Drop Spectra Research Articles

A worldwide data science investigation of rainfall

Abstract We perform a worldwide analysis of the rain drop size distribution using 166 dis-drometer datasets from 76 distinct sites for a total of 1,527,963 1-minute drop counts, 428,410 2-minute drop counts, and a total of 988,922,720 drops. Following data science tenets, we adopt a functional-agnostic description of the rain drop size distribution. In this way, we uncover the presence of an invariant structure of statistical relationship among the distribution parameters, not depending on location, synoptic origin, or type of disdrometer. The features of this structure are: 1) count-shape independence: there is no dependence between the drop count N and the shape of rain drop spectra. 2) mean-skewness prominence: the variability of the shape of rain drop spectra can be fully captured by its mean µ and skewness γ. 3) mean-skewness invariant parametrization: we derive empirical invariant functional forms expressing all other shape describing parameters in terms of the free parameters (µ, γ). The presented analysis reveals the global and local properties of the rain drops size distribution offering a coherent and universally applicable methodology to describe the rain drop size distribution.

Raindrop size distribution of stratiform precipitation over Southwest India – The Gateway of Indian Summer Monsoon

The study focuses on exploring the bright band (BB) characteristics, using Micro Rain Radar and ceilometer during the monsoon season of 2019 over three stations - coastal (20 m above mean sea level (MSL)), mid (400 m above MSL), and high (1820 m above MSL) altitude cloud physics observatories, setup by the National Centre for Earth Science Studies. The raindrop spectral characteristics are analyzed by classifying the BB events into three time periods, (i) prior to the occurrence of the bright band (pre-BB), (ii) during bright band (BB), and (iii) the subsequent hours after the dissipation of bright band (post-BB). Stronger BB events are noted in the high-altitude station with reflectivity of 33 dBZ at the melting layer while the occurrence of high duration (~2 h) events are noted at the mid-altitude station. Pre-BB hours are contributing the highest surface rainfall from all stages of BB evolution at the coastal and mid-altitude stations. The vertical profile of high rain rates indicates the presence of shallow and mixed-phase precipitating systems during pre-BB hours. At the coastal station, light to moderate rainfall with narrow raindrop spectra having dominance of mid-size drops are noticed. The efficiency of collision-coalescence processes is relatively more at the mid- and high-altitude stations resulting in small to mid-size drops and larger drops respectively. The raindrops generated from the melting layer fall into the shallow feeder clouds formed at the saturated surface in the high-altitude station that creates a seeder feeder mechanism of orographic rain enhancement. The distribution of mass-weighted mean diameter (Dm) varies from 0.5 to 1 mm with wider generalized intercept parameter (log10Nw) values (3–5 m−3 mm−1) at the mid- and high-altitude stations during pre-BB hours. In BB hours, the distribution of Dm values at the high-altitude station extends to 1.5 mm with high log10Nw values and the shape-slope (μ-Λ) relation also shows the extension of raindrop spectra to larger drops. The radar reflectivity-rain rate relations coherently explain the results concerning the evolution of drop spectra. The analysis provides a comprehensive understanding of the precipitation microphysical processes in the elevated terrain of the southwest coast of India, which acts as the gateway of the Indian summer monsoon.

Pathways of precipitation formation in different thermodynamic and aerosol environments over the Indian Peninsula

This study examines precipitation pathways in different thermodynamic and aerosol environments over the Indian peninsula. A three-dimensional idealized Large-Eddy Simulation (LES) model with spectral bin microphysics is used to simulate convective clouds growing under different monsoon environments. Numerical simulations were evaluated using in-situ measurements from the Cloud-Aerosol Interactions and Precipitation Enhancement Experiment (CAIPEEX). Sensitivity studies were carried out using varied combinations of water vapor mixing ratio (by ±10 and ± 20%) and cloud condensation nuclei number concentrations (100 and 3000 cm−3). The response of various microphysical process rates to the changes in moisture and aerosol is analyzed. The clouds developed in a moist environment have a greater cloud depth, more supercooled liquid, broader cloud drop spectra, earlier onset of mixed-phase regimes, and enhanced precipitation. The amount of moisture is an important factor in determining the aerosol effects on cloud phase and precipitation, with a more significant aerosol impact in the moist environment (+ 20%). An increase in aerosol number concentration results in an enhanced mixed-phase fraction, updraft velocity, and precipitation, mainly through enhancing deposition and aggregation rates. The overall change in moisture by ±20% in cloud properties and precipitation is much more significant than the ten-times change in aerosol concentrations. The secondary ice formation via the Hallet-Mossop (H-M) process plays a significant role in determining the total ice crystal concentration, and its importance increases with the increases of both moisture and aerosol number concentration. The impact of this process on precipitation is minimal. These results provide an important understanding of how co-varied moisture and aerosols over the Indian region would affect the monsoon clouds and precipitation.

