Graupel Particles Research Articles

A stochastic model for density-dependent microwave Snow- and Graupel scattering coefficients of the NOAA JCSDA community radiative transfer model

A structural model is developed for the single-scattering properties of snow and graupel particles with a strongly heterogeneous morphology and an arbitrary variable mass density. This effort is aimed to provide a mechanism to consider particle mass density variation in the microwave scattering coefficients implemented in the Community Radiative Transfer Model (CRTM). The stochastic model applies a bicontinuous random medium algorithm to a simple base shape and uses the Finite-Difference-Time-Domain (FDTD) method to compute the single-scattering properties of the resulting complex morphology.

Settling and collision between small ice crystals in turbulent flows

In cold clouds, small crystals of size $\ensuremath{\sim}$ 100 $\ensuremath{\mu}$m settling in a turbulent fluid need to aggregate through collisions to form large hail or graupel particles. Using fully resolved direct numerical simulations, the role of the two main physical processes responsible for collisions is clarified, namely, turbulence and the difference in settling velocity due to different crystals orientations.

Simulation of W‐band radar reflectivity for model validation and data assimilation

This article describes a reflectivity forward operator developed for the validation and assimilation of W‐band radar data into the regional AROME class of numerical weather prediction models. The forward operator is consistent with the AROME ICE3 one‐moment microphysical scheme and is devised for vertically pointing radars. A new neighbourhood validation method, the Most Resembling Column (MRC) method, is designed to disentangle spatial location model errors from errors in the forward operator. This novel method is used to validate the forward operator using data collected in diverse conditions by the airborne cloud radar RASTA (Radar Airborne System Tool for Atmosphere) during a 2 month period over a region of the Mediterranean. The MRC method is then applied to retrieve the optimal effective shapes (i.e. the mean axis ratios) of the predicted graupel, snow and pristine ice, by minimizing the standard deviation between observations and simulations. The optimal mean axis ratio is approximately 0.7 for snow and 0.8 for graupel. It is shown that treating snow and graupel particles as oblate spheroids with axis ratios close to their optimal values leads to good agreement between the observations and simulations of the ice levels. Conversely, there is a large bias if snow and graupel particles are considered to be either spherical or overly flattened. The results also indicate that pristine ice can be approximated by a sphere, but this conclusion should be taken cautiously since the amount of pristine ice particles is probably overestimated in the ICE3 microphysical scheme.

Numerical Study of a Southwest Vortex Rainstorm Process Influenced by the Eastward Movement of Tibetan Plateau Vortex

A number of studies revealed the possible eastward movement of the Tibetan Plateau low-pressure system in summer and indicated the enhancement effect of this process on the southwest vortex in the Sichuan Basin, which can induce strong convective precipitation and flood events in China. In this study, a numerical simulation of a southwest vortex rainstorm process was performed. The results show that the low-pressure system originated from the Tibetan Plateau affects the southwest vortex mainly at the middle level, causing the strength increase of southwest vortex (SWV), and acts as a connection between the positive vorticity centers at the upper and lower layers. For the microscopic cloud structure, the vertical updraft of the cloud cluster embedded in the SWV increases as the low-pressure system from the plateau arrives at the Sichuan Basin. Vapor and liquid cloud water at the lower level are transported upward, based on which the ice cloud at the upper level and the warm cloud at the lower level are joined to create favorable conditions for the growth of ice crystals. As the ice crystals grow up, snow and graupel particles form, which substantially elevates the precipitation. This effect leads to the rapid development of SWV rainstorm clouds and the occurrence of precipitation. In addition to the effect of the plateau vortex, the subsequent merging of the convective clouds is another important factor for heavy rainfall because it also leads to development of convective clouds, causing heavy rainfall.

Numerical Study of Motion of Falling Conical Graupel

In the present study, the attitudes of freely-falling conical graupel with a realistic range of densities are investigated numerically by solving the transient Navier-Stokes equations and the body dynamics equations representing the 6-degrees-of-freedom motion. This framework allows us to determine the position and orientation of the graupel in response to the hydrodynamic force of the flow fields. The results show more significant horizontal movements than those cases with a fixed bulk density of ice assumed in our previous study. This is because the real graupel particles possess the density less than the bulk density of ice, which, in turn, leads to a relatively small mass and a relatively small set of moments of inertia. We demonstrate that, with the six degrees of freedom considered together, when Reynolds number is small, a typical damped oscillation occurs, whereas when Reynolds number is high, amplifying oscillation may occur which leads to more complicated and unpredictable flying attitudes such as tumbling. The drag coefficients obtained in the present study agree with the previous studies and can be approximated by that of spheres of the same Reynolds numbers. We also show that conical graupel can perform significant horizontal translations which can be on the order of 1km in 1h.

