scholarly journals Drop Axis Ratios from a 2D Video Disdrometer

2005 ◽  
Vol 22 (7) ◽  
pp. 966-978 ◽  
Author(s):  
Merhala Thurai ◽  
V. N. Bringi
Keyword(s):  
Equilibrium Shape ◽  
Terminal Velocity ◽  
Drop Diameter ◽  
Fall Velocity ◽  
Axis Ratio ◽  
Axisymmetric Mode ◽  
Spreading Function ◽  
Radar Applications ◽  
Linear Fit ◽  
The Mean

Abstract Results from an experiment to measure the drop shapes using a 2D video disdrometer (2DVD) are reported. Under calm conditions, drops were generated from a hose located on a bridge 80 m above ground, this height being sufficient to allow drop oscillations to reach a steady state. The disdrometer data had to be carefully processed so as to eliminate the drops mismatched by the instrument and to remove the system spreading function. The total number of drops analyzed was around 115 000. Their axis ratio distributions were obtained for diameters ranging from 1.5 to 9 mm. The mean axis ratio decreases with increasing drop diameter, in agreement with the upper bound of the Beard and Chuang equilibrium shape model. The inferred mode of oscillation appears to be dominated by the oblate–prolate axisymmetric mode for the diameter range of 1.5 to 9 mm. The mean axis ratio agrees well with two empirically fitted formulas reported in earlier studies. In addition, a linear fit was applied to the data for radar applications relating to rain retrievals from dual-polarization measurements. The 2DVD data taken in moderate stratiform rain were also analyzed in a similar way and the results agree with the artificially generated drop experiment, at least up to 4 mm. No data for larger diameters were available for stratiform precipitation. Finally, the fall velocity was examined in terms of drop diameter. The results closely follow an empirical formula fitted to the Gunn and Kinzer data as well as the Beard and Pruppacher data including a slight decrease in the terminal velocity with a diameter beyond 7 mm.

scholarly journals Drop Shapes and Axis Ratio Distributions: Comparison between 2D Video Disdrometer and Wind-Tunnel Measurements

2009 ◽  
Vol 26 (7) ◽  
pp. 1427-1432 ◽  
Author(s):  
M. Thurai ◽  
V. N. Bringi ◽  
M. Szakáll ◽  
S. K. Mitra ◽  
K. V. Beard ◽  
...  
Keyword(s):  
Wind Tunnel ◽  
Close Agreement ◽  
Drop Diameter ◽  
Wind Tunnel Experiments ◽  
Axis Ratio ◽  
Wind Tunnel Measurements ◽  
Mean Shape ◽  
The Mean ◽  
Ratio Versus ◽  
Made In

Abstract Comparisons of drop shapes between measurements made using 2D video disdrometer (2DVD) and wind-tunnel experiments are presented. Comparisons are made in terms of the mean drop shapes and the axis ratio distributions. Very close agreement of the mean shapes is seen between the two sets of measurements; the same applies to the mean axis ratio versus drop diameter. Also, in both sets of measurements, an increase in the oscillation amplitudes with increasing drop diameter is observed. In the case of the 2DVD, a small increase in the skewness was also detected. Given that the two sets of measurements were conducted in very different conditions, the agreement between the two sets of data implies a certain “robustness” in the mean shape of oscillating drops that may be extended to natural raindrop oscillations, at least in steady rainfall and above the surface layer.

scholarly journals Raindrop fall velocity in turbulent flow: an observational study

2021 ◽  
Vol 18 ◽  
pp. 33-39
Author(s):  
Merhala Thurai ◽  
Viswanathan Bringi ◽  
Patrick Gatlin ◽  
Mathew Wingo
Keyword(s):  
Turbulent Flow ◽  
Gold Standard ◽  
Sonic Anemometer ◽  
Terminal Velocity ◽  
Laboratory Measurements ◽  
Air Velocity ◽  
Fall Velocity ◽  
Velocity Fluctuations ◽  
The Mean ◽  
Rain Events

Abstract. Laboratory measurements of drop fall speeds by Gunn–Kinzer under still air conditions with pressure corrections of Beard are accepted as the “gold standard”. We present measured fall speeds of 2 and 3 mm raindrops falling in turbulent flow with 2D-video disdrometer (2DVD) and simultaneous measurements of wind velocity fluctuations using a 3D-sonic anemometer. The findings based on six rain events are, (i) the mean fall speed decreases (from the Gunn–Kinzer terminal velocity) with increasing turbulent intensity, and (ii) the standard deviation increases with increase in the rms of the air velocity fluctuations. These findings are compared with other observations reported in the literature.

