Fall Velocity Research Articles

Hydrodynamic Model-Based Evaluation of Sediment Transport Capacity for the Makhool-Samarra Reach of Tigris River

This paper implemented an HEC-RAS-based steady two-dimensional hydrodynamic model to evaluate the hydraulic characteristics and Sediment Transport Capacity (STC) of the Makhool-Samarra Reach of the Tigris River, where this part is not already investigated. This model was prepared with the aid of topographical surveys, hydraulic measurements, and laboratory tests, as well as recorded data. The main findings show that the flow velocities within most of the studied reach are considered high compared to the local and worldwide flow velocity ranges in natural rivers. Also, the Toffaleti sediment transport function with the Van Rijn fall velocity and the effective depth-to-width computation methods are the most compatible and suitable methods for computing the Sediment Transport Potential (STP) in the ‎studied reach. With the maximum flow, the computed STP was less than the average (5.7×106 tons/day) in 45% of the reach length, where the maximum STP is 8.7×106 tons/day. However, with the minimum flow, the STP intermittently fluctuates below the average (0.2×106 tons/day) in 48% of the reach length, where the minimum STP is 0.14×106 tons/day. Generally, these STPs are little more than the Tigris River in Mosul and much higher than that in Baghdad. Also, these STPs are considered high on a global scale, especially during high flow. Medium gravel is the larger grain gradation class that can be transported. It constitutes less than 0.00005 % of the STP. Furthermore, the STC computations clearly show that the suspended sediment is the dominant part of the STC and the bed material constitutes less than 0.002% of the STC of the total load. However, most of the STC ‎concerns the clay and very fine silt.

3D trajectories and velocities of rainfall drops in a multifractal turbulent wind field

Abstract. Weather radars measure rainfall in altitude, whereas hydro-meteorologists are mainly interested in rainfall at ground level. During their fall, drops are advected by the wind, which affects the location of the measured field. The governing equation of a rain drop's motion relates the acceleration to the forces of gravity and buoyancy along with the drag force. It depends non-linearly on the instantaneous relative velocity between the drop and the local wind, which yields complex behaviour. Here, the drag force is expressed in a standard way with the help of a drag coefficient expressed as a function of the Reynolds number. Corrections accounting for the oblateness of drops greater than 1–2 mm are suggested and validated through a comparison of the retrieved “terminal fall velocity” (i.e. without wind) with commonly used relationships in the literature. An explicit numerical scheme is then implemented to solve this equation for a 3+1D turbulent wind field, and hence analyse the temporal evolution of the velocities and trajectories of rain drops during their fall. It appears that multifractal features of the input wind are simply transferred to the drop velocity with an additional fractional integration whose level depends on the drop size, and a slight time shift. Using an actual high-resolution 3D sonic anemometer and a scale invariant approach to simulate realistic fluctuations of wind in space, trajectories of drops of various sizes falling form 1500 m are studied. For a strong wind event, drops located within a radar gate in altitude during 5 min are spread on the ground over an area of the size of a few kilometres. The spread for drops of a given diameter is found to cover a few radar pixels. Consequences on measurements of hydro-meteorological extremes that are needed to improve the resilience of urban areas are discussed.

Mathematical Modeling-Based Management of a Sand Trap throughout Operational and Maintenance Periods (Case Study: Pengasih Irrigation Network, Indonesia)

Surface irrigation networks in Indonesia are damaged by several factors, and sedimentation is among the most severe challenges. Sand traps play a substantial role in improving irrigation system efficiency by reducing sedimentation. There are two periods in sand trap operation: the operational and maintenance periods. Pengasih is one of the irrigation schemes implemented in the Progo Opak Serang (POS) River Basin, which has a high level of erosion. This study aimed to propose an appropriate management strategy for the Pengasih sand trap as the first barrier in irrigation network sedimentation based on mathematical modeling. The HEC-RAS simulation software was used to simulate the sand trap hydraulic behaviour. The results show that the validated Manning’s coefficient was 0.025. The optimal transport parameters were Laursen for the potential function, Exner 5 for the sorting method, and Rubey for the fall velocity method. The recommended flushing timeframe is 315 min, with a discharge of 2 m3/s. We suggest that the sand trap flushing frequency be performed twice a year, and it can be performed at the end of March and October. This coincides with the end of the first and third planting seasons of the irrigation scheme.

