Hailstone Diameter Research Articles

Verification of weather-radar-based hail metrics with crowdsourced observations from Switzerland

Abstract. Remote hail detection and hail size estimation using weather radar observations has the advantage of wide spatial coverage and high spatial and temporal resolution. Switzerland's National Weather Service (MeteoSwiss) uses two radar-based hail metrics: the probability of hail on the ground (POH) to assess the presence of hail and the maximum expected severe hailstone size (MESHS) to estimate the largest hailstone diameter. However, radar-based metrics are not direct measurements of hail and have to be calibrated with and verified against ground-based observations of hail, such as crowdsourced hail reports. Switzerland benefits from a particularly rich and dense dataset of crowdsourced hail reports from the MeteoSwiss app. We combine a new spatiotemporal clustering method (Density-Based Spatial Clustering of Applications with Noise, ST-DBSCAN) with radar reflectivity to filter the reports and use the filtered reports to verify POH and MESHS in terms of the hit rate, false-alarm ratio (FAR), critical success index (CSI), and Heidke skill score (HSS). Using a 4 km × 4 km maximum upscaling approach, we find FAR values between 0.3 and 0.7 for POH and FAR > 0.6 for MESHS. For POH, the highest CSI (0.37) and HSS (0.52) are obtained using a 60 % threshold, while for MESHS the highest CSI (0.25) and HSS (0.4) are obtained using a 2 cm threshold. We find that the current calibration of POH does not correspond to a probability and suggest a recalibration based on the filtered reports.

Numerical Simulation of a Giant-Hail-Bearing Mediterranean Supercell in the Adriatic Sea

On 10 July 2019, a giant hail-bearing supercell hit the Adriatic coast of central Italy. Hailstones with a maximum diameter of 14 cm were reported in the city of Pescara between 10:00 and 11:00 UTC. In this work, the main synoptic and mesoscale features, responsible for the triggering and the development of the supercell, are analyzed using the WRF model. The intrusion of Bora wind over the northern and central Adriatic was relevant for two reasons: on the one side, the arrival of low-level cold air produced an uplift of the pre-existing warm air and favored the triggering of convection; on the other side, the strong vertical wind shear, also due to the presence of intense upper-level southwesterlies, created conditions favorable to the formation of the supercell. The predictability of the event is also discussed, comparing simulations starting at different initial times and forced with GFS and IFS forecasts. The model results show that the runs initialized at earlier times reproduced more accurately the track and the time evolution of the supercell. The HAILCAST module of WRF was also used to simulate hailstorm characteristics, such as the average hailstone diameter. WRF-HAILCAST simulations proved to be in fair agreement with the radar reflectivity retrievals and with local reports.

Review of Impact Factors of the Velocity of Large Hailstones for Laboratory Hail Impact Testing Consideration

The terminal velocity of hailstones is an essential quantity in hail research. It is an important value for the kinetic energy of hailstones and has to be carefully considered when laboratory hail impact testing is being planned. Many standardized hail impact testing procedures require specific impact velocities. Several empirical models for various hailstone diameter ranges have been developed to calculate the velocity based on the diameter, indicating somewhat inconsistent results. The velocity is influenced by factors such as their density, drag coefficient, shape or air density, and strong winds. This article takes a closer look into each of these factors and how they influence the velocity of hailstones and the damage potential of hailstorms. Implications are made for laboratory testing of hail impact testing procedures and how the geographic location of hailstorms might be a considerable factor when designing a test.

Observational analysis and simulations of a severe hailstorm in northeastern Italy

AbstractHailstorms are relatively frequent in the Friuli Venezia Giulia region, northeastern Italy and, for that reason, a network of manual hailpads has been set up there since the late 1980s. On July 4, 2007, a record number of hailpads in the network were impacted by hail (115 out of about 360 total stations). To the best of the authors' knowledge, no other single hailstorm case‐study has information coming from such a large number of hailpads impacted by hail. These hailpads are analysed using automatic software, able to interpret each hail dent as an ellipse. From these data, the distribution of hailstone diameters, areal hailstone density on the pads and total kinetic energy flux are computed and analysed in the present study. A parallel analysis of radar maximum reflectivity and cloud‐to‐ground lightning shows that the evolution of the hailstorm can be divided into three different phases, the first being the most severe in terms of lightning and hail. Finally, numerical simulations performed with a non‐hydrostatic mesoscale model, implementing a prognostic module for the explicit simulation of hail (WRF‐HAILCAST), show that the simulation system provides useful information on the severity of the event in terms of maximum hailstone diameter and reflectivity shape.

