Hail can cause significant damage to property and agriculture, as well as injuries or even deaths; in part, the risk associated with hail increases with increasing hailstone size, which generally leads to greater impact kinetic energy. The current definitions for hail size based on the National Weather Service include “sub-severe,” “severe,” and “significantly severe” (Table 1). In the scientific literature, some studies have also identified “giant” hail as those stones with maximum dimensions exceeding 10 cm. In the taxonomy of hail sizes listed in Table 1, we also propose a new size class for hailstones with maximum dimensions exceeding 15 cm or 6 in. (referred to here as “gargantuan hail”) to represent the upper extreme of hail sizes. Owing to the rarity of such extreme events, however, only a few studies have specifically documented giant or gargantuan hail events (e.g., Knight and Knight 2005; Blair and Leighton 2012; Pojorlie et al. 2013; Witt et al. 2018), and most are individual case studies rather than multicase comparisons. Knight and Knight (2005) described the physical characteristics of giant and gargantuan hailstones from the Aurora, Nebraska, storm of 2003. Every stone they studied exhibited an outer (i.e., final) growth layer indicating wet growth, and in some cases this layer was of quite substantial thickness. This implies that the hailstones went through heavy wet growth in their last moments in the updraft, just above the in-storm 0°C level. They suggested that an extremely strong updraft in the lower portion of the hail growth zone is required to produce such large hail. Blair and Leighton (2012) used social media reports to survey a more extensive sample of giant hail reports than was present in the Storm Data database alone, yielding hailstones that ultimately became certified state records. They suggest that the occurrence of giant or gargantuan hail is significantly underreported. Using crowdsourced photographs and videos from a tornadic supercell in El Reno, Oklahoma, Seimon et al. (2016) claim a storm chaser video captured a hailstone that may have been >20 cm in maximum diameter; however, there was no further discussion or analysis.
Proposed hail size naming convention, based on previous usage and operational terminology.
None of the aforementioned studies of giant or gargantuan hail focused on observed storm properties or environments. In contrast, Pojorlie et al. (2013) documented the synoptic and mesoscale environment of the supercell that produced the Vivian, South Dakota, hailstone (which registers as the world record for maximum dimension: 20 cm or 8 in.) and analyzed some of the storm’s radar characteristics. Their evaluation of the environment indicated that it was indeed supportive of severe convective storms, but it was not obviously supportive of such large hail as was observed. Other studies focused on radar observations of storms producing giant or gargantuan hail. Blair et al. (2011) compared equivalent radar reflectivity factor at horizontal polarization (ZH) and radial velocity (υr) signatures in giant-hail-producing storms to those in storms producing smaller hail. They found that giant hail was virtually always associated with supercells (>99% of cases), and that the best discriminators of hail sizes were strong midlevel azimuthal shear in υr associated with the mesocyclone and large values of storm-top divergence. Witt et al. (2018) performed an analysis of the 2013 El Reno, Oklahoma, storm using crowdsourced observations (see Seimon et al. 2016), WSR-88D data, and measurements from a mobile, polarimetric X-band radar. They focused on the fallout locations of hail >7 cm in maximum dimension and associated dual-polarization radar signatures, finding that large hailstones (including several ≥15 cm in maximum dimension) often fell outside the low-level maximum ZH. Some of this hail occurred in regions of ZH < 50 dBZ, but within ∼10 km of the updraft, consistent with findings by Kumjian et al. (2010) and Picca and Ryzhkov (2012). Additionally, Jiang et al. (2019) performed electromagnetic scattering calculations for real hailstone shapes that suggest giant- and gargantuan-sized hail may have similar dual-polarization radar characteristics to hail of smaller sizes, complicating radar-based hail detection of such large hail.
To synthesize the findings from these prior studies, the available evidence suggests that gargantuan-hail-producing storms typically would be supercells that form in environments that do not stand out among those associated with more “typical” supercells producing smaller hail. The radar signatures of storms with gargantuan or giant hail often are not particularly noteworthy, either, except perhaps stronger mesocyclonic rotation and divergence aloft. This implies that features commonly used by operational meteorologists to forecast and monitor severe storms may only be subtly different for extreme-hail-producing storms, making anticipation and warning for such storms a substantial challenge. Hailstone fallout locations are beneath the supecell’s main updraft, and the hailstone physical characteristics themselves suggest that the hailstones undergo significant wet growth in their final growth stage within the updraft. The fact that supercells most often occur in the relatively sparsely populated U.S. Great Plains, coupled with the likely sparse concentrations of extremely large hail within individual storms, suggests that gargantuan hail may be more common than is reported (albeit still rare compared to smaller hail sizes). Thus, documentation of such extreme events is important, as discussed by Knight and Knight (2001), and a necessary first step toward understanding how such hail is produced, ultimately unlocking clues toward improved prediction and detection of such events.