The influence of rainfall interception on the erosive power of raindrops under the birch tree

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.

Ice microphysical processes in the dendritic growth layer: a statistical analysis combining multi-frequency and polarimetric Doppler cloud radar observations

Abstract. The dendritic growth layer (DGL), defined as the temperature region between −20 and −10 ∘C, plays an important role for ice depositional growth, aggregation and potentially secondary ice processes. The DGL has been found in the past to exhibit specific observational signatures in polarimetric and vertically pointing radar observations. However, consistent conclusions about their physical interpretation have often not been reached. In this study, we exploit a unique 3-months dataset of mid-latitude winter clouds observed with vertically pointing triple-frequency (X-, Ka-, W-band) and polarimetric W-band Doppler radars. In addition to standard radar moments, we also analyse the multi-wavelength and polarimetric Doppler spectra. New variables, such as the maximum of the spectral differential reflectivity (ZDR) (sZDRmax), allows us to analyse the ZDR signal of asymmetric ice particles independent of the presence of low ZDR producing aggregates. This unique dataset enables us to investigate correlations between enhanced aggregation and evolution of small ice particles in the DGL. For this, the multi-frequency observations are used to classify all profiles according to their maximum average aggregate size within the DGL. The strong correlation between aggregate class and specific differential phase shift (KDP) confirms the expected link between ice particle concentration and aggregation. Interestingly, no correlation between aggregation class and sZDRmax is visible. This indicates that aggregation is rather independent of the aspect ratio and density of ice crystals. A distinct reduction of mean Doppler velocity in the DGL is found to be strongest for cases with largest aggregate sizes. Analyses of spectral edge velocities suggest that the reduction is the combined result of the formation of new ice particles with low fall velocity and a weak updraft. It appears most likely that this updraft is the result of latent heat released by enhanced depositional growth. Clearly, the strongest correlations of aggregate class with other variables are found inside the DGL. Surprisingly, no correlation between aggregate class and concentration or aspect ratio of particles falling from above into the DGL could be found. Only a weak correlation between the mean particle size falling into the DGL and maximum aggregate size within the DGL is apparent. In addition to the correlation analysis, the dataset also allows study of the evolution of radar variables as a function of temperature. We find the ice particle concentration continuously increasing from −18 ∘C towards the bottom of the DGL. Aggregation increases more rapidly from −15 ∘C towards warmer temperatures. Surprisingly, KDP and sZDRmax are not reduced by the intensifying aggregation below −15 ∘C but rather reach their maximum values in the lower half of the DGL. Also below the DGL, KDP and sZDRmax remain enhanced until −4 ∘C. Only there, additional aggregation appears to deplete ice crystals and therefore reduce KDP and sZDRmax. The simultaneous increase of aggregation and particle concentration inside the DGL necessitates a source mechanism for new ice crystals. As primary ice nucleation is expected to decrease towards warmer temperatures, secondary ice processes are a likely explanation for the increase in ice particle concentration. Previous laboratory experiments strongly point towards ice collisional fragmentation as a possible mechanism for new particle generation. The presence of an updraft in the temperature region of maximum depositional growth might also suggest an important positive feedback mechanism between ice microphysics and dynamics which might further enhance ice particle growth in the DGL.

Measurements of Size and Electrical Charges Carried by Precipitation Particles During RELAMPAGO Field Campaign

AbstractThe electrical charge carried by raindrops provides significant information about thunderstorm electrification mechanisms, since the charge acquired by hydrometeors is closely related to the microphysical processes that they undergo within clouds. Investigation of charges on raindrops was conducted during the Remote sensing of Electrification, Lightning, And Meso‐scale/micro‐scale Processes with Adaptive Ground Observations field campaign. A newly designed instrument was used to determine simultaneously the fall velocity and charge for precipitating particles. Hydrometeor size and charge were measured in Córdoba city, Argentina, during electrified storms. Temporal series of size‐charge of single raindrops were recorded for two storms, which were also monitored with a Parsivel disdrometer and Lightning Mapping Array. The results show that the magnitude of the electric charges range between 1 and 50 pC and more than 90% of the charges are mainly carried by raindrops >1 mm, even though most of the raindrops are smaller than 1 mm. Furthermore, the measurement series show charged hydrometeors of both signs all the time. A correlation between the sizes and the charges carried by the raindrops was found in both storms.