A numerical study on the ventilation coefficients of falling lobed hailstones

The ventilation coefficients that represent the enhancement of mass transfer rate due to the falling motion of spherical hailstones in an atmosphere of 460hPa and 248K are computed by numerically solving the unsteady Navier‐Stokes equation for airflow past hailstones and the convective diffusion equation for water vapor diffusion aroundthe fallinghailstones.The diameters of the hailstonesinvestigated are from 1to 10cm, corresponding to Reynolds number from 5935 to 177148. The calculated ventilation coefficients vary approximately linearly with the hailstone diameter, from about 19 for a 1-cm hailstone to about 208 for a 10-cm hailstone. Empirical formulas for ventilation coefficient variation with hailstone diameter as well as Reynolds and Schmidt numbers are given. Implications of these ventilation coefficients are discussed.

Large Hail Incidence and Its Economic and Societal Impacts across Europe

Abstract By 31 December 2018, 39 537 quality-controlled reports of large hail had been submitted to the European Severe Weather Database (ESWD) by volunteers and ESSL. This dataset and the NatCatSERVICE Database of Munich RE jointly allowed us to study the hail hazard and its impacts across Europe over a period spanning multiple decades. We present a spatiotemporal climatology of the ESWD reports, diurnal and annual cycles of large hail, and indicate where and how they may be affected by reporting biases across Europe. We also discuss which hailstorms caused the most injuries and present the only case with hail fatalities in recent times. Additionally, we address our findings on the relation between hail size to the type of impacts that were reported. For instance, the probability of reported hail damage to roofs, windows, and vehicles strongly increases as hail size exceeds 5 cm, while damage to crops, trees, and greenhouses is typically reported with hailstone diameters of 2–3 cm. Injuries to humans are usually reported with hail 4 cm in diameter and larger, and number of injuries increases with increasing hail size. Using the NatCatSERVICE data, we studied economic losses associated with hailstorms occurring in central Europe and looked for long-term changes. The trend in hail losses and the annual number of hail loss days since 1990 to 2018 are compared to that of meteorological conditions favorable for large hail as identified by ESSL’s Additive Regression Convective Hazards model. Both hail loss days and favorable environments show an upward trend, in particular since 2000.

How to Express Hail Intensity—Modeling the Hailstone Size Distribution

AbstractThe local intensity of hail can be expressed by a variety of variables, such as hail kinetic energy, maximum hailstone size, and radar reflectivity–driven algorithms. All of these variables are connected by the hailstone size distribution. In the United States, the Community Collaborative Rain, Hail and Snow Network (CoCoRaHS) provides more than 37 000 observations that describe the diameter of the smallest, average, and largest hailstone; duration of hail; and overall hailstone number density. We use these data and the assumption of an exponential hailstone size distribution aloft to model the rate of hailstones hitting the ground per unit area, time, and hailstone size bin during the passage of a hailstorm. We found that total hail kinetic energy is proportional to the diameter of the largest hailstone to a power of less than 2. To validate these results, we compared them with hailpad observations of the largest hailstone diameters and hail kinetic energies in southwestern France. As an example application, we calculate the probability mass function for the largest hailstone observed by a hailpad given the largest hailstone occurring in its vicinity. We use this model at Risk Management Solutions, Ltd. (RMS), to calculate the vulnerability of subjects at risk as a function of the diameter of the largest hailstone.