Relationship between cloud‐to‐ground lightning polarity and the space‐time distribution of solid hydrometeors in isolated summer thunderclouds observed by X‐band polarimetric radar

AbstractTo understand the charge distribution in thunderclouds associated with solid hydrometeors, the relationship between cloud‐to‐ground (CG) lightning polarity and the space‐time distribution of solid hydrometeors for isolated summer thunderclouds in Japan is examined using X‐band polarimetric radar. Hydrometeor classification was conducted to examine the space‐time distribution of dry snow, ice crystal, dry graupel (DG), and wet graupel in thunderclouds. Two thunderstorm cases were selected for analysis: 26 July 2010, which generated few positive CG flashes, and 25 August 2010, which generated positive CG flashes in the reflectivity cores. In both cases, negative CG flashes were observed in all reflectivity cores when a large volume of DG was identified above the height of −10°C level. This is consistent with previous studies showing that graupel particles have a negative charge below temperatures of −10°C. Reflectivity cores with positive CG flashes had a large volume of DG up to high altitudes (around or above the −45°C level). Further, reflectivity cores that sustained large DG volumes at high altitudes had a relatively large number of positive CG flashes. The top height of the DG volume reached lower altitudes for reflectivity cores without positive CG flashes compared with those with positive CG flashes. These results suggest that the persistence of graupel particles in reflectivity cores at high altitudes, implying the existence of strong updraft, is a necessary condition for positive CG flashes in summer thunderclouds. This effect would likely be caused by the positively charged graupel particles under high rime accretion rates.

Evaluation of radar reflectivity factor simulations of ice crystal populations from in situ observations for the retrieval of condensed water content in tropical mesoscale convective systems

Abstract. This study presents the evaluation of a technique to estimate cloud condensed water content (CWC) in tropical convection from airborne cloud radar reflectivity factors at 94 GHz and in situ measurements of particle size distributions (PSDs) and aspect ratios of ice crystal populations. The approach is to calculate from each 5 s mean PSD and flight-level reflectivity the variability of all possible solutions of m(D) relationships fulfilling the condition that the simulated radar reflectivity factor (T-matrix method) matches the measured radar reflectivity factor. For the reflectivity simulations, ice crystals were approximated as oblate spheroids, without using a priori assumptions on the mass–size relationship of ice crystals. The CWC calculations demonstrate that individual CWC values are in the range ±32 % of the retrieved average CWC value over all CWC solutions for the chosen 5 s time intervals. In addition, during the airborne field campaign performed out of Darwin in 2014, as part of the international High Altitude Ice Crystals/High Ice Water Content (HAIC/HIWC) projects, CWCs were measured independently with the new IKP-2 (isokinetic evaporator probe) instrument along with simultaneous particle imagery and radar reflectivity. Retrieved CWCs from the T-matrix radar reflectivity simulations are on average 16 % higher than the direct CWCIKP measurements. The differences between the CWCIKP and averaged retrieved CWCs are found to be primarily a function of the total number concentration of ice crystals. Consequently, a correction term is applied (as a function of total number concentration) that significantly improves the retrieved CWC. After correction, the retrieved CWCs have a median relative error with respect to measured values of only −1 %. Uncertainties in the measurements of total concentration of hydrometeors are investigated in order to calculate their contribution to the relative error of calculated CWC with respect to measured CWCIKP. It is shown that an overestimation of the concentration by about +50 % increases the relative errors of retrieved CWCs by only +29 %, while possible shattering, which impacts only the concentration of small hydrometeors, increases the relative error by about +4 %. Moreover, all cloud events with encountered graupel particles were studied and compared to events without observed graupel particles. Overall, graupel particles seem to have the largest impact on high crystal number-concentration conditions and show relative errors in retrieved CWCs that are higher than for events without graupel particles.

