•Drop-size distribution measurements below and above tree canopy were conducted.•Effect of birch tree on the soil erosion related processes was analysed.•A decrease in the raindrops erosivity below ...the canopy was demonstrated.•Impact of leafed and leafless period on the rainfall erosivity was significant.
Healthy soils are one of the key priorities of the EU Soil Strategy, and soil erosion can present a threat to soils around the world. Rainfall erosivity is the main driver of soil erosion by water. Rainfall interception by vegetation can reduce the erosive power of raindrops, and consequently measures involving vegetation can mitigate soil erosion losses. In this study, the effect of rainfall interception on the erosive power of raindrops under the birch tree in an urban park in the city of Ljubljana is investigated. More than one year of measurements of drop-size distribution using two optical disdrometers placed above and below the birch tree canopy were used to investigate the impact of rainfall interception on the erosive power of raindrops. The number of drops, fall velocity, and drop diameter were, on average smaller below the canopy in comparison to the measurements above the canopy for 20%, 7% and 27%, respectively. This also resulted in a reduction in the rainfall kinetic energy (3% and 30% in the leafless and leafed periods, respectively) and rainfall erosivity (21% and 50% for the leafless and leafed periods, respectively). The results demonstrate that rainfall interception has a significant seasonal influence on the erosive power of raindrops. Therefore, vegetation characteristics should be considered as time-varying rather than constant parameters in soil erosion modelling studies.
Understanding drop size distribution (DSD) variability has important implications for remote sensing and numerical modeling applications. Twelve disdrometer datasets across three latitude bands are ...analyzed in this study, spanning a broad range of precipitation regimes: light rain, orographic, deep convective, organized midlatitude, and tropical oceanic. Principal component analysis (PCA) is used to reveal comprehensive modes of global DSD spatial and temporal variability. Although the locations contain different distributions of individual DSD parameters, all locations are found to have the same modes of variability. Based on PCA, six groups of points with unique DSD characteristics emerge. The physical processes that underpin these groups are revealed through supporting radar observations. Group 1 (group 2) is characterized by high (low) liquid water content (LWC), broad (narrow) distribution widths, and large (small) median drop diameters D
0
. Radar analysis identifies group 1 (group 2) as convective (stratiform) rainfall. Group 3 is characterized by weak, shallow radar echoes and large concentrations of small drops, indicative of warm rain showers. Group 4 identifies heavy stratiform precipitation. The low latitudes exhibit distinct bimodal distributions of the normalized intercept parameter N
w
, LWC, and D
0
and are found to have a clustering of points (group 5) with high rain rates, large N
w
, and moderate D
0
, a signature of robust warm rain processes. A distinct group associated with ice-based convection (group 6) emerges in the midlatitudes. Although all locations exhibit the same covariance of parameters associated with these groups, it is likely that the physical processes responsible for shaping the DSDs vary as a function of location.
Quantifying the influence of rain in LiDAR performance Filgueira, A.; González-Jorge, H.; Lagüela, S. ...
Measurement : journal of the International Measurement Confederation,
January 2017, 2017-01-00, 20170101, Volume:
95
Journal Article
Peer reviewed
•Methodology to evaluate the relationship between LiDAR performance under rainy weather.•LiDAR intensity decreases with rain increasing. Pavement is the less affected material.•LiDAR detected points ...decrease with rain increasing.
LiDAR systems emerge as one of the key systems for autonomous vehicles. The present work quantifies the influence of rain in different LiDAR parameters: range, intensity, and number of detected points. Six areas with different target materials are used for the study. Range measurements appear stable even under important rain affectation. The variations are always lower than 20cm. These variations come from the experimental procedure (averaging of points detected from a surface) and not from the instabilities in the LiDAR detection with rain. The detected LiDAR intensity and the sampled points attenuate with the increasing of rain intensity. Drop size distribution is assumed constant along the study area. The highest decrease in the number of points appears for pavement. However, the intensity returned from pavement is not specially influenced by rain. The rest of the materials show similar trend in the intensity and the number of detected points.