II.—The Effect of Concentration on the Terminal Fall Velocity of Particles in Suspension

1965 ◽  
Vol 67 (1) ◽  
pp. 9-24
Author(s):  
Hugh Rankin Thorpe
Keyword(s):  
Power Series ◽  
Terminal Velocity ◽  
Fall Velocity ◽  
Dispersed Particles ◽  
Nominal Diameter ◽  
The Mean ◽  
The Individual ◽  
Image Position ◽  
Particle Shapes ◽  
Individual Particles

SynopsisThe paper describes an investigation of the terminal velocity of uniformly dispersed particles of various shapes, sizes and densities falling through water.It is concluded that for concentrations above 0–5 per cent by weight, the suspension as a whole behaves as though it were viscous even though the individual particles lie well outside the Stokes range. The shape of the particles has a significant effect only when the concentration is less than 0·5 per cent, and for concentrations between 0·5 and 7·0 per cent, the relative changes in velocity of descent are adequately described for a range of particle shapes from highly angular to spherical and for sizes at least up to 0·65 mm. nominal diameter, by the power seriesin which U is the velocity of the suspension, U0 that of a single particle, d the nominal diameter (i.e. that of a sphere having the same volume) and s the mean spacing of the particles.If the concentration is lower than 4 per cent, the equation may be assumed linear in (d/s) without serious error.

scholarly journals Distributions of Raindrop Sizes and Fall Velocities in a Semiarid Plateau Climate: Convective versus Stratiform Rains

2010 ◽  
Vol 49 (4) ◽  
pp. 632-645 ◽  
Author(s):  
Shengjie Niu ◽  
Xingcan Jia ◽  
Jianren Sang ◽  
Xiaoli Liu ◽  
Chunsong Lu ◽  
...  
Keyword(s):  
Terminal Velocity ◽  
Systematic Change ◽  
Drop Diameter ◽  
Size Distributions ◽  
Fall Velocity ◽  
Convective Clouds ◽  
Stratiform Clouds ◽  
The Difference ◽  
Raindrop Size ◽  
Rain Rates

Abstract Joint size and fall velocity distributions of raindrops were measured with a Particle Size and Velocity (PARSIVEL) precipitation particle disdrometer in a field experiment conducted during July and August 2007 at a semiarid continental site located in Guyuan, Ningxia Province, China (36°N, 106°16′E). Data from both stratiform and convective clouds are analyzed. Comparison of the observed raindrop size distributions shows that the increase of convective rain rates arises from the increases of both drop concentration and drop diameter while the increase of the rain rate in the stratiform clouds is mainly due to the increase of median and large drop concentration. Another striking contrast between the stratiform and convective rains is that the size distributions from the stratiform (convective) rains tend to narrow (broaden) with increasing rain rates. Statistical analysis of the distribution pattern shows that the observed size distributions from both rain types can be well described by the gamma distribution. Examination of the raindrop fall velocity reveals that the difference in air density leads to a systematic change in the drop fall velocity while organized air motions (updrafts and downdrafts), turbulence, drop breakup, and coalescence likely cause the large spread of drop fall velocity, along with additional systematic deviation from terminal velocity at certain raindrop diameters. Small (large) drops tend to have superterminal (subterminal) velocities statistically, with the positive deviation from the terminal velocity of small drops being much larger than the negative deviation of large drops.

scholarly journals Geometrical Properties of Hydrometeors during the Refreezing Process and Their Effects on Dual-Polarized Radar Signals

2019 ◽  
Vol 147 (5) ◽  
pp. 1753-1768 ◽  
Author(s):  
Nobuhiro Nagumo ◽  
Ahoro Adachi ◽  
Hiroshi Yamauchi
Keyword(s):  
Spherical Shape ◽  
Single Mode ◽  
Terminal Velocity ◽  
Winter Storm ◽  
Fall Velocity ◽  
Axis Ratio ◽  
Geometrical Properties ◽  
Initial Stage ◽  
Dry Conditions ◽  
Differential Reflectivity