Simulation of Latent Heating Rate From the Microphysical Process Associated With Mesoscale Convective System Over Korean Peninsula

AbstractTo investigate the structure of latent heating rate during different stages of mesoscale convective system (MCS) over the Korean Peninsula, a four nested domain Weather Research and Forecasting (WRF) model simulation with two‐way nesting method has performed. Advanced physics options from the Korean Integrated model physics are considered for simulation. Latent heating rate associated with condensation, evaporation, deposition, sublimation, freezing and melting process is computed based on different microphysical process using WRF double moment (WDM6) microphysics scheme. During the development/mature stage of the convective system, the warming associated with latent heat release by condensation is dominant below 400 hPa, and deposition above 500 hPa. The maximum contribution of warming processes from condensation, cloud nucleation, graupel accretion, and ice, snow deposition is evident. Cooling processes from rain, cloud evaporation followed by graupel, and snow melting are also noticed. Major updrafts associated with ascent of air parcel and large hydrometeor distribution is apparent along with downdrafts in the lower levels. In the dissipating stage, the cooling due to latent heat absorption by evaporation below 600 hPa and melting around 600 hPa are dominant. Contribution from cooling processes is dominant with major downdraft motion in the lower levels corresponding to the hydrometeor fall velocity. Dominant positive (negative) total latent heating provided (restrained) the energy for convection during developing and dissipating stages of system. Varied distribution of total cloud hydrometeors in different stages is apparent. Consistency between cloud hydrometeors, net heating rates, major upward (downward) motion and corresponding rainfall in developing (dissipating) stages of MCS is noticed. Model simulated latent heating (WDM6) profiles are consistent with computed moisture source and sink terms (ERA5).

A Catalog of Molecular Clumps and Cores with Infall Signatures

The research of infall motion is a common means to study molecular cloud dynamics and the early process of star formation. Many works had been done in-depth research on infall. We searched the literature related to infall study of molecular cloud since 1994, summarized the infall sources identified by the authors. A total of 456 infall sources are cataloged. We classify them into high-mass and low-mass sources, in which the high-mass sources are divided into three evolutionary stages: prestellar, protostellar and H ii region. We divide the sources into clumps and cores according to their sizes. The H2 column density values range from 1.21 × 1021 to 9.75 × 1024 cm−2, with a median value of 4.17 × 1022 cm−2. The H2 column densities of high-mass and low-mass sources are significantly separated. The median value of infall velocity for high-mass clumps is 1.12 km s−1, and the infall velocities of low-mass cores are virtually all less than 0.5 km s−1. There is no obvious difference between different stages of evolution. The mass infall rates of low-mass cores are between 10−7 and 10−4 M ⊙yr−1, and those of high-mass clumps are between 10−4 and 10−1 M ⊙yr−1 with only one exception. We do not find that the mass infall rates vary with evolutionary stages.

Studying the chemical and kinematical structures of dense cores TMC-1C, L1544, and TMC-1 in the Taurus molecular cloud using CCS and NH3 observations

ABSTRACT The measurement of chemical and kinematic structures in pre-stellar cores is essential for better understanding of the star-formation process. Here, we study three pre-stellar cores (TMC-1C, L1544, and TMC-1) of the Taurus molecular cloud by means of the thioxoethenylidene (CCS) radical and ammonia (NH3) molecule observed with the Karl G. Jansky Very Large Array telescope in the D, C, and CNB configurations. Our main results are based on the CCS observation of the TMC-1C core, showing that complex structures are present. A spatial offset relative to dust emission is observed in the CCS radical. Across a wide region around the dust peak, inward motion is found through the CCS radical. We have calculated the infall velocity and measured the turbulence inside the core. The turbulence is found to be subsonic. We obtain that the virial parameter α is < 1. Thus, thermal and non-thermal motions cannot prevent the collapse. Spatial incoherence of the CCS and NH3 is observed from the integrated intensity maps in these cores, suggesting that these molecules trace different environments in the cores. We compare the integrated flux densities of CCS with previous single-dish data and find that a small amount of flux is recovered in the interferometric observations, indicating the presence of significant diffuse emission in favourable conditions for producing CCS.