Dual-Polarized Radar and Surface Observations of a Winter Graupel Shower with Negative Zdr Column

AbstractComprehensive analysis of an unusual graupel-shower event recorded by an S-band polarimetric radar and two optical-imaging surface instruments is presented. The primary radar characteristic was negative differential reflectivity Zdr values along a vertical column. During the afternoon hours of 16 February 2015, a sequence of three showers that were composed primarily of small (8–15-mm diameter) graupel affected the ground instrumentation site that was established for the Multi-Angle Snowflake Camera and Radar (MASCRAD) experiment in the high plains of Colorado. While these showers passed the instrumentation site, the CSU–CHILL radar conducted high-time-resolution (~2.5-min cycle time) range–height indicator (RHI) scans from a range of 13 km. The RHI data show that the negative Zdr values extended vertically through much of the reflectivity cores, implying that the reflectivity-weighted mean axis ratios of the graupel particles in this event remained somewhat prolate throughout their lifetime. To be specific, the cores of the convective showers only extended to heights of ~3.5 km AGL and had fractionally negative (from ~−0.3 to −0.7 dB) Zdr levels in those cores. Particle-image data obtained by the MASC system and by a collocated 2D video disdrometer measured the diameters, shapes, and fall speeds of the graupel particles as they reached the surface. The graupel particles were found to be primarily of the lump type with a slightly prolate mean shape (especially for the larger-diameter particles). Microwave backscatter calculations confirm that the graupel-particle shape and orientation characteristics are consistent with the observed slightly, but consistently, negative Zdr values.

Riming Electrification in Hokuriku Winter Clouds and Comparison with Laboratory Observations

Abstract Riming electrification is the main charge separation mechanism of thunderstorms, occurring mainly during graupel particle–ice crystal collisions. Laboratory experiments have found that charge separation polarity and magnitude depend critically on cloud water content and temperature. Several groups have mapped this dependence, but there are substantial differences between their results. These conflicting laboratory-derived riming electrification topographies can be tested by comparing them to field observations. Here, direct and simultaneous sonde-based measurement of both precipitation particle type and charge (videosonde) and cloud water content [hydrometeor videosonde (HYVIS)] in lightning-active Hokuriku winter clouds at Kashiwazaki, Niigata Prefecture, Japan, are reported. With decreasing height, summed graupel charge transitioned from negative to positive at a mean temperature of −11°C, and the mean peak cloud water content in the positive graupel domain was 0.4 g m−3. Thus, in cloud regions of relatively high temperature (≥−11°C) and low cloud water content (CWC; ≤0.4 g m−3), graupel particles were mainly positively charged. This result can be compared with those of laboratory riming experiments; for example, in this temperature/cloud water content domain, graupel electrification has been reported to be positive by Takahashi, largely negative in early reports using the Manchester cloud chamber, positive in later reports using the Cordoba and Manchester modified cloud chambers, and partially positive in a more recent report using the Cordoba cloud chamber.

Factors controlling secondary ice production in cumulus clouds

Aircraft measurements of two cumulus clouds were made during the Ice and Precipitation Initiation in Cumulus campaign over the British Isles. The 18 May 2006 cloud had high concentrations of ice particles and conditions were conducive for the Hallett–Mossop (HM) process of secondary ice production, but the 13 July 2005 cloud had low concentrations. A bin‐resolved cloud model was used to investigate several factors that are known to control the HM process using the observations of the two clouds. For the 2006 cloud, the model results show that the fast production of graupel by directly freezing of supercooled raindrops through collisional collection with ice particles was crucial to the activation of the HM process. Switching off raindrop freezing led to much delayed and suppressed formation of graupel particles, and hence a negligible HM process. Sensitivity studies were performed on the concentration of primary ice particles required to kick‐start the HM process. It was found that a concentration of the first ice as low as 0.01 L−1 could be sufficient, as long as there was a large enough concentration of cloud droplets (small and large) available when a significant number of graupel particles developed in the HM temperature zone. For the modelled 2005 cloud, the HM process did not operate effectively mainly because of the low concentration of supercooled raindrops and hence graupel. The HM process was also hindered by the relatively greater number of aerosols, and higher temperatures at cloud base and top.