Abstract This paper describes an observational study for the geometrical properties of hydrometeors during a refreezing process initiated by a winter storm in the northern Kanto Plain, Japan, on 29 January 2016, in which a subfreezing layer developed below a melting layer. The observations by using dual-polarization radar showed consistency between high values of differential reflectivity ZDR signals in midair and ice-pellet reports at the surface. The high ZDR was indicative of the sideways-oriented particles with a small axis ratio. The low ZDR signals in midair corresponded with the reports of rain or rain/ice-pellet mixtures. Observations by using a two-dimensional video disdrometer (2DVD) near the ground showed different microphysics corresponding to high ZDR and low ZDR periods. The high ZDR periods of 2DVD observations indicated that the hydrometeors exhibited dual modes of fall velocities, namely, fast-falling and slow-falling modes. The fast-falling particles were found to be deformed ice pellets with long sideways orientations that contributed to the high ZDR. The slow-falling particles were also deformed ice pellets but with a variety of orientations. This feature was rather close to that of general dry conditions of ice particles in the atmosphere. Meanwhile, the low ZDR periods of 2DVD observations indicated that the hydrometeors exhibited a single mode of fall velocity close to the terminal velocity of raindrops, but with a more spherical shape compared to raindrops. Hence, it is suggested that the high ZDR signal occurs during freezing between the initial stage of spherical ice forming and completely freezing stage of ice pellets with a variety of orientations.

scholarly journals The Terminal Velocity of Axisymmetric Cloud Drops and Raindrops Evaluated by the Immersed Boundary Method

2021 ◽  
Vol 78 (4) ◽  
pp. 1129-1146
Author(s):  
Chia Rui Ong ◽  
Hiroaki Miura ◽  
Makoto Koike
Keyword(s):  
Density Dependence ◽  
Laboratory Data ◽  
Terminal Velocity ◽  
Immersed Boundary ◽  
Atmospheric Conditions ◽  
Fall Velocity ◽  
Axis Ratio ◽  
Model Calculations ◽  
Air Density ◽  
Free Falling

AbstractThe terminal velocity of cloud drops and raindrops used in numerical model calculations can significantly affect weather predictions. Current formulations rely on laboratory experiments made in the 1940s and 1960s. Because these experiments were performed only at typical environmental conditions of 20°C and 1013 hPa, parameterizations have been introduced to deduce the terminal velocity aloft without rigorous evaluation. In this study, an incompressible two-phase flow direct numerical simulation model is used to calculate the free-falling motion of axisymmetric drops with diameters between 0.025 and 0.5 mm to study the terminal fall velocity. Simulated terminal fall velocities of free-falling drops at 20°C and 1013 hPa agree within 3.2% with the previous empirical parameterization (Beard formula), and 4.5% with existing laboratory data in the diameter range between 0.3 and 0.5 mm. The velocities converge to the analytic Hadamard–Rybczynski solution within 2% for small Reynolds numbers, demonstrating the robustness of our simulations. Simulations under various atmospheric conditions show that existing empirical parameterizations that account for the air density dependence of the terminal velocity have errors up to 11.8% under the conditions examined in this study. We propose a new empirical formula that describes the air density dependence of the terminal velocity. It is also shown that the falling speed of a small drop is not sensitive to shape oscillation, and the terminal velocity decreases by only less than 1.3% when the axis ratio increases by 12% with reduced surface tension. Internal circulation within falling drops is also presented and compared with previous studies.

scholarly journals A Wind Tunnel Study on the Shape, Oscillation, and Internal Circulation of Large Raindrops with Sizes between 2.5 and 7.5 mm

2009 ◽  
Vol 66 (3) ◽  
pp. 755-765 ◽  
Author(s):  
Miklós Szakáll ◽  
Karoline Diehl ◽  
Subir K. Mitra ◽  
Stephan Borrmann
Keyword(s):  
Wind Tunnel ◽  
Oscillation Frequency ◽  
Force Balance ◽  
Equivalent Diameter ◽  
Drop Diameter ◽  
Detailed Knowledge ◽  
Shape Parameters ◽  
Internal Circulation ◽  
Frequency Method ◽  
Axis Ratio

Abstract Precipitation prediction using weather radars requires detailed knowledge of the shape parameters of raindrops falling at their terminal velocities in air. Because the raindrops undergo oscillation, the most important shape parameters from the radar prediction point of view are the equilibrium drop shape, the time-averaged axis ratio, and the oscillation frequency. These parameters for individual water drops with equivalent diameter from 2.5 to 7.5 mm were investigated in a vertical wind tunnel using high-speed video imaging. A very good agreement was found between the measured and the theoretically determined raindrop shape calculated by a force balance model. A new method was developed to determine the equivalent drop diameter with the help of the oscillation frequency. The drop size determination by means of the frequency method was found to be three times more precise than by volumetric methods. The time-averaged axis ratio was found to be equal to the equilibrium axis ratio in the investigated raindrop size range. The analysis of the oscillation frequency of the raindrops revealed that the drops undergo multimode oscillations and are oscillating in a transverse mode in addition to an axisymmetric oblate–prolate mode. Experiments are included in which the internal circulation associated with drop oscillation was investigated and compared to theory.