Experimental and numerical investigations on the hydrodynamics of gas-solid fluidized bed with an inclined agitator

The effect of inclined agitation on the fluidization quality of a gas-solid fluidized bed is experimentally and numerically investigated. The pressure characteristics are characterized by the standard deviation of pressure drop and power spectra. The particle velocity, solid volume fraction and granular temperature distributions are investigated by numerical modeling. It is found that inclined agitation enhances the gross circulation of solid particles and enlarges the affect region of the agitator, improving the gas-solid fluidization quality. The increase in the agitation speed leads to higher frequency of collisions and more activity of particles, resulting in decreases of bubble sizes and a more homogeneous solid volume fraction distribution. Wide-blade agitator can also improve the fluidization performance.

A Model Approach to Infer the Quality in Agricultural Sprayers Supported by Knowledge Bases and Experimental Measurements

This paper presents a method to infer the quality of sprayers based on data collection of the drop spectra and their physical descriptors, which are used to generate a knowledge base to support decision-making in agriculture. The knowledge base is formed by collected experimental data, obtained in a controlled environment under specific operating conditions, and the semantics used in the spraying process to infer the quality in the application. The electro-hydraulic operating conditions of the sprayer system, which include speed and flow measurements, are used to define experimental tests, perform calibration of the spray booms and select the nozzle types. Using the Grubbs test and the quartile-quartile plot an exploratory analysis of the collected data was made in order to determine the data consistency, the deviation of atypical values, the independence between the data of each test, the repeatability and the normal representation of them. Therefore, integrating measurements to a knowledge base it was possible to improve the decision-making in relation to the quality of the spraying process defined in terms of a distribution function. Results shown that the use of advanced models and semantic interpretation improved the decision-making processes related to the quality of the agricultural sprayers.

Relationships between giant sea salt particles and clouds inferred from aircraft physicochemical data

AbstractThis study uses airborne data from multiple field campaigns off the California coast to determine the extent to which a size distribution parameter and a cloud water chemical measurement can capture the effect of giant cloud condensation nuclei (GCCN), specifically sea salt, on marine stratocumulus cloud properties. The two GCCN proxy variables, near‐surface particle number concentration for diameters >5 µm and cloud water chloride concentration, are significantly correlated (95% confidence) with each other, and both exhibit expected relationships with other parameters (e.g., surface wind) that typically coincide with sea salt emissions. Factors influencing the relationship between these two GCCN proxy measurements include precipitation rate (R) and the standard deviation of the subcloud vertical velocity owing likely to scavenging effects and improved mixing/transport of sea salt to cloud base, respectively. When comparing 12 pairs of high and low chloride cloud cases (at fixed liquid water path and cloud drop number concentration), the average drop spectra for high chloride cases exhibit enhanced drop number at diameters exceeding 20 µm, especially above 30 µm. In addition, high chloride cases coincide with enhanced mean columnar R and negative values of precipitation susceptibility. The difference in drop effective radius between high and low chloride conditions decreases with height in cloud, suggesting that some GCCN‐produced raindrops precipitate before reaching cloud tops. The sign of cloud responses (i.e., R) to perturbations in giant sea salt particle concentration, as evaluated from Modern Era Retrospective Analysis for Research and Applications version 2 reanalysis data, is consistent with the aircraft data.