Using high-resolution mass spectrometry, we have studied the synthesis of isoquinoline in a charged electrospray droplet and the complexation between cytochrome c and maltose in a fused droplet to ...investigate the feasibility of droplets to drive reactions (both covalent and noncovalent interactions) at a faster rate than that observed in conventional bulk solution. In both the cases we found marked acceleration of reaction, by a factor of a million or more in the former and a factor of a thousand or more in the latter. We believe that carrying out reactions in microdroplets (about 1–15 μm in diameter corresponding to 0·5 pl – 2 nl) is a general method for increasing reaction rates. The mechanism is not presently established but droplet evaporation and droplet confinement of reagents appear to be two important factors among others. In the case of fused water droplets, evaporation has been shown to be almost negligible during the flight time from where droplet fusion occurs and the droplets enter the heated capillary inlet of the mass spectrometer. This suggests that (1) evaporation is not responsible for the acceleration process in aqueous droplet fusion and (2) the droplet–air interface may play a significant role in accelerating the reaction. We argue that this ‘microdroplet chemistry’ could be a remarkable alternative to accelerate slow and difficult reactions, and in conjunction with mass spectrometry, it may provide a new arena to study chemical and biochemical reactions in a confined environment.
Nowadays, numerical simulations of atomization has reached a mature state through interface capturing approaches. With these approaches, the liquid–gas flow with its complex interface morphology can ...be precisely described but at the price of high mesh resolution, that makes these simulations very intensive in terms of computational cost. Practically, for most of the injection system, it is not possible to achieve a simulation up to a point where the spray formation is completed. Thus, there are several attempts based on the transformation of liquid elements such as ligaments, liquid sheet or other non-spherical to equivalent droplets. Overall, these approaches assumed that before the complete finalization of the atomization process the spray under formation carries enough information to be representative of the final spray. In this work, we have pushed this idea a step further by using global variables such as the liquid volume fraction and the surface density available in ELSA models to ensure the transports and the conservation of the main features of the spray. Then, the surface curvature distribution is analysed assuming that a part of the interface carries already curvatures that are relevant with respect to the final spray. A well-known academic test case representative of an aeronautical injection system and based on a planar prefilmer atomizer with a gas co-flow has been selected to evaluate our proposal. This configuration was studied both experimentally and numerically thanks to high-fidelity simulations. Our purpose has been to follow the already validated numerical approaches but with less computationally intensive simulations. Then, new analysis based on surface density and surface curvature distribution have been tested to recover spray characteristics and even the spray diameter distribution. It appears that these variables are meaningful even when there is no droplet yet formed, and thus they allow the description of the full atomization process from the early stage even on the liquid film. Finally, a procedure has been proposed where the spray is sufficiently atomized to reconstruct the diameter distribution from the curvature distribution. At our best knowledge, this last step is a first attempt on practical injection system. We have proposed a very simple method to detect which part of the interface carries the relevant curvature, thus obtaining a reconstructed diameter distribution that fit well with available experimental data. Although the proposed approach cannot be still considered universally applicable to any case, this preliminary assessment clearly shows its potential.
•Numerical simulation of the atomization occurring in an air blast atomizer.•Numerical determination of the amount of liquid–gas surface along that atomization process.•Numerical determination of the surface curvature distribution along the atomization process.•A new approach is proposed to obtain the drop size distribution from curvature distribution and liquid volume fraction.•This approach allows the determination of the spray size distribution at an early stage of the atomization process.
Abstract In situ data collected by three research aircraft in four geographical locations are analyzed to determine the relationship between cloud-base temperature, drop size distribution, and the ...development of supercooled water drops and ice in strong updraft cores of convective clouds. Data were collected in towering cumulus and feeder cells in the Caribbean, over the Gulf of Mexico, over land near the Gulf Coast, over land in the southeastern United States, and the high plains in Colorado and Wyoming. Convective clouds in the Caribbean, over the Gulf of Mexico and its coast, and over the southeastern United States all develop millimeter-diameter supercooled drops in updraft cores. Clouds over the high plains do not generate supercooled large drops, and rarely are drops >70 μm observed in updraft cores. Commensurate with the production of supercooled large drops, ice is generated and rapidly glaciates updraft cores through a hypothesized secondary ice process that is based on laboratory observations of large drops freezing and emitting tiny ice particles. Clouds over the high plains do not experience the secondary ice process and significant concentrations of supercooled liquid in the form of small drops are carried much higher (up to −35.5°C) in the updraft cores. An empirical relationship that estimates the maximum level to which supercooled liquid water will be transported, based on cloud-base drop size distribution and temperature, is developed. Implications have applications for modeling the transport of water vapor and particles into the upper troposphere and hygroscopic seeding of cumulus clouds.