Operational Forecasts of Maximum Hailstone Diameter in Mendoza, Argentina

A coupled one-dimensional cloud and hail growth model was used to simulate the growth of hailstones in Mendoza, Argentina. The model-derived maximum hail size forecasts were based on 118 representative soundings released during the 1999-2000 hail season. Model ensemble, persistence and subjective hail forecasts were also verified against daily observations of the maximum size. The model control and ensemble showed promising skill when forecasting the occurrence of hail as measured by the Heidke’s Skill Score (HSS=0.60). On days with severe hail (diameter of 2 cm or more), the model control forecasts showed the best skill (HSS=0.59). The model showed improved forecast skill when run using sounding and surface data from the Alberta Hail Project. This was likely attributable to the stringent criteria placed on the proximity soundings and the availability of real-time surface data in Alberta. Although certain cloud model parameters were useful for inferring the potential (and size) of hail in Mendoza, the best results were achieved using the coupled cloud and hail model. The data also suggest that the ensemble technique improves the accuracy and skill of the hail forecasts on some days.

Change in hailstone size distributions with an increase in the melting level height

In France, the melting level height varies, during the hail season, from about 1km in the spring to more than 4km in the summer, and this parameter appears to have a connection with the mean characteristics of hailfalls. In fact, a large 25-year sample of hail size distribution measurements, made with hailpad networks along the southern Atlantic coast, reveals strong correlations between the melting level height and the mean hailstone number in different hailstone diameter ranges. On average, an increase in the melting level height is associated with a decrease in the number of small hailstones (5 to 7mm diameter), has no apparent effect in the middle range (7 to 9mm), and is concomitant with an increase in the number of larger hailstones, particularly in the 11 to 21mm ranges. When the relationships giving the hailstone number in each diameter range as a function of the melting level height are applied to a given hailfall, an increase in the melting level height is then found to go together with changes in the total hailstone number and kinetic energy. From this observation, it is possible to estimate the potential effect of global warming on hailfalls with different hailstone size distributions. The trends in the monthly surface temperature and in the hail day melting level height observed at Bordeaux during the 1988–2012 period and the global warming projections made for the present century suggest that an increase in the melting level height of 500m is forecast to occur in this region between 2000 and 2040. The consequences of such an increase for a standard hailfall in the region will be a slight decrease (−12%) in the total number of hailstones, and a significant increase (40%) in total kinetic energy, but with no significant change in the hail frequency. This analysis is compared to the already observed increase in hail intensity in the area under study, and to observations, measurements, and numerical models in other parts of the world.

A Numerical Study on the Ventilation Coefficients of Falling Hailstones

Abstract The ventilation coefficients that represent the enhancement of mass transfer rate due to the falling motion of spherical hailstones in an atmosphere of 460 hPa and 248 K are computed by numerically solving the unsteady Navier–Stokes equation for airflow past hailstones and the convective diffusion equation for water vapor diffusion around the falling hailstones. The diameters of the hailstones investigated are from 1 to 10 cm, corresponding to Reynolds number from 5935 to 177 148. The calculated ventilation coefficients vary approximately linearly with the hailstone diameter, from about 19 for a 1-cm hailstone to about 208 for a 10-cm hailstone. Empirical formulas for ventilation coefficient variation with hailstone diameter as well as Reynolds and Schmidt numbers are given. Implications of these ventilation coefficients are discussed.

Observations of the 14 July 2011 Fort Collins Hailstorm: Implications for WSR-88D-Based Hail Detection and Warnings

Abstract The issuance of timely warnings for the occurrence of severe-class hail (hailstone diameters of 2.5 cm or larger) remains an ongoing challenge for operational forecasters. This study examines the application of two remotely sensed data sources between 0100 and 0400 UTC 14 July 2011 when pulse-type severe thunderstorms occurred in the jurisdiction of the Denver/Boulder National Weather Service (NWS) Forecast Office in Colorado. First, a developing hailstorm was jointly observed by the dual-polarization Colorado State University–University of Chicago–Illinois State Water Survey (CSU–CHILL) research radar and by the operational, single-polarization NWS radar at Denver/Front Range (KFTG). During the time period leading up to the issuance of the initial severe thunderstorm warning, the dual-polarization radar data near the 0 °C altitude contained a positive differential reflectivity ZDR column (indicating a strong updraft lofting supercooled raindrops above the freezing level). Correlation coefficient ρHV reductions to ~0.93, probably due to the presence of growing hailstones, were observed above the freezing level in portions of the developing >55-dBZ echo core. Second, data from the National Lightning Detection Network (NLDN), including the locations and polarity of cloud-to-ground (CG) discharges produced by several of the evening’s storms, were processed. Some association was found between the prevalence of positive CGs and storms that produced severe hail. The analyses indicate that the use of the dual-polarization data provided by the upgraded Weather Surveillance Radar-1988 Doppler (WSR-88D), in combination with the NLDN data stream, can assist operational forecasters in the real-time identification of thunderstorms that pose a severe hail threat.