Estimates of the statistical two-dimensional spatial structure in rain over a small network of disdrometers

Microphysical understanding of the variability in rain requires a statistical characterization of different drop sizes both in time and in all dimensions of space. Temporally, there have been several statistical characterizations of raindrop counts. However, temporal and spatial structures are neither equivalent nor readily translatable. While there are recent reports of the one-dimensional spatial correlation functions in rain, they can only be assumed to represent the two-dimensional (2D) correlation function under the assumption of spatial isotropy. To date, however, there are no actual observations of the (2D) spatial correlation function in rain over areas. Two reasons for this deficiency are the fiscal and the physical impossibilities of assembling a dense network of instruments over even hundreds of meters much less over kilometers. Consequently, all measurements over areas will necessarily be sparsely sampled. A dense network of data must then be estimated using interpolations from the available observations. In this work, a network of 19 optical disdrometers over a 100 m by 71 m area yield observations of drop spectra every minute. These are then interpolated to a 1 m resolution grid. Fourier techniques then yield estimates of the 2D spatial correlation functions. Preliminary examples using this technique found that steadier, light rain decorrelates spatially faster than does the convective rain, but in both cases the 2D spatial correlation functions are anisotropic, reflecting an asymmetry in the physical processes influencing the rain reaching the ground not accounted for in numerical microphysical models.

Comparison of different fittings of drop spectra for rainfall retrievals

It is widely acknowledged that a thorough characterization of the raindrop size distribution (DSD) may address many needs regarding the remote sensing of precipitation, which is central to new research challenges related to the estimation and management of water resources. In particular, the third and higher moments (up to the sixth) of DSD are proportional to relevant hydrological and meteorological parameters (i.e., rain rate, liquid water content, radar reflectivity, and kinetic energy). Therefore, the retrieval process of these quantities is usually based upon higher-order statistics that are mainly influenced by the upper part of the DSD (i.e., its tail behavior). In this study, we first investigate the effects on rainfall integral parameters of truncating the DSD at upper drop diameters when assuming heavy- and light-tailed distributions. Then, we compare both the tails (i.e., large drops only) and the entire empirical distributions of thousands of disdrometer-measured raindrop spectra with four common theoretical distributions characterized by different tail behaviors (i.e., heavy- and light-tailed distributions): the Pareto, lognormal, gamma, and Weibull distributions. In particular, we analyze the relative quality of each distribution (relative ranking) by means of a straightforward method. Observational data consist of 1-min spectra collected by two-dimensional video disdrometers (2DVD) during three pre-launch field campaigns of the NASA Global Precipitation Measurement (GPM) mission located in (i) Rome (HyMeX SOP 1, (ii) Central Oklahoma (MC3E, ), and (iii) Eastern Iowa (IFloodS). The results obtained from the analysis of the three datasets were consistent with each other, and they show that the lighter-tailed distributions are in better agreement with the observed size spectra than the heavier-tailed distributions. However, we also found significant departures of empirical drop spectra from light-tailed distributions, especially when fitting only the tail of the distributions. These departures may imply a dramatic increase of uncertainty in the statistical estimation of high-order DSD moments, thus making the retrieval process unreliable.

Measurement of oil entrainment rates and drop size spectra from coalescence filter media

Liquid entrainment from coalescence filter media—i.e. flow induced “blow-off” of previously deposited oil—in the form of droplets is poorly understood, for one because the generated spectrum is very wide and difficult to characterize with temporal and size resolution, especially for very large drops which carry most of the mass. Such filters operate at much lower flow rates than classical demisters, often in vertical orientation, with much finer geometries, and gravity plays no direct role for entrainment. We present a novel combination of four measurement techniques used to capture the entrained oil drop spectrum from filters during operation in the size range of 0.01–2400µm. The diameter range below 10μm combines two established real-time methods including an electrical mobility particle spectrometer (EMPS; <1μm) and an optical particle counter (OPC; 0.3 to 10μm). The diameter range above 170μm is covered by a newly developed “large drop detection system” (LDDS) based on single particle light scattering. OPC and LDDS continuously count and classify all drops originating from the entire filter coupon with a time resolution of 1min. The EMPS was operated intermittently, following brief switches from aerosol to clear air flow. Drops in the size range between OPC and LDDS were collected and sized by an off-line method.This measuring system was applied to two representative types of glass microfiber media operated with oil mist generated from compressor oil, in order to characterize time resolved drop formation rates and spectra in the range of nanometers to millimeters. Wettable and non-wettable filter media were found to show similar entrainment characteristics, each with multi-modal drop spectra having two pronounced peaks in the ranges of 1–2μm and 200–300μm, respectively. During steady-state operation both modes were generated quasi-continually, the large drops at the rate of a few drops per hour and cm2 of filter surface, the micron size drops 103–104 times more frequently. Available indirect evidence suggests the same underlying entrainment mechanism for both types of fibrous media, namely the break-up of air bubbles formed periodically on the oil that drains on the downstream filter face. Direct detachment (blow-off) of large drops is unlikely at the prevailing operating conditions.