The dynamics of magnetized viscous-resistive ADAFs under a self-similar evolution

In this paper, we explore the self-similarity time evolution of a hot accretion flow around a compact object in the presence of a toroidal magnetic field. We focus on a simplified model which is axisymmetric, rotating, unsteady viscous-resistive under an advection-dominated stage. In this work, we suppose that both the kinematic viscosity and the magnetic diffusivity to be a result of turbulence in the accretion flow. To describe such a flow, we apply magneto-hydrodynamics equations in spherical coordinates, [Formula: see text] and adopt unsteady self-similar solutions. By neglecting the latitudinal dependence of the flow, we obtain a set of one-dimensional differential equations governing the accretion system. In this research, we encounter two parameters related to the magnetic field; one of them is, [Formula: see text], defined as the ratio of the magnetic pressure to the gas pressure and the other one, [Formula: see text] applied in the magnetic diffusivity definition. Our results show that [Formula: see text] is a function of position, and increases towards outer layers. On the other hand, we examine different strength of magnetic field by choosing different value of [Formula: see text] which is the value of [Formula: see text] at the inner edge of disc. We see that both [Formula: see text] and [Formula: see text] have positive effect on growing the radial infall velocity but density and gas pressure decrease by larger values of these parameters. Moreover, the rotational velocity and temperature of accreting material reduce considerably under the influence of a stronger magnetic field. We also focus on the behavior of the mass accretion rate appearing as a descending function of position. Finally, our solutions confirm that the radial trend of the physical quantities in a dynamical accretion flow is different from the ones in a steady flow. However, the effect of various parameters on the physical quantities in our model is qualitatively consistent with similar steady models.

Continuously Operable Simulator and Forecasting the Deposition of Volcanic Ash from Prolonged Eruptions at Sakurajima Volcano, Japan

At Sakurajima volcano, frequent Vulcanian eruptions have been seen at the summit crater of Minamidake since 1955. In addition to this eruption style, the eruptive activities of Strombolian type and prolonged ash emission also occur frequently. We studied the design of a simulator of advection-diffusion-fallout of volcanic ash emitted continuously. The time function of volcanic ash eruption rate is given by a linear combination of the volcanic tremor amplitude and the volume change of the pressure source obtained from the ground motion. The simulation is repeated using discretized values of the eruption rate time function at an iteration time interval of the simulation. The integrated value of the volcanic ash deposition on the ground obtained from each individual simulation is used to estimate the value of the ash fallout. The plume height is given by an empirical equation proportional to a quarter of the power of the eruption rate. Since the wind velocity field near the volcano is complicated by the influence of the volcanic topography, the predicted values published by meteorological organizations are made in high resolution by Weather Research and Forecasting (WRF) for the simulation. We confirmed that an individual simulation can be completed within a few minutes of iteration interval time, using the PUFF model as the Lagrangian method and FALL3D-8.0 as the Eulerian method on a general-purpose PC. Except during rainfall, the radar reflectivity, the count of particles per particle size, and fall velocity obtained by the disdrometers can be used for the quasi-real time matching of the plume height calculated from the eruption rate and the ash fall deposition rate obtained from the simulation.

