Raindrop Shape Research Articles

AIMHNet: An Attribute-Insensitive Multiscale Hourglass Network for Rain Streak and Raindrop Removal

CNN-based methods have made great progress in single-image rain removal. Most recent methods improve performance by increasing the depth of the network. To fully extract local and global features while reducing inference time, we propose a top-to-down attribute-insensitive multiscale hourglass network for rain streak and raindrop removal. For the rain removal task, we expect that the constructed network can accurately identify the various attributes of the rain information characteristics of the small target. Considering the difference in the size, shape, direction and density of rain streak and raindrop, inspired by the performance of hourglass architecture to capture multiscale features in human pose estimation, we introduce an attribute-insensitive hourglass module to recognize the attributes of rain streak and raindrop in a unified framework. This feature extraction module could capture the characteristics of rain streak and raindrop with different attributes. This stacked hourglass blocks down-sample features and then up-samples them back to the original resolution based on discrete wavelet transform and inverse discrete wavelet transform. We perform extensive experiments on five synthetic and real-world de-raining datasets to validate the effectiveness of our proposed network on rain streak and raindrop removal. The qualitative and quantitative results show that our method is suitable for removing rain streak and raindrop in a unified framework. We present the results of generalization and ablation study for key components, we also report the accuracy of semantic segmentation after preprocessing with all rain removal methods. Our source code will be available on the GitHub: https://github.com/Ruini94/AIMHNet .

A database of the raindrop scattering properties at millimeter and sub-millimeter wavelengths

Currently, the majority of publicly raindrop scattering database are calculated with spherical particles. However, as the raindrop grows, the bottom of the raindrop gradually flattens out. The differences of raindrop shape will lead to a difference of the calculated scattering parameters from reality, and further influencing the accuracy of calculations in the fields of radar detection, microwave transmission and satellite remote sensing. In this regard, we developed a non-spherical raindrop scattering parameter database (RP database) with frequencies from 3GHz to 1000GHz. It has been published in an open-access repository. We compared the RP database with the database of Ekelund et al., and found that their difference is basically under 5%; Further the comparisons were also made between the scattering parameters of ellipsoids, spherical raindrops and the non-spherical raindrops, both spherical and ellipsoidal particles have clear difference from non-spherical particles. Finally, we analyzed the group particle attenuation at different frequencies.

Vortex-induced vibration at low Reynolds numbers: Vortex shedding modes transitions

A numerical model based on the finite difference method is presented to investigate the vortex-induced vibration of a circular cylinder with two degrees of freedom. The immersed boundary method is used to deal with boundary conditions at fluid–solid interfaces. Systematic studies are conducted for Ur = 3–8 (reduced velocity), Re = 170–210 (Reynolds number), m* = 3–10 (mass ratio) and ζ = 0–0.01 (damping ratio). The numerical results indicate that the “P + S” vortex shedding mode and the raindrop-shaped trajectory are more likely to appear in the range of Ur = 4.7–5.1, Re > 180, m*>5, and ζ < 0.007, which correspond to the streamwise resonance. The streamwise displacement exhibits two frequencies fx/fn = 1 and fx/fn = 2, and their amplitude ratios RA = Afx/fn = 1/Afx/fn = 2 can be applied to identify the motion trajectory of the structure and the vortex shedding mode. The trajectory is a raindrop shape and the vortex shedding mode is asymmetric “P + S” for RA > 1.6; the trajectory is an asymmetric “8” shape and the vortex shedding mode is asymmetric “2P” for RA < 1.6; and the trajectory is a symmetric “8” shape and the vortex shedding mode is “2S” or “2P” for RA = 0. These findings contribute to a better understanding of the formation conditions of the “P + S” vortex shedding mode and its transitions with the other vortex shedding modes.