Precipitation Sensitivity to the Mean Radius of Drop Spectra: Comparison of Single- and Double-Moment Bulk Microphysical Schemes

In this study, two bulk microphysical schemes were compared across mean radius values of the entire drop spectra. A cloud-resolving mesoscale model was used to analyze surface precipitation characteristics. The model included the following microphysical categories: water vapour, cloud droplets, raindrops, ice crystals, snow, graupel, frozen raindrops and hail. Two bulk schemes were used: a single-moment scheme in which the mean radius was specified as a parameter and a double-moment scheme in which the mean radius of drops was calculated diagnostically with a fixed value for the cloud droplet number concentration. Experiments were conducted out for three values of the mean radius (in the single-moment scheme) and two cloud droplet number concentrations (in the double-moment scheme). There were large differences in the surface precipitation for the two schemes, the simulated precipitation generated by the double-moment scheme had a higher sensitivity. The single-moment scheme generated an unrealistic collection rate of cloud droplets by raindrops and hail as well as unrealistic evaporation of rain and melting of solid hydrometeors; these processes led to inaccurate timing and amounts of surface precipitation.

Dynamics of sessile drops. Part 2. Experiment

High-speed images of driven sessile water drops recorded under frequency scans are analysed for resonance peaks, resonance bands and hysteresis of characteristic modes. Visual mode recognition using back-lit surface distortion enables modes to be associated with frequencies, aided by the identifications in Part 1 (Bostwick &amp; Steen, J. Fluid Mech., vol. 760, 2014, pp. 5–38). Part 1 is a linear stability analysis that predicts how inviscid drop spectra depend on base state geometry. Theoretically, the base states are spherical caps characterized by their ‘flatness’ or fraction of the full sphere. Experimentally, quiescent shapes are controlled by pinning the drop at a circular contact line on the flat substrate and varying the drop volume. The response frequencies of the resonating drop are compared with Part 1 predictions. Agreement with theory is generally good but does deteriorate for flatter drops and higher modes. The measured frequency bands agree better with an extended model, introduced here, that accounts for forcing and weak viscous effects using viscous potential flow. As the flatness varies, regions are predicted where modal frequencies cross and where the spectra crowd. Frequency crossings and spectral crowding favour interaction of modes. Modal interactions of two kinds are documented, called ‘mixing’ and ‘competing’. Mixed modes are two pure modes superposed with little evidence of hysteresis. In contrast, modal competition involves hysteresis whereby one or the other mode disappears depending on the scan direction. Perhaps surprisingly, a linear inviscid irrotational theory provides a useful framework for understanding observations of forced sessile drop oscillations.

Triangular-Shaped Coupled Microrings for Robust Wavelength Multi-/Demultiplexing in Si Photonics

We propose and demonstrate triangular-shaped coupled Si microrings for robust wavelength multi-/demultiplexing. Thanks to the geometry, stable interring couplings are obtained at straight waveguides, and heaters for thermal tuning can be isolated maximally with each other so that the thermal crosstalk is suppressed between rings. To suppress the bend loss and enhance the free spectral range (FSR), 2-μm-radius low-loss sharp bends with an optimum waveguide offset are employed. A CMOS-compatible process is used for fabricating second- and fourth-order coupled-microrings designed for the Butterworth and Chebyshev spectral responses, respectively. In the second-order rings, flat-top Box-like drop spectra with fluctuations <; ±0.1 dB, insertion loss of 1.2 dB, out-of-band suppression >30 dB, bandwidth of 2.2 nm, FSR of 19 nm, and thermal tuning up to 8 nm are obtained experimentally. The efficiency of the thermal tuning is better and the thermal crosstalk is lower than those in conventional circular and race-track rings for such wide FSR.