Explosive Ice Multiplication Induced by Multiplicative-Noise Fluctuation of Mechanical Breakup in Ice–Ice Collisions

Abstract The number of ice fragments generated by breakup of large graupel in collisions with small graupel fluctuates randomly owing to fluctuations in relative sizes and densities of colliding graupel particles and the stochastic nature of fracture propagation. This paper investigates the impact of the stochasticity of breakup on ice multiplication. When both the rate of generation of primary ice and the initial number concentration of ice crystals are low, the system most likely loses all the initial ice and graupel owing to a lack of sustaining sources. Even randomness does not change this mean evolution of the system in its phase space. However, a fluctuation of ice breakup number gives a small but finite chance that substantial ice crystal fragments are generated by breakup of large graupel. That, in turn, generates more large graupel. This multiplicative process due to fluctuations potentially leads to a small but finite chance of explosive growth of ice number. A rigorous stochastic analysis demonstrates this point quantitatively. The randomness considered here belongs to a particular category called “multiplicative” noise, because the noise amplitude is proportional to a given physical state. To contrast the multiplicative-noise nature of ice breakup with a standard “additive” noise process, fluctuation of the primary ice generation rate is also considered as an example of the latter. These processes are examined by taking the Fokker–Planck equation that explicitly describes the evolution of the probability distribution with time. As an important conclusion, stability in the phase space of the cloud microphysical system of breakup in ice–ice collisions is substantially altered by the multiplicative noise.

싸락눈 종단 속도의 불확실성이 구름 모의에 미치는 영향

In spite of considerable progress in the recent decades, there still remain large uncertainties in numerical cloud models. In this study, effects of uncertainty in terminal velocity of graupel on cloud simulation are investigated. For this, a two-dimensional bin microphysics cloud model is employed, and deep convective clouds are simulated under idealized environmental conditions. In the sensitivity experiments, the terminal velocity of graupel is changed to twice and half the velocity in the control experiment. In the experiment with fast graupel terminal velocity, a large amount of graupel mass is present in the lower layer. On the other hand, in the experiment with slow graupel terminal velocity, almost all graupel mass remains in the upper layer. The graupel size distribution exhibits that as graupel terminal velocity increases, in the lower layer, the number of graupel particles increases and the peak radius in the graupel mass size distribution decreases. In the experiment with fast graupel terminal velocity, the vertical velocity is decreased mainly due to a decrease in riming that leads to a decrease in latent heat release and an increase in evaporative cooling via evaporation, sublimation, and melting that leads to more stable atmosphere. This decrease in vertical velocity causes graupel particles to fall toward the ground easier. By the changes in graupel terminal velocity, the accumulated surface precipitation amount differs up to about two times. This study reveals that the terminal velocity of graupel should be estimated more accurately than it is now.

A preliminary numerical study on the time-varying fall attitudes and aerodynamics of freely falling conical graupel particles

The flow fields and dynamic motions of conical graupel of diameters 0.5–5mm falling in air of 800hPa and −8°C are studied by solving the transient Navier-Stokes equations numerically for flow past the conical graupel and the body dynamics equations representing the 6-degrees-of-freedom motion that determines the position and orientation of the graupel in response to the hydrodynamic force of the flow fields. The shape of conical graupel made through a simple but practical existing mathematical equation allows us to have an uneven mass distribution, which is generally believed to have great influence on ice particles' orientations while falling when inertial force becomes increasingly dominant over other effects. The simulated motions include vertical fall, lateral translation, sailing, rotation and pendulum swing. The computed flow fields are characterized in terms of streamtrace patterns as well as the vorticity magnitude fields, and the corresponding motions of the conical graupel is physically featured by looking upon the graupel surface distributions of pressure coefficient, torques contributed by both pressure as well as viscous effects. Tumbling doesn't occur when an initial orientation of the graupel is either 20° or 160° about Y axis, and the torque contributed by the pressure effect is dominant over that contributed by the viscous effect.

