Drop Fall Velocity Research Articles

A small portable rainfall simulator for reproducible experiments on soil erosion

The importance of distinguishing and discretely studying the subprocesses of runoff generation and erosion has led to the development of rainfall simulations on small plots.We methodically upgraded a small portable rainfall simulator with particular respect to (1) rainfall characteristics that include homogeneous spatial rainfall distribution and drop spectrum, (2) handling, and (3) control of test conditions. We measured simulator characteristics with rain gauges, calibration plate and Laser Precipitation Monitor by Thies (LPM). The upgraded small rainfall simulator, and measurements of the improved rainfall characteristics are presented in this paper.The upgraded configuration shows the desired improvements: regarding drop size distribution, a close relationship to natural rainfall (Marshall & Palmer Distribution) can be observed. Due to low fall heights, measured drop fall velocities are slow; maximum velocities range between 3.4 and 5ms−1. Mean kinetic energy expenditure, mean kinetic energy per unit area and unit depth of rainfall and mean momentum are 214Jm−2h−1, 5.8Jm−2mm−1 and 0.016kgms−1, respectively. The spatial rainfall distribution of the upgraded simulator is homogenous with a Christiansen-Uniformity Coefficient of 91%. The measured variables show extremely low variation throughout all tests and should therefore be reproducible in field investigations at any time.

Sensitivity Analysis of Rain Characteristics on HTO Concentration in Drops

Precipitation scavenging coefficients, such as the HTO washout rate, are often derived from raindrop intensity and average velocity of drop. In this study, several Drop Size Distribution (DSD) functions based on experimental data are compared to calculate HTO concentration, by using Chamberlain equation, then the washout rate. Variability of Gamma, Lognormal and Marshall-Palmer distributions on the rain concentrations and washout rates are estimated. The fall velocities of drops are computed for every DSD with Kessler, Andronache, Seinfield and Loosmore and empirical formula used in the CEA code called CERES which is the CEA reference computational tool for impact assessment. Results show that the HTO concentration in drops and the scavenging coefficient depends very closely on rain characteristics. More than the choice of velocity drop calculations, which represent an uncertainty less than 20%, the choice of the distribution is important and depends on the rain observed. Certain assumptions can lead to uncertainty about a factor 2 to 3 on HTO concentration in rain and on the washout rate.

Application of piezoelectric transducers in simulated rainfall erosivity assessment

Rainfall simulators have often been used to mimic natural rainfall for studies of various land-surface and water interaction processes. The characteristics of the simulated rainfall are the main indicators used to judge the performance of the rainfall simulators. The aim of this study is to investigate the potential of piezoelectric transducers for measuring and evaluating a dripper-type simulated rainfall drop-size distribution (DSD) and kinetic energy (KE). The directly measured KE was significantly correlated with the estimated KE using the drop-size distribution and empirical rain drop fall velocity relationships. This result emphasizes the potential use of the piezoelectric sensor to directly measure and evaluate rainfall kinetic energy. Also, the relationship between rainfall intensity and KE showed good patterns of agreement between simulated rainfall and natural rainfall. Citation Abd Elbasit, M. A. M., Yasuda, H. & Salmi, A. (2011) Application of piezoelectric transducers in simulated rainfall erosivity assessment. Hydrol. Sci. J. 56(1), 187–194.

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

AbstractJoint 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.

Calibration of simulated rainfall characteristics for the study of soil erosion on agricultural land

Rainfall simulation is a widely used method for soil erosion studies on agricultural land. Major problem of this experimental research method is the comparability between different simulators due to differences in simulated rainfall. Therefore the purpose of this study is to characterize the rainfall produced by a rainfall simulator which was widely used during the last decades. Four different calibration methods were used to assess the drop size distribution: (1) Indication Paper, (2) Plaster Micro Plot, (3) Joss–Waldvogel Disdrometer and (4) Laser Distrometer (Thies). Additionally, the latter one was used to measure drop fall velocity in combination with drop diameter. The spatial rainfall distribution pattern on the plot was measured with 100 rainfall gauges. The spatial rainfall distribution pattern clearly shows a heterogeneity, which is caused by the used nozzle configuration. Considerable differences in drop-size distribution can be observed depending on the used measurement technique. Laser Disdrometer and Plaster Micro Plot cover the whole produced drop size spectrum ranging from <0.5 mm to >3.0 mm, whereas Indication Paper as well as the Joss–Waldvogel Disdrometer primarily show drops smaller than 2.0 mm. Characterisation of rainfall is therefore strongly dependent on the used method and if different methods are used, may lead to contradictory results. The volume drop size distribution reflected by the Laser Distrometer is very similar to that one produced by rain with an intensity of 40 mm h −1. Nevertheless, with maximum velocities above 10 m s −1 small drops are by far too fast and large drops with velocities dominantly below 5 m s −1 are too slow compared to natural rainfall. As an overall result, the simulator can be characterised as suitable for runoff and infiltration measurements, but with constraints due to the low reproducibility of the spatial rain distribution. As a consequence of the produced drop spectrum and fall velocity the erosion quantities may be underestimated systematically. For this, methodological development has to be focussed on homogeneous spatial rainfall distributions and on increasing the amount of large drops with higher fall velocities.