Herein, we document a case from 8 February 2018 that, unlike the other cases described above, featured gargantuan hail in a populated urban region. This includes a hailstone estimated photogrammetrically to be very close to or exceeding the Vivian hailstone world record for maximum dimension. The storm occurred in Villa Carlos Paz, in the Córdoba Province of Argentina (Fig. 1), making it the first well-documented case of gargantuan hail outside the U.S. Great Plains, and the first in the Southern Hemisphere. This region is known to be prone to hail (e.g., Torre et al. 2011; Mezher et al. 2012; Cecil and Blankenship 2012; Bruick et al. 2019), though hail of giant or gargantuan size has until now not been documented there. Although the authors were aware of this storm based on social media reports, this study is the result of poststorm forensic meteorological research made possible by the Remote Sensing of Electrification, Lightning, and Mesoscale/Microscale Processes with Adaptive Ground Observations (RELAMPAGO; S. W. Nesbitt et al. 2020, unpublished manuscript) field campaign, which also happened to be based in Villa Carlos Paz from 1 November through 15 December 2018.
(a) Map of the region of interest in South America, including WRF simulation innermost domain (cyan box) and the approximate RELAMPAGO study region (light purple dashed box). The locations of Córdoba and Villa Carlos Paz are indicated as white square and circle markers, respectively. The map shows terrain elevation, shaded according to the outset scale on the right. Black solid lines are country boundaries. (b) A zoomed-in view of downtown Villa Carlos Paz, Argentina, and the location of three hailstones analyzed herein (with maximum dimensions indicated, in cm). Map imagery courtesy of Google.
Citation: Bulletin of the American Meteorological Society 101, 8; 10.1175/BAMS-D-19-0012.1
The remainder of this paper is laid out as follows. The second section provides a detailed overview of the environment leading up to the Villa Carlos Paz supercell storm, with the aid of a high-resolution numerical simulation of the event. The evolution of observed radar signatures and the simulated storm are discussed in the third section. The analysis of giant and gargantuan hail reports in Villa Carlos Paz is provided in the fourth section. A discussion of the results and conclusions is provided in the final section.
Description of the environment
At 1200 UTC (0900 local time) on 8 February 2018, a broad upper-level trough was located off the west coast of Chile (not shown), which favors the presence of a warm, humid, conditionally unstable air mass conducive to severe convection over central Argentina. The 1200 UTC radiosonde launched at Córdoba Ingeniero Aeronáutico Ambrosio L.V. Taravella Airport (SACO; about 30 km east-northeast of Villa Carlos Paz) reveals several elevated mixed layers above a low-level nocturnal inversion (Fig. 2a), although the most unstable parcel convective available potential energy (MUCAPE) is only about 300 J kg‒1 at this time. These broadscale features are consistent with the composite pattern for supercells in this region shown by Mulholland et al. (2018), though their composite also indicates a northerly low-level jet parallel to the Sierras de Córdoba, which was absent at this time (the observed 1200 UTC SACO sounding shows only weak winds in the lower troposphere). Unsurprisingly, such environmental factors that promote hailstorms in the U.S. have also been identified as important in Argentina (e.g., Mezher et al. 2012; Bruick et al. 2019).
(a) Thermodynamic soundings at SACO (observed at 1200 UTC, in red) and Villa Carlos Paz (simulated at 1200 and 2000 UTC, in blue and black, respectively), with temperature given by the solid line and dewpoint temperature given by the dashed line. (b) 1200 UTC SACO observed 0–10-km hodograph, with the Bunkers et al. (2000) left-moving supercell motion shown by the red star. (c),(d) As in (b), but simulated at Villa Carlos Paz at 1200 and 2000 UTC.