Raindrop-Removal Image Translation Using Target-Mask Network with Attention Module

Image processing plays a crucial role in improving the performance of models in various fields such as autonomous driving, surveillance cameras, and multimedia. However, capturing ideal images under favorable lighting conditions is not always feasible, particularly in challenging weather conditions such as rain, fog, or snow, which can impede object recognition. This study aims to address this issue by focusing on generating clean images by restoring raindrop-deteriorated images. Our proposed model comprises a raindrop-mask network and a raindrop-removal network. The raindrop-mask network is based on U-Net architecture, which learns the location, shape, and brightness of raindrops. The rain-removal network is a generative adversarial network based on U-Net and comprises two attention modules: the raindrop-mask module and the residual convolution block module. These modules are employed to locate raindrop areas and restore the affected regions. Multiple loss functions are utilized to enhance model performance. The image-quality assessment metrics of proposed method, such as SSIM, PSNR, CEIQ, NIQE, FID, and LPIPS scores, are 0.832, 26.165, 3.351, 2.224, 20.837, and 0.059, respectively. Comparative evaluations against state-of-the-art models demonstrate the superiority of our proposed model based on qualitative and quantitative results.

Versatile Approach of Silicon Nanofabrication without Resists: Helium Ion-Bombardment Enhanced Etching.

Herein, we report a helium ion-bombardment enhanced etching method for silicon nanofabrication without the use of resists; furthermore, we demonstrate its unique advantages for straightforward fabrication on irregular surfaces and prototyping nano-electro-mechanical system devices, such as self-enclosed Si nanofluidic channels and mechanical nano-resonators. This method employs focused helium ions to selectively irradiate single-crystal Si to disrupt the crystal lattice and transform it into an amorphous phase that can be etched at a rate 200 times higher than that of the non-irradiated Si. Due to the unique raindrop shape of the interaction volumes between helium ions and Si, buried Si nanofluidic channels can be constructed using only one dosing step, followed by one step of conventional chemical etching. Moreover, suspended Si nanobeams can be fabricated without an additional undercut step for release owing to the unique raindrop shape. In addition, we demonstrate nanofabrication directly on 3D micro/nano surfaces, such as an atomic force microscopic probe, which is challenging for conventional nanofabrication due to the requirement of photoresist spin coating. Finally, this approach can also be extended to assist in the etching of other materials that are difficult to etch, such as silicon carbide (SiC).

Analysis of Pre-Monsoon Convective Systems over a Tropical Coastal Region Using C-Band Polarimetric Radar, Satellite and Numerical Simulation

Analysis of pre-monsoon convective systems over the southern peninsular India has been performed using C-band radar and numerical simulation. Statistics on the radar polarimetric measurements show that the distribution of differential reflectivity (Zdr) and specific differential phase (Kdp) have much higher spread over convective regions. The distribution of Kdp is almost uniform across the vertical over the stratiform regions. The mean profile of Zdr over stratiform regions shows a distinct local maxima near melting level. A comprehensive analysis has been done on an isolated deep convective system on 13 May 2018. Plan position indicator (PPI) diagrams and satellite measured cloud top temperature demonstrate that pre-monsoon deep convective systems can develop very rapidly within a very short span of time over the region. Heavy precipitation near the surface is reflected in the high value of Kdp (&gt;5° km−1). High values of Zdr (&gt;3 dB) were measured at lower levels indicating the oblate shape of bigger raindrops. A fuzzy logic-based hydrometeor identification algorithm has been applied with five variables (Zh, Zdr, ρhv, Kdp, and T) to understand the bulk microphysical properties at different heights within the storm. The presence of bigger graupel particles near the melting layer indicates strong updrafts within the convective core regions. The vertical ice hydrometeor signifies the existence of a strong electric field causing them to align vertically. Numerical simulation with the spectral bin microphysics (SBM) scheme could produce most of the features of the storm reasonably well. In particular, the simulated reflectivity, graupel mixing ratio and rainfall were in good agreement with the observed values.

Comparison of a laser precipitation monitor, piezoelectric transducer and particle imaging transient visual measurement technology under simulated rainfall in laboratory conditions