Effects of Mixed Phase Microphysical Process on Precipitation in a Simulated Convective Cloud

The effects of the liquid water content (LWC) and mixing ratio of hydrometeors in the simulation of convective precipitation in Wuhan, Hubei Province, China, are investigated using a three-dimensional convective rainstorm model. The microphysical processes of warm and cold clouds are considered into microphysical parameterization. The warm-cloud process is dominated by the combined effects of condensation and drop coalescence. The cold-cloud process is initiated mainly by production of graupel, and the microphysical parameterizations are used to predict the mixing ratio of cloud droplets, rain, ice crystals, snow, and graupel. The simulations results show that 80% rainfall is derived from warm cloud microphysical processes, and the rest is produced by cold cloud microphysical processes. The mixed phase microphysical process can invigorate the production of convective rainfall and enhance the liquid water content (LWC). In addition, the vertical distribution of LWC is mainly concentrated at the height isotherms of −10 to −20 °C in precipitation and the concentration area of LWC matches the distribution range of graupel particles. However, the growth of graupel particles depend on the microphysical processes of nucleation and propagation between rain and graupel particles (NUrg) and collision and coalescence between cloud droplets and graupel (CLcg), in which NUrg is a major source of graupel particles and the contribution of the process accounts for 77% of the amount of graupel particles.

Thunderstorm graupel charging in the absence of supercooled water droplets

This study reports new measurements of the charge separation during ice crystal–graupel collisions in the absence of supercooled water droplets. Experimental measurements of the average charge acquired by a simulated graupel, under condition of subsaturation with respect to water and supersaturation with respect to ice, were performed. The relative humidity of the air was controlled and measured during the experiments. The experiments were carried out in the temperature range between −7 and −21 °C, with a relative velocity of 3 m s−1 between the ice particles and the graupel. The aim of the measurements was to study the dependence of the non‐inductive mechanism on the presence/absence of supercooled water droplets. Present results together with previous findings indicate that the sign of the graupel charging depends on the presence/absence of supercooled water droplets and on the relative humidity (subsaturation/supersaturation) of the environment in the case of absence of water droplets. This laboratory study is the first to map the non‐riming conditions with low velocity impact and these new results confirm the negative charging of graupel under these conditions. It was found that the magnitude of the charge separated per collision in non‐riming conditions is of the same order of magnitude as in riming conditions. This suggests that the non‐inductive mechanism could be operating in the storms even in the absence of water droplets, emphasizing the importance of knowledge of the environmental thermodynamic conditions in order to know the charging behaviour of the graupel particles.

Model analysis of radar echo split observed in an artificial cloud seeding experiment

An artificial cloud seeding experiment was performed over the Japan Sea in winter to show how massive seeding could be effective to mitigate heavy snowfall damage. The results showed that 20 min after cloud seeding, a portion of the radar echo beneath the seeding track was weakened to divide the radar echo into two parts. In order to analyze the results, a numerical simulation was conducted by using the Weather Research and Forecasting model verion 3.5.1. In this simulation, the seeding effects were represented as phenomena capable of changing rain particles by accreting cloud ice and snow to form graupel particles and by changing cloud liquid water to snow particles. The graupel particles fell rapidly, thus temporarily intensifying the rainfall, which subsequently decreased. Therefore, the weakened radar echo in the field experiment is deemed to have been caused by the increase in rapidly falling graupel particles.

Numerical simulation of precipitation formation in the case orographically induced convective cloud: Comparison of the results of bin and bulk microphysical schemes

In this study, results of bulk and bin microphysical schemes are compared in the case of idealized simulations of pre-frontal orographic clouds with enhanced embedded convection. The description graupel formation by intensive riming of snowflakes was improved compared to prior versions of each scheme. Two methods of graupel melting coincident with collisions with water drops were considered: (1) all simulated melting and collected water drops increase the amount of melted water on the surface of graupel particles with no shedding permitted; (2) also no shedding permitted due to melting, but the collision with the water drops can induce shedding from the surface of the graupel particles.The results of the numerical experiments show: (i) The bin schemes generate graupel particles more efficiently by riming than the bulk scheme does; the intense riming of snowflakes was the most dominant process for the graupel formation. (ii) The collision-induced shedding significantly affects the evolution of the size distribution of graupel particles and water drops below the melting level. (iii) The three microphysical schemes gave similar values for the domain integrated surface precipitation, but the patterns reveal meaningful differences. (iv) Sensitivity tests using the bulk scheme show that the depth of the melting layer is sensitive to the description of the terminal velocity of the melting snow. (v) Comparisons against Convair-580 flight measurements suggest that the bin schemes simulate well the evolution of the pristine ice particles and liquid drops, while some inaccuracy can occur in the description of snowflakes riming. (vi) The bin scheme with collision-induced shedding reproduced well the quantitative characteristics of the observed bright band.