Citation: Bulletin of the American Meteorological Society 101, 8; 10.1175/BAMS-D-19-0012.1
To further examine the storm environment, we ran a doubly nested 3–1-km-grid-spacing Weather Research and Forecasting (WRF; Skamarock et al. 2008) Model v4.1 simulation using boundary conditions from the ECMWF fifth-generation atmospheric reanalysis (ERA5) (Copernicus Climate Change Service 2019). The ERA5 is an hourly reanalysis dataset with 0.25° horizontal resolution and 37 vertical layers from 1000 to 1 hPa. Here, the simulation primarily is used to downscale the reanalysis boundary conditions with better-resolved topography and to examine the storm and the rapidly evolving environment leading up to it. The WRF simulation outer domain, covering much of subtropical South America, was initialized at 0000 UTC 8 February and run for 24 h; the inner 1-km nest covered central Argentina, Chile, and the adjacent Pacific Ocean, and was initialized at 1800 UTC. The WRF simulation had 80 vertical levels and used the Thompson microphysics scheme (Thompson et al. 2004).
The WRF-simulated thermodynamic profiles and hodographs at 1200 and 2000 UTC at Villa Carlos Paz are shown in Fig. 2, for comparison with the 1200 UTC observed sounding at SACO. The thermodynamic and kinematic profiles at Villa Carlos Paz and SACO at 1200 UTC are qualitatively similar, with minimal MUCAPE (∼300 J kg‒1), although a weaker capping inversion than observed) and a hodograph indicating small values (−47 m2 s‒2) of 0–3-km storm-relative helicity (SRH). However, the simulation indicates an evolution toward an environment more conducive to supercells at Villa Carlos Paz during the subsequent 8-h period. Instability increases through the development of a deep mixed layer, while the boundary layer moistened and a midlevel cap in the 625–500-hPa layer eroded. MUCAPE increased to 2241 J kg‒1 at 2000 UTC, while most unstable convective inhibition (MUCIN) decreased in magnitude to just –10.4 J kg‒1. Hodograph length increased over time in response to the approaching upper-level trough, indicating an increase in deep-layer vertical wind shear that is known to be supportive of supercell hailstorms (e.g., Marwitz 1972; Browning 1977; Nelson 1983; Foote 1984) and favorable for hail production owing to the resultant increase in updraft breadth (Dennis and Kumjian 2017). The 0–6-km bulk wind difference is ∼20.1 m s‒1 (39.0 kt); however, an additional ∼5 m s‒1 (10 kt) of shear is found between 6 and 9 km that can contribute to storm organization (e.g., Warren et al. 2017). Further, hodograph curvature increases during this time. Assuming the Bunkers et al. (2000) left-moving supercell storm motion (shown with the red star in Figs. 2b–d), 0–3-km SRH increased in magnitude from −47 to –126 m2 s‒2. Especially rapid changes in the simulated environment occur from 1930 to 2100 UTC, in part a result of upslope flow along the east side of the Sierras de Córdoba. This anabatic flow transports high-CAPE (>3,000 J kg‒1) air up the eastern slopes of the Sierras de Córdoba over a ∼1-h period prior to the storm arriving in Villa Carlos Paz (Fig. 3). As in Mulholland et al. (2019), the upslope flow also helps erode the MUCIN, and enhances low-level flow, leading to a corridor of enhanced 0–3-km SRH values, as shown in Fig. 4. Small environmental low-level shear (<5 m s‒1 over the lowest 2 km AGL), however, suggests weak organization of the low-level mesocyclone of any supercellular convection that would develop (e.g., Markowski and Richardson 2014; Coffer and Parker 2017), resulting in a primarily severe hail and wind threat for this event. In summary, the simulation shows the rapid mesoscale development of storm parameters favorable for supercells along the eastern terrain slope, and that the environment is not well represented by the individual operational 1200 UTC sounding collected at SACO.
Output of the WRF simulation of this event, shown at (a) 1930, (b) 2000, (c) 2030, and (d) 2100 UTC. Shaded are values of MUCAPE (J kg‒1, with color bar at lower right), contour values are selected values of simulated ZH (20 and 40 dBZ in white and black contours, respectively). Terrain at 1,000 and 2,000 m is shown with the green and blue contours, respectively. 10-m wind barbs are shown (full barb = 10 m s‒1). The white star shows the location of Villa Carlos Paz.