On the response of hailstorms to enhanced CCN concentrations

In this study we explore the role that cloud base height (or cloud base temperature) plays in the response of hailstorms to increased concentrations of cloud condensation nuclei (CCN). A rather large number of cloud-resolving simulations were performed which explored the parameter space varying both potential CCN concentration and cloud base heights, using a microphysical scheme that predicts 3-moments of the hail size distribution. Several simulated quantities exhibited distinct responses to enhanced CCN concentrations depending on the cloud base altitudes. For instance, positive changes prevailed in the number concentration and the predominant diameter of hailstones on the surface, the integral mass of hail precipitation and the peak rates for high cloud base heights or a deeper layer between cloud base and the freezing level. Conversely, weak effects prevailed for numerical experiments initialized with lower cloud bases.

A point hailfall classification based on hailpad measurements: The ANELFA scale

The ANELFA scale for hailfall intensity is proposed on the model of the 6-class Fujita scale for tornadoes. It is based on more than three thousand point hailfalls measured by hailpads over a 16-year period in France. The class number of a hailfall is determined by the integer value of the largest measured hailstone diameter in cm, or by equivalence with current objects: A0 to A5 for pea, grape, pigeon's egg, walnut, hen's egg, orange. The class number is followed by a plus or minus sign if the ground is significantly more or less than half-covered by hailstones respectively. When the scale is applied to the ANELFA data, a log-normal distribution is found for the class distribution, allowing the frequency determination of the upper class ever observed until now at the hailpad stations.

Climatological Estimates of Daily Local Nontornadic Severe Thunderstorm Probability for the United States

Abstract The probability of nontornadic severe weather event reports near any location in the United States for any day of the year has been estimated. Gaussian smoothers in space and time have been applied to the observed record of severe thunderstorm occurrence from 1980 to 1994 to produce daily maps and annual cycles at any point. Many aspects of this climatology have been identified in previous work, but the method allows for the consideration of the record in several new ways. A review of the raw data, broken down in various ways, reveals that numerous nonmeteorological artifacts are present in the raw data. These are predominantly associated with the marginal nontornadic severe thunderstorm events, including an enormous growth in the number of severe weather reports since the mid-1950s. Much of this growth may be associated with a drive to improve warning verification scores. The smoothed spatial and temporal distributions of the probability of nontornadic severe thunderstorm events are presented in several ways. The distribution of significant nontornadic severe thunderstorm reports (wind speeds ≥ 65 kt and/or hailstone diameters ≥ 2 in.) is consistent with the hypothesis that supercells are responsible for the majority of such reports.

Comparison of Hail Characteristics in NW Croatia for Two Periods

This paper presents hail characteristics for two separate periods (1981–1990,1991–1999) in the western part of the hail protected area along the Croatianand Slovenian border. The first period is characterised by full application ofhail suppression technology. The protected area was covered with a sufficientnumber of hail suppression rockets, rockets launching was permitted across theSlovenian border aimed at early seeding, and Slovenia in that time had also theoperative hail suppression system. The second period is characterised withsignificantly lower application of hail suppression technology compared withfirst period. During the independence war in Croatia, because of a lack ofrockets and other objective circumstances, appropriate rocket hail suppressionwas missing. This work examines whether hail characteristics on the border areahave changed, taking into account the changes in suppression. Results show thatthe average diameter of hailstones and duration of hailfall slightly increasedin second period. Also, there is an increase in the number of areas with alarger ratio of hail producing heavy damage, compared with first period. On thebasis of data from five meteorological stations, it is visible that, as a resultof implementation of the hail suppression system, the average number of dayswith solid precipitation on that area in period April–October has reduced,compared with the period when there was no hail suppression system.

The Electrification of Severe Storms

Abstract No Abstract available.