• Three instruments were compared in the lab under simulated rainfall conditions. • The raindrop size was most accurately described using the geometric mean diameter. • The raindrop visual measurement method could provide accurate raindrop shapes. • The laser precipitation monitor was more sensitive to measuring raindrops of < 1 mm. • Based on rainfall tests, the applicability of the three instruments was discussed. Precipitation microphysics, which describes the basic characteristics of rainfall, is important for agricultural praxis, weather prediction, aviation safety, and soil erosion prediction. In this study, three types of instruments (a laser precipitation monitor, LPM; piezoelectric transducer, PT; and particle imaging transient visual measurement technology, PIV) were employed to measure and compare the raindrop size distribution (DSD) and rainfall kinetic energy rate ( KE t ) under simulated rainfall conditions. Comparisons of the results indicated that under the same simulated rainfall conditions, the number of raindrops per unit area measured by LPM was larger than that measured by PIV. The DSD measured by PIV was more uniform than that of the PT and LPM under the same rainfall conditions. The raindrop size range measured by the LPM was smaller than that measured by the PT and PIV. In addition, the geometric mean diameter was a more accurate representation of raindrop size because PIV can capture the true irregular shape of raindrops. Compared to the PIV sensor, the LPM underestimates the raindrop diameter. The median raindrop diameter measured and calculated by PIV using the geometric mean diameter was approximately 1.61 times that of LPM. The KE t values measured by PIV and PT were approximately similar, while the KE t calculated by LPM was 0.51 and 0.57 times that of PIV and PT for the same rainfall conditions, respectively. A correction factor of 1.75 provided an approximate reference for the calibration of the kinetic energy calculation of the LPM instrument. The above results can provide basic insights for calibration and application of the three instruments.

Analysis of precipitation microstructure characteristic during Madden Julian Oscillation (MJO) using micro rain radar (MRR) and disdrometer in South Tangerang

Rain microstructure is a critical aspect to understand the dynamics and microphysics character of the clouds. It is characterized by the distribution of size, fall velocity and shape of raindrop. Raindrop size distribution (DSD) explains the detail of the microphysical process because it represents a process of rain to the surface. One of the phenomena that influence the rain patterns in Indonesia is Madden Julian Oscillation (MJO). Therefore, observing rain microstructure with its relation to MJO can determine the differences in rainfall characteristic and microphysical processes during active and inactive MJO period. The data used in this study are Micro Rain Radar (MRR), disdrometer, and real-time multivariate (RMM) index data. The period/date selection of active MJO event performed using RMM index method is more than 1 in phases 4 and 5 and otherwise for inactive MJO. Types of rain are divided into stratiform and convective rain based on disdrometer data. From that, there are 46 active and 52 inactive MJO events. Rain microstructure in this study focuses on DSD from disdrometer and micro rain radar data analyzed with liquid water content profile, fall velocity, reflectivity, and rain rate from MMR. Besides, there are parameters of DSD, which are the mass-weighted diameter (Dm) and total concentration (Nw), calculated using the moment and gamma distribution method. The result shows that DSD and other parameters are greater during inactive MJO period. It means that process of collision-coalescence, evaporation, and updraft is dominant during inactive MJO period.

Simulation of raindrop shapes and its interaction with electromagnetic wave

Radio waves propagating through a rain zone will undergo scattering, depolarization, absorption and delay in time. All these effects on wave propagation are related to the frequency at which the signal is transmitted and as well as the rain parameters, a good knowledge of these hydrometeor parameters is therefore of great importance in the modeling of their effects on radio communication. These parameters can be measured with micro rain radar (MRR). The MRR instrument is a versatile equipment for collecting rain parameter. In this study these parameters were obtained using a MATLAB software to develop a rain radar simulator, this was found necessary because of the inadequacy of micro rain radar instrument in many locations of interest, thus the simulator will facilitate ubiquitous use and application in weather and radio wave propagation and augment the available MRR equipment. The simulation result shows the oblate spherical raindrop shape. The axis ratio was obtained and compared with those obtained by other researcher. This raindrop parameter was further used to simulate the interaction of raindrop with electromagnetic wave and the raindrop reflectivity was obtained. The simulation was validated by comparing the reflectivity obtained with that measured by the MRR equipment.