European small portable rainfall simulators: A comparison of rainfall characteristics

Small-scale portable rainfall simulators are an essential research tool for investigating the process dynamics of soil erosion and surface hydrology. There is no standardisation of rainfall simulation and such rainfall simulators differ in design, rainfall intensities, rain spectra and research questions,which impede drawing a meaningful comparison between results. Nevertheless, these data become progressively important for soil erosion assessment and therefore, the basis for decision-makers in application-oriented erosion protection. The artificially generated rainfall of the simulators used at the Universities Basel, La Rioja, Malaga, Trier, Tubingen, Valencia, Wageningen, Zaragoza, and at different CSIC (Spanish Scientific Research Council) institutes (Almeria, Cordoba, Granada, Murcia and Zaragoza) was measured with the same methods (Laser Precipitation Monitor for drop spectra and rain collectors for spatial distribution). Data are very beneficial for improvements of simulators and comparison of simulators and results. Furthermore, they can be used for comparative studies, e.g. with measured natural rainfall spectra. A broad range of rainfall data was measured (e.g. intensity: 37–360 mm h−1; Christiansen Coefficient for spatial rainfall distribution: 61–98%; median volumetric drop diameter: 0.375–6.5 mm; mean kinetic energy expenditure: 25–1322 J m−2 h−1;meankinetic energy per unit area and unit depth of rainfall: 0.77–50 J m−2 mm−1). Similarities among the simulators could be found e.g. concerning drop size distributions (maximum drop numbers are reached within the smallest drop classes b1 mm) and low fall velocities of bigger drops due to a general physical restriction. The comparison represents a good data-base for improvements and provides a consistent picture of the different parameters of the simulators that were tested.

The effect of rain characteristics on scavenging rate of tritium-oxide from the atmosphere

The results of field experiments, involving HTO scavenging from the atmosphere by precipitation in the vicinity of HT and HTO emission sources, are presented. The experiments were aimed at obtaining direct experimental data on atmospheric HTO scavenging for a variety of rain characteristics (rain intensity and drop spectra).The most reliable are the calculations of the rate of wash-out with precipitation with the use of the method of integration of the constant exchange for a spectrum of drops. The results of such calculations are in good agreement with the experimental data and can serve as a basis for the generalized parameterization dependences. It is shown that the exact calculation can be replaced by a simpler formula using the mean-value theorem.For the known approximations of the spectra of the rain drops, formulas were obtained to give parameterization dependence of the rate of wash-out Λ on the intensity of precipitation p. This approach can be used for rapid assessment, as well as to determine parameters of wash-out of gases with precipitation in the numerical complexes, which are used for the calculation of the transfer and removal of impurities from the atmosphere.

Takahashi 구름모형에서의 얼음입자 충돌효율 개선

The collision efficiency data for collision between graupel or hail particles and cloud drops that take into account the differences of particle density are applied to the Takahashi cloud model. The original setting assumes that graupel or hail collision efficiency is the same as that of the cloud drops of the same volume. The Takahashi cloud model is run with the new collision efficiency data and the results are compared with those with the original. As an initial condition, a thermodynamic profile that can initiate strong convection is provided. Three different CCN concentration values and therefore three initial cloud drop spectra are prescribed that represent maritime (CCN concentration = 300 <TEX>$cm^{-3}$</TEX>), continental (1000 <TEX>$cm^{-3}$</TEX>) and extreme continental (5000 <TEX>$cm^{-3}$</TEX>) air masses to examine the aerosol effects on cloud and precipitation development. Increase of CCN concentration causes cloud drop sizes to decrease and cloud drop concentrations to increase. However, the concentration of ice particles decreases with the increase of CCN concentration because small drops are difficult to freeze. These general trends are well captured by both model runs (one with the new collision efficiency data and the other with the original) but there are significant differences: with the new data, the development of cloud and raindrop formation are delayed by (1) decrease of ice collision efficiency, (2) decrease of latent heat from riming process and (3) decrease of ice crystals generated by ice multiplication. These results indicate that the model run with the original collision efficiency data overestimates precipitation rates.