Instrumental Uncertainties inZ–RRelationships and Raindrop Fall Velocities

Abstract This study demonstrates the sensitivity of reflectivity–rainfall rate (Z–R) relationships, which were derived from disdrometer-based drop size distribution measurements, to the fall velocity of the drops. The dataset used comes from the simultaneous observation of a series of five moderate rainfall events with a Joss and Waldvogel disdrometer and an optical spectropluviometer. The signal-processing software of the latter was able to measure the residence time of the drops in its sampling volume, enabling computation of the volumetric drop concentration without any assumption on the fall velocity of the drops. The Z–R relationships derived from the two instruments are significantly different. This difference is shown to essentially arise from the drop fall velocities in a twofold manner. First, it comes from the use of a theoretical terminal fall velocity in still air to compute the drop concentration from the drop flux. Second, it comes from the principle of drop size measurement by the impact-ty...

Rain properties controlling soil splash detachment

Laboratory experiments have been conducted to study the effects of various rain properties on sand detachment resulting from raindrop impact. Splash cups were exposed to simulated rainfall intensities ranging between 10 and 140 mm h−1. The detached sand was collected and weighed whereas rain intensity, equivalent drop diameter and fall velocity of raindrops were measured with an optical spectro-pluviometer (OSP). The properties of the simulated rain (i.e. median volume diameter and kinetic energy) were compared with those observed in natural conditions. Statistical analyses have been undertaken in order to evaluate which rain variable best predicts the mass of sand detached. Linear and non-linear correlations between the mass of detached sediment and the product of drop size (d) by drop velocity (v), i.e. DαVβ, with values of α varying between 1 and 6 and β between 0 and 3, have been computed. The results indicate that the coefficient of determination (R2) for α ranging between 3 and 5 and β lower or equal to 2 are satisfying. Although kinetic energy (D3V2) described splash detachment relatively well, the product of momentum by drop diameter (D4V) was slightly superior in describing splash detachment. Therefore, the momentum multiplied by the drop diameter is recommended as the best rain variable to describe splash detachment. Copyright © 2000 John Wiley & Sons, Ltd.

Performance of an optical spectro pluviometer in measuring basic rain erosivity characteristics

The Optical Spectro Pluviometer (OSP), an infrared optical device, is used to measure the rain drop size, fall velocity and intensity. The OSP, which works on the principle of measuring an optical shadow, enables independent measurement of drop size and drop velocity in real time and thus any parameter linked to rain erosivity. Simulated rain characteristics measured with the OSP are reported. Calibrations made in the laboratory using single raindrops allow an estimation of the accuracy of drop size and velocity measurements. The single drop calibration shows that drop diameters are measured with an accuracy that is at least 6% in the range 0.3–4.7 mm. Larger drops are detected but without quantification of their diameter. The accuracy in the drop fall velocity measurements varies from 1% for the lowest velocity (0.2 m s −1) to 20% for the highest velocity (10 m s −1). Simulated and natural raindrop-size distributions measured by the OSP are compared with distribution obtained by the filter paper method. Taking into account the uncertainties associated with the two methods, the raindrop-size distributions derived from the OSP and the filter paper method are in good agreement. To complete the calibration, rain intensity is derived from the raindrop-size distribution measurements and compared with values measured by a nearby rain gauge. For rain intensities less than 35 mm h −1 the OSP underestimates intensity by 12%. However, for rain intensities in excess of 35 mm h −1, the OSP underestimates intensity by 38%. The errors on the two widely used rain erosivity indices, the kinetic energy and the momentum, are estimated to be less than 4%.

The Potential of 8-mm Radars for Remotely Sensing Cloud Drop Size Distributions

Abstract This paper describes the use of a vertically pointing 8.6-mm-wavelength Doppler radar for measuring drop size spectra in clouds. The data used were collected in the Atlantic Stratocumulus Transition Experiment in 1992. This paper uses the full Doppler velocity spectrum from the time series of Doppler radial velocities to extract information farther into the small-drop regime than previously attempted. The amount of liquid residing in the cloud regime is compared with that found in the precipitation regime where drop fall velocities are resolvable. Total liquid is compared with that measured with a collocated three-channel microwave radiometer. Examples of number density spectra, liquid water spectra, and flux spectra are shown and compared with what is known of these quantities from various in situ measurements by aircraft in similar clouds. Error estimates and uncertainties are discussed. It is concluded that 8-mm Doppler radars have the potential for broader use in cloud and precipitation studi...