Estimating raindrop size distributions and vertical air motions with spectral difference using vertically pointing radar

AbstractDoppler spectra measured by vertically pointing radars are inherently linked to raindrop size distributions (DSDs). But, accurate estimation of DSDs remains challenging because raindrop spectra are broadened by atmospheric turbulence and shifted by vertical air motions. This paper presents a novel method to estimate vertical air motions that there is no need to assume a model for DSD at each range gate. The theory of the new method is that the spectral difference between the adjacent range gates is contributed by vertical air motions and the variability of DSDs. The contribution of the change of DSDs is estimated by looking up the prepared tables (LUTs) of raindrop velocity difference and shape function difference. Then, the vertical air motions can be estimated by minimizing the cost function of the two spectra between the adjacent range gates. The retrieval algorithm is applied to three cases including a stratiform and two convective observed by a C band vertically pointing radar in Longmen, Guangdong province of China in June 2016. Before that, the spectrum broadening effect is removed by the traditional deconvolution method with a wind profiler. The vertical profiles of precipitation parameters are also retrieved to investigate the microphysical process. The precipitation parameters retrieved near the surface are compared with the ground data collected by a two-dimensional video disdrometer(2DVD) and the results show good agreements.

Microphysical Characteristics of Rainfall Observed by a 2DVD Disdrometer during Different Seasons in Beijing, China

The seasonal variations of raindrop size distribution (DSD) and rainfall are investigated using three-year (2016–2018) observations from a two-dimensional video disdrometer (2DVD) located at a suburban station (40.13° N, 116.62° E, ~30 m AMSL) in Beijing, China. The annual distribution of rainfall presents a unimodal distribution with a peak in summer with total rainfall of 966.6 mm, followed by fall. Rain rate (R), mass-weighted mean diameter (Dm), and raindrop concentration (Nt) are stratified into six regimes to study their seasonal variation and relative rainfall contribution to the total seasonal rainfall. Heavy drizzle/light rain (R2: 0.2~2.5 mm h−1) has the maximum occurrence frequency throughout the year, while the total rainfall in summer is primarily from heavy rain (R4: 10~50 mm h−1). The rainfall for all seasons is contributed primarily from small raindrops (Dm2: 1.0~2.0 mm). The distribution of occurrence frequency of Nt and the relative rainfall contribution exhibit similar behavior during four seasons with Nt of 10~1000 m−3 registering the maximum occurrence and rainfall contributions. Rainfall in Beijing is dominated by stratiform rain (SR) throughout the year. There is no convective rainfall (CR) in winter, i.e., it occurs most often during summer. DSD of SR has minor seasonal differences, but varies significantly in CR. The mean values of log10Nw (Nw: mm−1m−3, the generalized intercept parameter) and Dm of CR indicate that the CR during spring and fall in Beijing is neither continental nor maritime, at the same time, the CR in summer is close to the maritime-like cluster. The radar reflectivity (Z) and rain rate (?) relationship (Z = ?R?) showed seasonal differences, but were close to the standard NEXRAD Z-R relationship in summer. The shape of raindrops observed from 2DVD was more spherical than the shape obtained from previous experiments, and the effect of different axis ratio relations on polarimetric radar measurements was investigated through T-matrix-based scattering simulations.

The Physics of Falling Raindrops in Diverse Planetary Atmospheres

AbstractThe evolution of a single raindrop falling below a cloud is governed by fluid dynamics and thermodynamics fundamentally transferable to planetary atmospheres beyond modern Earth's. Here, we show how three properties that characterize falling raindrops—raindrop shape, terminal velocity, and evaporation rate—can be calculated as a function of raindrop size in any planetary atmosphere. We demonstrate that these simple, interrelated characteristics tightly bound the possible size range of raindrops in a given atmosphere, independently of poorly understood growth mechanisms. Starting from the equations governing raindrop falling and evaporation, we demonstrate that raindrop ability to vertically transport latent heat and condensible mass can be well captured by a new dimensionless number. Our results have implications for precipitation efficiency, convective storm dynamics, and rainfall rates, which are properties of interest for understanding planetary radiative balance and (in the case of terrestrial planets) rainfall‐driven surface erosion.