A study of microphysical properties within a precipitation system using wind profiler spectra

This study investigates microphysical properties from wind profiler Doppler spectra observed within a precipitation system that produced high rainfall rates up to 40 mm hr−1 near the southern coast of the Korean Peninsula on 25∼26 June 2010. A 1290-MHz wind profiler located in the National Center for Intensive Observation of Severe Weather at Boseong, Korea, observed a widespread stratiform region and short-lived convective cells from 1850 UTC 25 to 0200 UTC 26 June 2010. By using a spectral model applied to observed profiler spectra, rainfall parameters and raindrop size distributions were retrieved below a melting layer during this period. Three representative periods during precipitation were selected based on intensities of bright band and characteristics in vertical profiles of radar reflectivity and Doppler velocity. During a brief convective period (∼30 min), radar reflectivity tended to be proportional to vertical air motion (positive upward), suggesting that updrafts up to ∼3 m s−1 over a large vertical extent through the melting layer probably contributed to increasing rainfall rates at the surface. In reflectivityrainfall rate distributions, large drop spectra (high reflectivity) were analyzed within downdrafts and small drop spectra (low reflectivity) within updrafts, similar to the large and small drop spectra but found in stratiform and convective regions, respectively, in previous studies. This indicates that the degree of spread between reflectivity and rainfall rate may be strongly dependent on positive and negative magnitudes of vertical air motion. For three categories of vertical air motion (i.e., updrafts, neutral, and downdrafts), physical relations between the retrieved rainfall parameters were examined.

The Coupled Effect of Mid-Tropospheric Moisture and Aerosol Abundance on Deep Convective Cloud Dynamics and Microphysics

The humidity of the mid troposphere has a significant effect on the development of deep convection. Dry layers (dry intrusions) can inhibit deep convection through the effect of a thermal inversion resulting from radiation and due to the reduction in buoyancy resulting from entrainment. Recent observations have shown that the sensitivity of cloud top height to changes in mid-tropospheric humidity can be larger than straightforward “parcel dilution” would lead us to expect. Here, we investigate how aerosol effects on cloud development and microphysics are coupled to the effects of mid-tropospheric dry air. The two effects are coupled because the buoyancy loss through entrainment depends on droplet evaporation, so is controlled both by the environmental humidity and by droplet sizes, which are, in turn, controlled in part by the aerosol size distribution. Previous studies have not taken these microphysical effects into account. Cloud development and microphysics are examined using a 2-D non-hydrostatic cloud model with a detailed treatment of aerosol, drop, and ice-phase hydrometeor size spectra. A moderately deep mixed-phase convective cloud that developed over the High Plains of the United States is simulated. We find that a dry layer in the mid troposphere leads to a reduction in cloud updraft strength, droplet number, liquid water content and ice mass above the layer. The effect of the dry layer on these cloud properties is greatly enhanced under elevated aerosol conditions. In an environment with doubled aerosol number (but still realistic for continental conditions) the dry layer has about a three-times larger effect on cloud drop number and 50% greater effect on ice mass compared to an environment with lower aerosol. In the case with high aerosol loading, the dry layer stops convective development for over 10 min, and the maximum cloud top height reached is lower. However, the effect of the dry layer on cloud vertical development is significantly reduced when aerosol concentrations are lower. The coupled effect of mid-tropospheric dry air and aerosol on convective development is an additional way in which long term changes in aerosol may impact planetary cloud processes and climate.

Studying an effect of salt powder seeding used for precipitation enhancement from convective clouds

Abstract. Experimental and theoretical studies of cloud microstructure modification with hygroscopic particles for obtaining additional precipitation amounts from convective clouds are performed. The experiment used salt powder with the particle sizes that gave the greatest effectiveness according to the simulations of Segal et al. (2004). The experiments were carried out in a cloud chamber at the conditions corresponding to the formation of convective clouds. The results have shown that the introduction of the salt powder before a cloud medium is formed in the chamber results in the formation on a "tail" of additional large drops. In this case seeding with the salt powder leads also to enlargement of the whole population of cloud drops and to a decrease of their total concentration as compared to a cloud medium that is formed on background aerosols. These results are the positive factors for stimulating coagulation processes in clouds and for subsequent formation of precipitation in them. An overseeding effect, which is characterized by increased droplet concentration and decreased droplet size, was not observed even at high salt powder concentrations. The results of numerical simulations have shown that the transformation of cloud drop spectra induced by the introduction of the salt powder results in more intense coagulation processes in clouds as compared to the case of cloud modification with hygroscopic particles with relatively narrow particle size distributions, and for the distribution of the South African hygroscopic flares. The calculation results obtained with a one-dimensional model of a warm convective cloud demonstrated that the effect of salt powder on clouds (total amounts of additional precipitation) is significantly higher than the effect caused by the use of hygroscopic particles with narrow particle size distributions at comparable consumptions of seeding agents, or with respect to the hygroscopic flares. Here we show that seeding at rather low consumption rate of the salt powder initiates precipitation from otherwise non precipitating warm convective clouds, mainly by the effect of adding large cloud drops to the tail of the distribution.