Modelled spongy growth and shedding process for spheroidal hailstones

Model calculations are carried out for spheroidal hailstone growth, by taking into account the results obtained by Garcı́a-Garcı́a and List from wind tunnel experiments. For the maximum hailstone diameter D varying from 2 to 4 cm, the liquid water contents at the onset of the wet growth and of shedding regimes ( W f,I and W f,E, respectively) are represented as functions of both the air temperature T a and D. The values of these parameters approach the experimental ones for D=2 cm, but show that, for D=4 cm, these transitions occur at 25% smaller values of W f. Using the obtained parameterized equation of W f,E, the dependence on T a of the number of drops N( d) shed per km of fall by a particle of D=2 and 4 cm is studied for different values of W f. For D=2 cm, it is shown that shedding is negligible at all temperatures for W f<3 g m −3 and, even for W f=4 g m −3 a value N( d) km −1=1000 is only reached near the melting level. According to these results, shedding from particles growing in the spongy regime would be much less pronounced than has been previously calculated. The effect of the variation of D on the shedding effect is also discussed.

Dual-Doppler and Multiparameter Radar Observations of a Bow-Echo Hailstorm

Abstract During the afternoon and evening hours of 23 May 1991 a hail-producing multicellular severe thunderstorm developed near Denver, Colorado, and tracked eastward for more than 100 km. Along this path, hailstone diameters of 2–7 cm (0.75–2.75 in.) were reported at several points. The storm was observed by both the CSU-CHILL (CHL) and NCAR Mile High (MUR) 10-cm Doppler radars. The general echo morphology evolved by way of cyclic, discrete new cell formation near an outflow boundary moving ahead of the storm's forward flank. As this new cell growth occurred, the shape of the storm's most intense core also evolved in a periodic fashion. On four separate occasions these cores briefly assumed a bow shape with peak reflectivity values of 65–70 dBZ. The evolution of one such bow echo was examined by a series of six CHL-MHR dual-Doppler analyses. The resultant airflow patterns suggested that the core reflectivity structure was deformed into the bowlike configuration by updraft-induced flow field perturbation...

Further Exploratory Evaluations of Grossversuch IV Using Hailpad Data: Analysis of Hail Patterns and Stratification by Storm Type for Seeding Effect

Abstract Further exploratory evaluations of the Grossversuch IV randomized hail suppression experiment using hailpad data are presented. These deal with the spatial distribution of the ground-measured variables within hail patterns. The possibility of a seeding effect limited to a part of a hailfall or to various objectively stratified types of precipitation cells is investigated. The results show an increase in the ratio of the hailed areas with maximum hailstone diameters greater than 10 mm versus the total hailed area (P value = 0.008) and an increase in Dmax, the maximum hailstone diameter, for type II repetitive evolution cells (P value = 0.04). Differences observed for the main test variables in relation to the various cell types are shown to largely exceed those that could stem from a possible seeding effect. A satisfactory physical interpretation of the differential tendency related to the storm type is impossible, however, due to the simple storm model on which the Grossversuch IV experiment is b...

Simulation des natürlichen hagelschlags an transparenten kunststoffplatten

AbstractNatural hail impact tests are scarcely available because hailstorms are limited to a small area and cannot be predicted regarding time, place and intensity. Therefore we relied on laboratory tests simulating natural hailstones by polyamide balls (5‐50 mm dia.) whose final velocity could be varied according to meteorological data. So the influence of diameter (weight) and velocity of the artificial hailstones could be taken into consideration independently.The energy of damage or equivalent natural hailstone diameter were determined for 16 mm double skin sheet from PMMA as 0.2 J (14 mm dia.) for star shaped cracks and 2 J (26 mm dia.) for holes. These values do not significantly decrease after 10 years weathering. 10 mm double skin sheets from PC show an extremly high energy of damage of abt. 10 J (38 mm dia.) which decreases to a medium level of 2 J (26 mm dia.) after several years weathering. This decrease is remarkably reduced by weather resistant protective coating.As hailstorms consist to more than 80% of hailstones below 10 mm dia. (0.04 J) the mentioned sheet materials are quite hail resistant also after long weathering periods, but they cannot withstand an extreme hail catastrophe as in Munich July 12, 1984.