Simulation of the size distribution and erosivity of raindrops and throughfall drops

AbstractThis paper presents a model that simulates the size distribution and erosivity of raindrops and throughfall drops. It utilizes existing models of rainfall drop size distribution and fall velocity and combines them with newly collated evidence of throughfall drop size distributions. A sensitivity analysis reveals that the model is sensitive to parameters that are easily measured or estimated: rainfall intensity, the mean volume drop diameter of the intercepted throughfall, canopy cover, and canopy height.The results of the model may be used at two levels. Firstly, to calculate specifically the size and fall velocity of individual drops, parameters that are needed in studies examining the response of soil surfaces to forces applied by rainfall. Secondly, to produce erosivity indices, based on rainfall intensity but which take account of the effects of a vegetation canopy. The paper shows that while the kinetic energy of rainfall (E(0), J mm−1 m−2) may be calculated from an equation of the familiar form: the kinetic energy of throughfall under any canopy may be calculated by combining this equation with another that relates the energy of drops under a 100 per cent canopy cover (E(100)) and the canopy height: .

Variations in raindrop size distributions measured at Rome

The knowledge of raindrop size distributions is of great interest to cloud physicists, radar meteorologists and communication engineers. Beginning from the Winter 1986–87, the Precipitation Group of the Atmospheric Physics Institute installed on the roof of the Institute building in Roma-Eur a disdrometer RD-69. The sensor is an electromechanical device with surface of 50 cm2 able to measure the raindrop diameter with a precision of 5% and a resolution (delay between two drops hitting the sensor) of one millisecond. After the calibration, significant precipitations were recorded continuously, with a sample period of 1 minute, during the whole life of the storms. It can be expected that a knowledge of the drop size distributions, and in particular of the differences between distrubitions, can be used to infer details of the storm and the environment. In this work the first results of the analyses are presented. Particular attention was conferred to the variation characteristics of the precipitation type (thunderstorm and widespread) as well as to the variations of the distributions in short time intervals, that may be caused by different effects, for example the growth and decay of a precipitation cell or the movement of a cell combined with the different fall velocities of small and large drops.

Effects of surface active agents on fall velocities of drops

AbstractTerminal velocities of drops undergoing mass transfer were measured in nine systems containing anionic, cationic, or nonionic surfactants. Variables included drop size, formation time, number of parallel droplet‐streams, and surfactant concentration. Results were interpreted in terms of visually observed internal circulation and oscillation of the drops and some success was achieved in correlating the data obtained.

Precipitation Characteristics at Vertical Incidence from Multiple Wavelength Doppler Radars

Abstract A vertically looking, multiple-wavelength Doppler radar technique to estimate vertical velocity, drop size distribution and turbulence is presented. The ratio of the Doppler spectra, corresponding to the drop fall velocities at two different wavelengths, is uniquely related to the ratio of the radar scattering cross sections at the appropriate temperature. These ratios are used to estimate drop fall velocities and therefore drop size distributions and vertical wind. Turbulence, which broadens the velocity power spectrum, can be estimated by deconvolution and the drop size distribution subsequently derived.

Determination of Salt Deposition Rates From Drift From Evaporative Cooling Towers

The purpose of this paper is to provide a method for estimating the deposition rate of the salt ejected in drift drops from a cooling tower in which salt water is circulated. The surface over which the salt will be distributed depends upon the wind speed, the maximum height the drops can reach, and the drop fall velocities. The drop fall velocity changes because of the evaporation in the air outside of the plume. The rate and extent of the evaporation is a function of the drop size, salt concentration, and ambient relative humidity. Three degrees of evaporation have been considered: (1) no evaporation; (2) evaporation to saturated solution and (3) evaporation to dry salt particles. For each degree of evaporation a graph was constructed that permits determination of the distance from the tower at which the salt contained in given drop size will fall. To use these graphs, the salt concentration, wind speed, and the maximum height a drop can reach must be known. The choice of the graph to be used for a particular case is made on the basis of humidity data for the site and the drift drop size. Corrections to compensate for the errors introduced by some of the assumptions are presented.

Equations for Calculating the Terminal Velocities of Water Drops

The experimental data of Gunn and Kinzer, Beard and Pruppacher, and Davies are used to curve-fit a polynomial for Re in terms of CdRe2. The resulting equations predict very accurately all known experimental data for drop fall velocities at sea level and at higher altitudes. Iteration of the equations is not necessary.

Electricity and rain

THE GENERATION OF ELECTRICITY and the formation of rain in thunderstorms are closely related through the physical and electrical interactions among colliding cloud particles, raindrops and hail. Our understanding of these processes started with the observations of Benjamin Franklin and Lord Rayleigh, in the 18th and 19th centuries, that electric fields exist in thunderclouds and that charged raindrops can coalesce. Modern theories indicate that different fall velocities of large and small drops are responsible for the separation of charge in the interior of the cloud.