A computational framework for coating fatigue analysis of wind turbine blades due to rain erosion

The rain-induced fatigue damage in the wind turbine blade coating has attracted increasing attention owing to significant repair and maintenance costs. The present paper develops an improved computational framework for analyzing the wind turbine blade coating fatigue induced by rain erosion. The paper first presents an extended stochastic rain field simulation model that considers different raindrop shapes (spherical, flat, and spindle), raindrop sizes, impact angles, and impact speeds. The influence of these raindrop characteristics on the impact stress of the blade coating is investigated by a smoothed particle hydrodynamics approach. To address the expensive computational time, a stress interpolation method is proposed to calculate the impact stress of all raindrops in a random rain event. Furthermore, coating fatigue analysis is performed by including the fatigue crack initiation in the incubation period and the fatigue crack propagation in the mass-loss-rate increasing period due to raindrop impact. Finally, the proposed computational framework is verified by comparing the estimated fatigue life with those obtained in literature. The results from the study show that by incorporating the statistics of rainfall data, the proposed framework could be used to calculate the expected fatigue life of the blade coating due to rain erosion.

Microwave single-scattering properties of non-spheroidal raindrops

Abstract. Falling raindrops undergo a change in morphology as they grow in size and the fall speed increases. This change can lead to significant effects in passive and active microwave remote sensing measurements, typically in the form of a polarization signal. Because previous studies generally only considered either passive or active measurements and a limited set of frequencies, there exist no general guidelines on how and when to consider such raindrop effects in scientific and meteorological remote sensing. In an attempt to provide an overview on this topic, this study considered passive and active remote sensing simultaneously and a wider set of frequencies than in previous studies. Single-scattering property (SSP) data of horizontally oriented raindrops were calculated using the T-matrix method at a large set of frequencies (34 in total). The shapes of the raindrops were calculated assuming an aerodynamic equilibrium model, resulting in drops with flattened bases. The SSP data are published in an open-access repository in order to promote the usage of realistic microphysical assumptions in the microwave remote sensing community. Furthermore, the SSPs were employed in radiative transfer simulations of passive and active microwave rain observations, in order to investigate the impact of raindrop shape upon observations and to provide general guidelines on usage of the published database. Several instances of noticeable raindrop shape-induced effects could be identified. For instance, it was found that the flattened base of equilibrium drops can lead to an enhancement in back-scattering at 94.1 GHz of 1.5 dBZ at 10 mm h−1, and passive simulations showed that shape-induced effects on measured brightness temperatures can be at least 1 K.

Hybrid capacitive/piezoelectric visualized meteorological sensor based on in-situ polarized PVDF-TrFE films on TFT arrays

A novel type of hybrid capacitive/piezoelectric sensor was designed and fabricated for real-time monitoring of the size, rate, and frequency of rainfall. The design is based on in situ polarized poly (vinylidene fluoride–trifluoroethylene) (PVDF-TrFE) films on thin-film transistor (TFT) arrays. The polarization of the PVDF-TrFE film with an area of 200 × 200 mm2 can be achieved within 5 min using an in-situ polarization process, and the piezoelectric coefficient d33 can reach an average value of 27 pC/N with a uniform distribution across the area. The PVDF-TrFE piezoelectric sensor exhibits high sensitivity, with a raindrop frequency error margin of 1%. The capacitive sensor is composed of the PVDF-TrFE films and TFT arrays, which can be used to determine the size and shape of raindrops on the sensor surface. In addition, the sensor can also be used for high-precision detection of water level, with a resolution of 50 μm.

Shapes and Fall Speeds of Freezing and Frozen Raindrops

AbstractThis study investigates the shapes and fall speeds of freezing and frozen raindrops through field observations using an instrument called the high-speed optical disdrometer (HOD) that we developed recently. Our field observations showed that while the shapes of all of the observed freezing raindrops and a portion of the frozen raindrops (39% of the frozen raindrops that are larger than 1.0 mm in volume equivalent diameter D) resemble the shapes of warm raindrops, majority of frozen raindrops (61% of the frozen raindrops with D &gt; 1.0 mm) exhibited a distinct feature such as a spicule, bulge, cavity, or aggregation. Field observations of axis ratios (i.e., ratio of the vertical to horizontal chord) and fall speeds were compared with the predictions of available models. Separate empirical axis ratio parameterizations were developed for the freezing and frozen raindrops using the HOD field observations and extensions to an available shape model were also incorporated. For the fall speeds of freezing and frozen raindrops, field observations demonstrated a good agreement with the predictions of the available parameterizations. Frozen raindrops showed a larger scatter of fall speeds around the mean fall speed of a given drop size than those of the freezing raindrops due to the shape variety among the frozen raindrops with the aforementioned distinct features. The drag coefficients for the observed hydrometeors were compared with the predictions of the available drag coefficient models. Separate “drag coefficient–Reynolds number” relationships for freezing and frozen raindrops were developed.

