Predictions of dust extinction coefficients at visible (0.55-microm) and IR (10.4-microm) wavelengths were made using the measured size distributions together with estimates of dust refractive indices. These predictions suggest that extinction should be nearly neutral (wavelength independent) in agreement with transmission measurements made during some of the tests. Predicted mass extinction coefficients, under the assumption of dust material density of 2.5 g cm-3, are of the order of 0.05 m2 g-1 at both visible and IR wavelengths. This value is also in good agreement with a test-averaged measured value of 0.03 m3 g-1 (at lambda = 10.6 microm) obtained using a short path transmissometer and hi-vol sampler.