In this paper, we study the primary atomization characteristics of liquid jet injected into a gaseous crossflow through direct numerical simulations (DNS) and large eddy simulations (LES). The DNS ...use a coupled level set volume of fluid (CLSVOF) sharp interface capturing method resolving all relevant scales to predict the drop size distribution (DSD) for drops larger than the grid spacing. The LES use a volume of fluid (VOF) diffused interface method modelling the sub grid droplets. The purpose of this paper is to provide a comparison of the results of drop data between DNS and LES. The simulations are performed for a liquid jet injection with liquid-gas momentum flux ratio of 6.6, liquid jet Reynolds number of 14,000 injected into a crossflowing air with Reynolds number 570,000 and Weber number of 330 at a liquid-to-gas density ratio of 10. Two distinct and simultaneous atomization/breakup mechanisms have been observed in the simulations: column/bag breakup and ligament/surface breakup. It was found that the DSDs obtained from the DNS and LES each follow a log-normal distribution based on their respective droplet diameter data. An overlap region exists between the individual DSDs from the DNS and LES when combined. The width of this overlap region decreases along the downstream direction. A log-normal distribution is found to be a good fit to the combined DSD incorporating both resolved and sub-grid droplets. This information is relevant for the secondary atomization simulations and modeling.
The variability in the microphysical properties of raindrop size distribution (DSD) over the rain shadow region of Indian peninsula is investigated in this study. Simultaneous in-situ measurements of ...precipitating cloud systems by an impact disdrometer, Micro Rain Radar (24 GHz), and wind profiler (1.28 GHz) are used in this study. The normalized Gamma distribution parameters for five DSD clusters of precipitating systems, corresponding to the non-convective, deep convective, shallow convective, mixed-type, and stratiform rain types were obtained using the k-means clustering algorithm. Four DSDs are nearly bounded by shape parameters, −3 < μ < 25, except for stratiform DSDs (−3 < μ < 45) indicating a wide variation in DSD shape. Bimodal (mono-modal) distributions in terms of frequency of occurrence of Nw are observed in the non-convective, deep convective and shallow convective (stratiform and mixed) clouds. The bimodality is attributed to the large raindrops formed by the collision and coalescence (CC) process, and the small raindrops due to breakup that survive evaporation and reach the ground. The mono-modality of stratiform rain is attributed to the in-efficient CC and CC-breakup process. These rain microphysical processes are further characterized using the radar reflectivity factor (Z)-Rainfall Intensity (R) relationships of the form Z = aRb. Finally, we found that the convective DSDs over the region appear to be a composite of both maritime and continental characteristics.
•Microphysical properties of non-convective, shallow, convective, mixed, and stratiform rains are described.•Parameterization of normalized Gamma DSD is done for DSD clusters. DSDs are maritime-continental origin•Collision-coalescence process dominates in non-convective, shallow, deep convective, and mixed rain.
Rain Attenuation at Millimeter Wave and Low-THz Frequencies Norouzian, Fatemeh; Marchetti, Emidio; Gashinova, Marina ...
IEEE transactions on antennas and propagation,
2020-Jan., 2020-1-00, 20200101, Volume:
68, Issue:
1
Journal Article
Peer reviewed
Wave attenuation through rain with different rainfall rates at millimeter wave ( f = 77 GHz) and low-terahertz (Low-THz) ( f = 300 GHz) frequencies is studied in this article. Rain has pronounced ...impacts on electromagnetic wave propagation and one of the well-known effects is attenuation of the transmitted wave. Attenuation at both frequencies and hydrometeor properties rainfall rate and drop size distribution (DSD) are measured simultaneously. The measured DSD is fit with gamma and Weibull distributions and is also compared to the frequently used distribution Marshall and Palmer (MP) model; Weibull is shown to be a better fit to the measured DSDs. Theoretical prediction of attenuation as a function of rainfall rate (up to about 20 mm/h) is determined using Mie scattering theory, and the fit gamma and Weibull, and MP distribution models; as well as using the International Telecommunications Union Radiocommunication Sector (ITU-R) recommendation. The calculations are evaluated by comparing them to the experiment. The measured results at 77 GHz best agree with the ITU-R recommendation whereas at 300 GHz, the calculation based on Mie scattering and the Weibull distribution exhibits the best fit to the measured data. The measured data that exceed the theoretical prediction are analyzed and interpreted based on their corresponding observed drop size properties, for the first time.
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