Light scattering by an infinite cylinder of arbitrarily smooth cross section based on vectorial complex ray model

This paper reports the extension of vectorial complex ray model allowing to account, in the high-frequency limit, for the direction, polarization, curvature of wave front, amplitude, phase and scattered intensity of the light rays interacting with an infinite cylinder of arbitrary while smooth cross section. Based on the proposed method, a numerical study is performed on the scattering patterns of composite elliptical cylinders (CEC). The effects of shape deformation, refractive index and the direction of incident wave on the scattering patterns of the CEC whose cross sections approximate the shapes of realistic raindrops are investigated and quantitatively analyzed, which provides insight into how these factors affect the appearance of a natural rainbow. Being flexible and numerically efficient (a full scattering diagram is obtained in few seconds on a laptop computer), the method proposed in this paper is thought to have important applications in calculating and analyzing the scattering characteristics of light by cylindrical objects of cross sections ranging from simple to complex.

Analysis of Raindrop Shapes and Scattering Calculations: The Outer Rain Bands of Tropical Depression Nate

Tropical storm Nate, which was a powerful hurricane prior to landfall along the US Gulf coast, traversed north and weakened considerably to a tropical depression as it moved near an instrumented site in Hunstville, AL. The outer rain bands lasted 18 h (03:00 to 21:00 UTC on 08 October 2017) and a 2D-video disdrometer (2DVD) captured the event which was shallow at times and indicative of pure warm rain processes. The 2DVD measurements are used for 3D reconstruction of drop shapes (including the rotationally asymmetric drops) and the drop-by-drop scattering matrix has been computed using Computer Simulation Technology integral equation solver for drop sizes &gt;2.5 mm. From the scattering matrix elements, the polarimetric radar observables are simulated by integrating over 1 min consecutive segments of the event. These simulated values are compared with dual-polarized C-band radar data located at 15 km range from the 2DVD site to evaluate the contribution of the asymmetric drop shapes, specifically to differential reflectivity. The drop fall velocities and drop horizontal velocities in terms of magnitude and direction, all being derived from each drop image from two orthogonal cameras of the 2DVD, are also considered.

A computational model of wind turbine blade erosion induced by raindrop impact

This research focuses on developing a computational model for analysing wind turbine blade erosion induced by raindrop impact. A stochastic rain texture model is used to simulated realistic rain events determined by a rain intensity and a rain duration. A new smoothed particle hydrodynamic approach is implemented to study the influence of the raindrop size, impact speed, impact angle, and raindrop shape on the impact stress. In addition, a stress interpolation method is proposed to accurately and efficiently calculate the impact stress under a random rain event. The fatigue damage of the top coating material is evaluated under a rain event based on the rainflow cycle counting method and the Miner’s rule.

Theoretical and Experimental Studies of the Rain Load for Transmission Tower Based on Single-Raindrop Impinging Force

Rain load has a substantial effect on the dynamic response of transmission tower-line systems, and studying the impinging force imparted by a single raindrop can provide a better understanding of the acting mechanism of rain load on these systems. In this study, a theoretical analysis of the impinging force of a raindrop is conducted, and a piezoelectric transducer has been developed to measure the impinging force versus time. Based on the principle of conservation of momentum, a formula for the force is derived under ideal conditions. However, the form of the equation is so complex that the force can only be obtained by numerical methods for non-spherical raindrop shapes. Then, the formula of raindrop impinging force was derived by considering the raindrop acceleration, the raindrop splashing, the water film, and the flow velocity of the impinged raindrop, of which the form is so complex that it cannot be solved directly. At last, a single-raindrop impinging experiment was carried out, and the results reveal that the raindrop impinging process exhibits both positive and negative forces and that the peak values are of the same order of magnitude. The negative force is caused primarily by the rebound of the raindrop. The positive force acts for only a short time, while the negative force acts much longer. When the transducer surface is wet, the positive force is greater than that under dry conditions, and the force acts for a longer period of time. This study is expected to expand the understanding of the rain load mechanism for transmission towers.