Dynamic analysis of rock fall impact for a cantilever rock shed with geofoam cushion

In this study, the effect of geofoam compressible cushions on the impact response of the rockfall protection galleries located in a main artery highway connecting the southern Turkey to central Anatolia is investigated. Due to high occurrence risk of rockfall hazard at the mountainous region, permanent protection structures are required to prevent damage. Numerical analyses were carried out to evaluate the performance of EPS geofoam compressible layers with different stiffness characteristics on reducing the transmissibility of the impact forces caused by rockfalls. The kinetic energy levels, expected fall velocities for rock masses and the typical rockfall paths were assessed considering the topography of the hill. Dynamic explicit finite element analyses were performed to investigate the effect of the rockfall impact on a reinforced concrete rock shed. Stresses, strains and displacements on the structural section and the cushioning zone were monitored through the analyses. Based on the results, it was observed that geofoam has a significant effect on reducing the transmitted forces to the structure while increasing the impact duration. In terms of structural displacements, up to 75% reduction is observed when EPS cushions are placed on top of rock shed structure.

A convolution neural network approach to Doppler spectra classification of 205 MHz radar

Wind profiler radars are capable of measuring three-dimensional wind profiles at various altitudes of the atmosphere, at very high temporal and spatial resolution. The Advanced Centre for Atmospheric Radar Research (ACARR), located at Cochin University of Science and Technology (CUSAT), operates the world’s first 205 MHz stratosphere-troposphere wind profiler radar which provides three-dimensional wind profiles for an altitude range of 315 m to 20 km. During non-rainy condition, the radar Doppler power spectrum bears the signature of ambient air motion whereas during rainy conditions, it contains signatures of both ambient air motion and fall velocity of rain droplets. The classification of Doppler power spectra for rainy (Precipitation) and non-rainy (Clear) conditions is necessary as wind profile retrieval from the former needs careful separation of ambient air motion from fall velocity of droplets. A manual classification of the power spectrum is cumbersome, time-consuming, and therefore not practical due to the vast database. This work intends to automate Doppler power spectra classification using the deep learning Convolutional Neural Network (CNN). The proposed Convolutional Neural Network model gives a k-fold validation accuracy of 99.77% and testing accuracy of 99.60% for power spectra classification. The performance of CNN is compared against other popular machine learning classifiers such as Support Vector Machine, Decision Tree, K Nearest Neighbour and Naive Bayes. The performance comparison results show that the proposed CNN outperforms other models in radar Doppler power spectra classification.

Algorithmic Differentiation for Sensitivity Analysis in Cloud Microphysics

AbstractThe role of clouds for radiative transfer, precipitation formation, and their interaction with atmospheric dynamics depends strongly on cloud microphysics. The parameterization of cloud microphysical processes in weather and climate models is a well‐known source of uncertainties. Hence, robust quantification of this uncertainty is mandatory. Sensitivity analysis to date has typically investigated only a few model parameters. We propose algorithmic differentiation (AD) as a tool to detect the magnitude and timing at which a model state variable is sensitive to any of the hundreds of uncertain model parameters in the cloud microphysics parameterization. AD increases the computational cost by roughly a third in our simulations. We explore this methodology as the example of warm conveyor belt trajectories, that is, air parcels rising rapidly from the planetary boundary layer to the upper troposphere in the vicinity of an extratropical cyclone. Based on the information of derivatives with respect to the uncertain parameters, the ten parameters contributing most to uncertainty are selected. These uncertain parameters are mostly related to the representation of hydrometeor diameter and fall velocity, the activation of cloud condensation nuclei, and heterogeneous freezing. We demonstrate the meaningfulness of the AD‐estimated sensitivities by comparing the AD results with ensemble simulations spawned at different points along the trajectories, where different parameter settings are used in the various ensemble members. The ranking of the most important parameters from these ensemble simulations is consistent with the results from AD. Thus, AD is a helpful tool for selecting parameters contributing most to cloud microphysics uncertainty.