Relationships between total lightning activity, microphysics and kinematics during the 24 September 2012 HyMeX bow‐echo system

The structure of a bow‐echo system that crossed southeast France on 24 September 2012 during the first Special Observation Period of the Hydrological cycle in the Mediterranean Experiment (HyMeX) is investigated from a three‐dimensional lightning mapping array (LMA), an operational lightning network, and operational polarimetric radar observations. Lightning observations reveal that 97% of flash initiation and 93% of flash propagation occurred within the convective region. Through combining LMA and radar‐derived microphysical data it is also shown that lightning initiation events and propagation mostly occurred in regions containing graupel (initiation = 70%, propagation = 58%), ice (initiation = 22%, propagation = 33%), and to a lesser extent hail (initiation = 6%, propagation = 5%). The detailed analysis of the lightning activity during two periods of contrasted electrical activity suggests that wet hail growth processes and/or charge redistribution within the cloud may have had a negative impact on lightning initiation occurrences and propagation. The most active periods in terms of lightning activity occurred in conjunction with a vigorous, terrain driven, updraught that transported graupel particles above a height of 12 km.

Comparison of Simulated Polarimetric Signatures in Idealized Supercell Storms Using Two-Moment Bulk Microphysics Schemes in WRF

Abstract Microphysics parameterization becomes increasingly important as the model grid spacing increases toward convection-resolving scales. The performance of several partially or fully two-moment (2M) schemes within the Weather Research and Forecasting (WRF) Model, version 3.5.1, chosen because of their well-documented advantages over one-moment (1M) schemes, is evaluated with respect to their ability in producing the well-known polarimetric radar signatures found within supercell storms. Such signatures include the ZDR and KDP columns, the ZDR arc, the midlevel ZDR and ρHV rings, the hail signature in the forward-flank downdraft, and the KDP foot. Polarimetric variables are computed from WRF Model output using a polarimetric radar simulator. It is found that microphysics schemes with a 1M rimed-ice category are unable to simulate the ZDR arc, despite containing a 2M rain category. It is also found that a hail-like rimed-ice category (in addition to graupel) may be necessary to reproduce the observed hail signature. For the microphysics schemes that only contain a graupel-like rimed-ice category, only very wet graupel particles are able to reach the lowest model level, which did not adequately reduce ZDR in this signature. The most realistic signatures overall are found with microphysics schemes that are fully 2M with a separate hail category.

Explosive ice multiplication by mechanical break‐up in ice–ice collisions: a dynamical system‐based study

Mechanical break‐up in ice–ice collisions observed in the laboratory can lead to explosive ice multiplication. The possibility is examined in detail by constructing an idealized analytical single‐point (zero‐dimension) model assuming spatial homogeneity within a cloud. The rate of generation of primary ice is fixed at a constant value over time, corresponding to a situation in which the relative humidity (at water saturation as for a mixed‐phase cloud) and updraught speed are fixed. This would further imply an infinite supply of vapour, if the crystal concentration were somehow infinite. Fixed times are assumed for the transformation of the generated primary ice into small graupel, which then grow into large graupel. Secondary ice particles produced by collisions between small and large graupel, in turn, potentially multiply explosively, because the secondary ice may eventually grow to become splintering graupel with a positive feedback.Two basic processes can lead to explosive ice multiplication. The first process is the generation of primary ice, which initiates ice multiplication only with a relatively low threshold (∼10−4 m−3 s−1) compared to typical observed values for deep convective clouds. Then the second process is induced by a sufficiently large initial number of ice crystals, which generate enough both small and large graupel particles, leading to explosive multiplication, even when the rate of generation of primary ice is below the threshold. These initial crystals at super‐critical amounts may be from any source. Only a low number density of crystals of the order of 1 m−3 (or 10−3 l−1) is required initially. In both cases, the ice number literally explodes within a finite time, with a time‐scale from 30 to 200 min under the idealized single‐point model studied. Importantly, in real clouds a typical number density of large graupel is above the threshold needed for explosive break‐up.