Calibration of radar reflectivity measurements from the KMA operational radar network

A systematic procedure for calibrating system gain bias (so-called “calibration error”) of radar reflectivity measurements from the Korea Meteorological Administration (KMA) operational radar network is presented. First, the RJNI radar located at Jindo Island is calibrated by comparing with radar reflectivities simulated theoretically by a scattering algorithm using drop spectra collected by a disdrometer from June 19 to 29, 2009. Once the RJNI radar is calibrated, the reflectivity measurements from nearby radars are compared with the RJNI radar reflectivities to determine existing gain biases of nearby radars. This radar-radar calibration procedure was repeated with the other radars within the network. For isolating a system gain bias, echoes affected by partial beam blockage due to ground clutter and by attenuation due to precipitation were removed. The system gain biases of the RJNI and RPSN radars were −3 and −4.2 dB, respectively, during the experimental period. The RBRI and RDNH radars revealed relatively large biases, above −8 dB. The other radars (RKSN, RGSN, RSSP, RKWK, RGDK, RIIA, and RMYN) revealed biases from −6 to −7 dB. Thus, the reflectivity measurements from all of the KMA radars were severely biased. New R-Z relations of R = 3.350 × 10−2Z0.624 (Z = 231.1R1.6) for stratiform and R=1.546 × 10−2Z0.714 (Z = 342.4R1.4) for convective precipitations were derived using disdrometer data. Using these R-Z relations, the radar-derived total rainfall amounts from the reflectivity measurements without calibration produced significant underestimations, compared to gauge measurements at about 80 sites, with a normalized bias of about −56%. On the other hand, after calibrating the above system biases, the radar-derived rainfall amounts corresponded well with the gauge measurements, with a normalized bias of about −3%. In conclusions, the radar reflectivity measurements from the KMA radar network are severely biased and the procedure presented in this study can be used to resolve the system gain biases.

Numerical Investigation of Collision-Induced Breakup of Raindrops. Part I: Methodology and Dependencies on Collision Energy and Eccentricity

AbstractBinary collisions of large raindrops moving with terminal fall velocity are numerically investigated using FS3D, a direct numerical simulation (DNS) code based on the “volume of fluid” method. The result of this process can be a permanent coalescence or a temporal coalescence followed by a breakup of the coalesced system into smaller-sized remnants of the original raindrops and a number of fragment droplets of different sizes. In total, 32 drop pairs are studied with sizes chosen to cover nearly completely the entire size parameter range relevant to breakup. This is an important extension of investigations performed in 1982 by Low and List, who studied 10 drop pairs only. Moreover, eccentricity has been introduced as an additional parameter controlling the collision outcome. Eccentricity is defined as the horizontal distance of the initial drops’ centers with values equal to approximately 0 for centric and 1 for grazing collisions.The main results include numerically calculated data of coalescence efficiencies and fragment size distributions with emphasis on eccentricity effects. It is shown that eccentricity largely determines the appearance of specific breakup modes and consequently the respective fragment size distributions. Comparisons are made with the main findings of Low and List. Coalescence efficiency values larger than those derived by Low and List show up for very small Weber numbers. Additionally, the existence of their definite limit value of collision kinetic energy necessary for coalescence could not be confirmed. The fragment size distributions are in some cases similar to those measured by Low and List but there are also major differences for other cases. The presented results are used for parameterizations of coalescence efficiencies and fragment size distributions as well as for calculations of stationary drop spectra shown in Part II of this study.