Predicting HCN, HCO+, multi-transition CO, and dust emission of star-forming galaxies

The interstellar medium is a turbulent, multiphase, and multi-scale medium that follows scaling relations that link the surface density, volume density, and velocity dispersion with the cloud size. Galactic clouds range from below 1 pc to about 100 pc in size. Extragalactic clouds appear to follow the same range, although they are only now becoming observable in atomic and molecular lines. Analytical models of galactic gaseous disks need to take the multi-scale and multiphase nature of the interstellar medium into account. They can be described as clumpy star-forming accretion disks in vertical hydrostatic equilibrium, with the mid-plane pressure balancing the gravity of the gaseous and stellar disk. Interstellar medium turbulence is taken into account by applying Galactic scaling relations to the cold atomic and molecular gas phases. Turbulence is maintained through energy injection by supernovae. With the determination of the gas mass fraction at a given spatial scale, the equilibrium gas temperature between turbulent heating and line cooling, the molecular abundances, and the molecular line emission can be calculated. The resulting model radial profiles of infrared, HI, CO, HCN, and HCO+ emission are compared to THINGS, HERACLES, EMPIRE, SINGS, and GALEX observations of 17 local spiral galaxies. The model free parameters were constrained for each galactic radius independently. The Toomre parameter, which measures the stability against star formation (cloud collapse), exceeds unity in the inner disk of a significant number of galaxies. In two galaxies it also exceeds unity in the outer disk. Therefore, in spiral galaxies Qtot = 1 is not ubiquitous. The model gas velocity dispersion is consistent with the observed HI velocity dispersion where available. Within our model, HCN and HCO+ is already detectable in relatively low-density gas (∼1000 cm−3). We derive CO and HCN conversion factors and molecular gas depletion times. Both conversion factors are consistent with values found in the literature. Whereas in the massive galaxies the viscous timescale greatly exceeds the star-formation timescale, the viscous timescale is smaller than the star-formation timescale within R ∼ 2 Rd, the disk scale length, in the low-mass galaxies. We suggest that massive spiral galaxies undergo starvation in the absence of gas accretion from the halo, whereas in low-mass galaxies the fuel for star formation reaches R ∼ 2 Rd from outside via a thick gas disk component with a high radial infall velocity observable in the HI line.

Automatic fall detection using Orbbec Astra 3D pro depth images

Falls particularly among the older population has always been a matter of concern. With the steady rise of small families, the elderly is very often left alone at home. Dedicated nurses or caretakers are quite expensive. Thus, intelligent monitoring systems with automatic fall detection systems installed at home or nursing homes could be a game changer in such applications. In this paper, a simple yet effective fall detection system based on computer vision. Novelty of this paper is that it uses the Yolo v2 network on the depth videos for extracting the subject from cluttered background. The robust performance of the YOLOv2 network ensures accurate subject detection and removes the need for any complicated fall detection algorithm. Fall detection is carried out using subject’s height to width ratio and fall velocity. These parameters are simple and easy to calculate and yet provide effective results. The input data is captured using the Orbbec Astra 3D camera.

Cloudlet Capture Model for Asymmetric Molecular Emission Lines Observed in TMC-1A with ALMA

TMC-1A is a protostellar source harboring a young protostar, IRAS 04365+2353, and shows highly asymmetric features of a few 100 au scale in its molecular emission lines. Blueshifted emission is much stronger in the CS (J = 5–4) line than redshifted emission. This asymmetry can be explained if the gas accretion is episodic and takes the form of cloudlet capture, given that the cloudlet is approaching toward us. The gravity of the protostar transforms the cloudlet into a stream and changes its velocity along the flow. The emission from the cloudlet should be blueshifted before the periastron, while it should be redshifted after the periastron. If a major part of cloudlet has not reached the periastron, the former should be dominant. We perform hydrodynamical simulations to examine the validity of the scenario. Our numerical simulations can reproduce the observed asymmetry if the orbit of the cloudlet is inclined to the disk plane. The inclination can explain the slow infall velocity observed in the C18O (J = 2–1) line emission. Such episodic accretion may occur in various protostellar cores since actual clouds could have inhomogeneous density distributions. We also discuss the implication of the cloudlet capture on observations of related objects.