Refinements to Ice Particle Mass Dimensional and Terminal Velocity Relationships for Ice Clouds. Part II: Evaluation and Parameterizations of Ensemble Ice Particle Sedimentation Velocities

2007 ◽  
Vol 64 (4) ◽  
pp. 1068-1088 ◽  
Author(s):  
Andrew J. Heymsfield ◽  
Gerd-Jan van Zadelhoff ◽  
David P. Donovan ◽  
Frederic Fabry ◽  
Robin J. Hogan ◽  
...  
Keyword(s):  
Climate Models ◽  
Doppler Radar ◽  
Terminal Velocity ◽  
Fall Velocity ◽  
Particle Sedimentation ◽  
Cloud Properties ◽  
Wide Range ◽  
Single Moment ◽  
Ice Water Content ◽  
Ice Water

Abstract This two-part study addresses the development of reliable estimates of the mass and fall speed of single ice particles and ensembles. Part I of the study reports temperature-dependent coefficients for the mass-dimensional relationship, m = aDb, where D is particle maximum dimension. The fall velocity relationship, Vt = ADB, is developed from observations in synoptic and low-latitude, convectively generated, ice cloud layers, sampled over a wide range of temperatures using an assumed range for the exponent b. Values for a, A, and B were found that were consistent with the measured particle size distributions (PSD) and the ice water content (IWC). To refine the estimates of coefficients a and b to fit both lower and higher moments of the PSD and the associated values for A and B, Part II uses the PSD from Part I plus coincident, vertically pointing Doppler radar returns. The observations and derived coefficients are used to evaluate earlier, single-moment, bulk ice microphysical parameterization schemes as well as to develop improved, statistically based, microphysical relationships. They may be used in cloud and climate models, and to retrieve cloud properties from ground-based Doppler radar and spaceborne, conventional radar returns.

The gravitational settling of aerosol particles in homogeneous turbulence and random flow fields

1987 ◽  
Vol 174 ◽  
pp. 441-465 ◽  
Author(s):  
M. R. Maxey
Keyword(s):  
Settling Velocity ◽  
High Strain ◽  
Free Fall ◽  
Terminal Velocity ◽  
Homogeneous Turbulence ◽  
Mean Flow ◽  
Particle Inertia ◽  
The Mean ◽  
Random Flow ◽  
Flow Particle

The average settling velocity in homogeneous turbulence of a small rigid spherical particle, subject to a Stokes drag force, is shown to depend on the particle inertia and the free-fall terminal velocity in still fluid. With no inertia the particle settles on average at the same rate as in still fluid, assuming there is no mean flow. Particle inertia produces a bias in each trajectory towards regions of high strain rate or low vorticity, which affects the mean settling velocity. Results from a Gaussian random velocity field show that this produces an increased settling velocity.

scholarly journals Measuring Hydrometeors with a Precipitation Microphysical Characteristics Sensor: Calibration and Field Measurements

2017 ◽  
Vol 2017 ◽  
pp. 1-16 ◽  
Author(s):  
Yuntao Hu ◽  
Xichuan Liu ◽  
Taichang Gao ◽  
Xiaojian Shu
Keyword(s):  
Field Measurements ◽  
Rain Gauge ◽  
Sensor Calibration ◽  
Threshold Method ◽  
Fall Velocity ◽  
Axis Ratio ◽  
Measurement Results ◽  
Accumulated Rainfall ◽  
Typical Shape ◽  
Particle Imaging

Aiming at the simultaneous measurement of the size, shape, and fall velocity of precipitation particles in the natural environment, we present here a new ground-based precipitation microphysical characteristics sensor (PMCS) based on the particle imaging velocimetry technology. The PMCS can capture autocorrelated images of precipitation particles by double-exposure in one frame, by which the size, axis ratio, and fall velocity of precipitation particles can be calculated. The PMCS is calibrated by a series of glass balls with certain diameters under varying light conditions, and a self-adaptive threshold method is proposed. The shape, axis ratio, and fall velocity of raindrops were calculated and discussed based on the field measurement results of PMCS. The typical shape of large raindrop is an oblate ellipsoid, the axis ratio of raindrops decreases linearly with the diameter, the fall velocity of raindrops approaches its asymptote, and the above observed results are in good agreement with the empirical models; the synchronous observation of a PMCS, an OTT PARSIVEL disdrometer, and a rain gauge shows that the PMCS is able to measure the rain intensity, accumulated rainfall, and drop size distribution with high accuracy. These results have validated the performance of PMCS.

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