Gravitational Collision of Small Nonspherical Particles: Swept Volumes of Prolate and Oblate Spheroids in Calm Air

Abstract The aggregation rate of ice crystals depends on their shape and intercrystal relative velocity. Unlike spherical particles, the nonspherical ones can have various orientations relative to the gravitational force in the vertical direction and can approach each other at many different angles. Furthermore, the fall velocity of such particles could deviate from the vertical direction velocity. These properties add to the computational complexity of nonspherical particle collisions. In this study, we derive general mathematical expressions for gravity-induced swept volumes of spheroidal particles. The swept volumes are shown to depend on the particles’ joint orientation distribution and relative velocities. Assuming that the particles are Stokesian prolate and oblate spheroids of different sizes and aspect ratios, the swept volumes were calculated and compared to those of equivalent volume spheres. Most calculated swept volumes were larger than the swept volumes of equivalent spherical particles, sometimes by several orders of magnitude. This was due to both the complex geometry and the side drift, experienced by spheroids falling with their major axes not parallel to gravity. We expect that the collision rate between nonspherical particles is substantially higher than that of equivalent volume spheres because the collision process is nonlinear. These results suggest that the simplistic approach of equivalent spheres might lead to serious errors in the computation of the collision rate.

Uncertainty Quantification of WRF Model for Rainfall Prediction over the Sichuan Basin, China

The mesoscale Weather Research and Forecasting (WRF) model has been widely employed to forecast day-ahead rainfalls. However, the deterministic predictions from the WRF model incorporate relatively large errors due to numerical discretization, inaccuracies in initial/boundary conditions and parameterizations, etc. Among them, the uncertainties in parameterization schemes have a huge impact on the forecasting skill of rainfalls, especially over the Sichuan Basin which is located east of the Tibetan Plateau in southwestern China. To figure out the impact of various parameterization schemes and their interactions on rainfall predictions over the Sichuan Basin, the Global Forecast System data are chosen as the initial/boundary conditions for the WRF model and 48 ensemble tests have been conducted based on different combinations of four microphysical (MP) schemes, four planetary boundary layer (PBL) schemes, and three cumulus (CU) schemes, for four rainfall cases in summer. Compared to the observations obtained from the Chinese ground-based and encrypted stations, it is found that the Goddard MP scheme together with the asymmetric convective model version 2 PBL scheme outperforms other combinations. Next, as the first step to explore further improvement of the WRF physical schemes, the polynomial chaos expansion (PCE) approach is then adopted to quantify the impacts of several empirical parameters with uncertainties in the WRF Single Moment 6-class (WSM6) MP scheme, the Yonsei University (YSU) PBL scheme and the Kain-Fritsch CU scheme on WRF rainfall predictions. The PCE statistics show that the uncertainty of the scaling factor applied to ice fall velocity in the WSM6 scheme and the profile shape exponent in the YSU scheme affects more dominantly the rainfall predictions in comparison with other parameters, which sheds a light on the importance of these schemes for the rainfall predictions over the Sichuan Basin and suggests the next step to further improve the physical schemes.

Effect of Vertical Downdraft on Rain Attenuation Modeling Over an Earth-Space Path

The impact of vertical downdraft on rain attenuation estimation from rain rate measurements has been studied. The rain rate (<i>R</i>) and specific attenuation (&#x03B3;) are derived from rain drop size distributions (DSD) by considering the widely accepted Gunn&#x2013;Kinzer (G-K) relationship between drop size and drop fall velocity that is valid for stagnant air. In the real atmosphere, drops are carried with the vertical wind draft, and the actual fall velocity of the raindrops deviates from the G&#x2013;K formulation. The ITU-R estimated rain attenuation values, based on DSDs, which are mostly valid for temperate regions, can deviate from actual rain attenuation measurements in tropical regions, especially at higher rain rates when downdraft is prominent. The present study uses disdrometer measurements of DSD, optical rain gauge measurements of rain rates, and Ku-band rain attenuation data over an earth-space path at a tropical location Kolkata (22&#x00B0;34&#x2032; N, 88&#x00B0;29&#x2032; E) to show that vertical downdrafts impact the rain attenuation estimation from rain rate measurements. At the present location, an overall representative vertical downdraft of 2 m&#x002F;s needs to be considered to match the experimental attenuation measurements with the estimated values using the modified &#x03B3;&#x2013;<i>R</i> relationship in the ITU-R model.