Citation: Bulletin of the American Meteorological Society 101, 8; 10.1175/BAMS-D-19-0012.1
As in Fig. 3, but 0–3-km SRH (m2 s‒2) is shaded.
Citation: Bulletin of the American Meteorological Society 101, 8; 10.1175/BAMS-D-19-0012.1
Overview of the Villa Carlos Paz supercell storm
Although there is significant radar beam blockage due to Córdoba’s tall buildings and the Sierras Chicas mountains between the radar and Villa Carlos Paz, the storm’s salient radar characteristics are still evident in low-level (<2° elevation angle) scans (Fig. 5) as observed with the operational C-band radar in Córdoba (RMA1). Throughout the analysis period (1819–2058 UTC), the storm displayed supercellular structure. At the beginning of the analysis period (1819 UTC; not shown), two distinct cells are evident, each with separate and strong updrafts, as indicated by differential reflectivity (ZDR) columns (e.g., Kumjian et al. 2014, and references therein) aloft. The cells separate further over the next 30 min (Fig. 5), with the left-moving cell being favored because of the orientation of the hodograph curvature (cf. Fig. 2d). By 1850 UTC, the left-moving storm starts to acquire a hook-echo-like appendage (Markowski 2002, and references therein), and continues to organize and advance northeastward over the next 15–20 min. By 1926:58 UTC, the heavy precipitation core is passing south and east of Villa Carlos Paz, and a well-defined hook echo is approaching the city. The main updraft moves directly over Villa Carlos Paz, as confirmed by the pronounced bounded weak echo region (BWER; see, e.g., Browning and Donaldson 1963; Marwitz 1972; Browning and Foote 1976, among many others) aloft in ZH (Fig. 6, left). Figure 6 (center) shows that the inner edge of the BWER is marked by a ZDR column, typical of polarimetric radar signatures in supercell storms (Kumjian and Ryzhkov 2008). Further, the inner edge of the BWER exhibits a pronounced reduction in copolar correlation coefficient ρhv (Fig. 6, right), which has been attributed to the presence of severe hail undergoing wet growth (Kumjian and Ryzhkov 2008; Kumjian 2013a). Similarly low ρhv values (<0.5) have been observed at C band in previous cases of significantly severe hail (Picca and Ryzhkov 2012) and are consistent with scattering calculations for significantly severe hailstones (e.g., Jiang et al. 2019). Further, evidence of a polarimetric three-body scattering signature (Hubbert and Bringi 2000; Picca and Ryzhkov 2012; Kumjian 2013b, 2018) aloft at this time in ZDR and ρhv confirms the presence of hail, though it gives no indication of its size (e.g., Zrnić et al. 2010; Kumjian 2013b). The maximum1 observed ZH from any elevation angle and at any time to pass over Villa Carlos Paz was nearly 12 dB lower than the storm-maximum ZH value. Additionally, these maxima were separated by 11.8 km, indicating that the gargantuan hail did not fall in the heaviest precipitation core of the storm, but rather beneath the updraft, consistent with the conceptual model of Browning and Foote (1976) and observations by Kumjian et al. (2010), Picca and Ryzhkov (2012), and Witt et al. (2018). After 1946 UTC, the storm passed beyond Villa Carlos Paz and continued to exhibit supercellular characteristics, including a hook echo. Storm structure produced in our WRF simulation is qualitatively similar to the overall storm evolution observed with the RMA1 radar, albeit with inaccurate timing. The simulated storm displayed a long track of minimum updraft helicity (UH) approaching −700 m2 s‒1 (Fig. 7), demonstrating a persistent rotating updraft characteristic of supercell storms.
Evolution of the storm as depicted by low-level ZH scans (shaded, in dBZ, according to the scale) from the RMA1 radar. The white star shows the location of Villa Carlos Paz in each panel. Elevation angles shown are 1.54°, 1.49°,1.59°, 1.85°, 1.80°, and 1.49°.
Citation: Bulletin of the American Meteorological Society 101, 8; 10.1175/BAMS-D-19-0012.1
A selected PPI scan from 11° elevation angle, taken at 1926:58 UTC, around the time the storm was producing gargantuan hail. Panels shown are (left) ZH, (center) ZDR, and (right) ρhv. The annotated arrow indicates the polarimetric three-body scattering signature. The black dot is Villa Carlos Paz.
Citation: Bulletin of the American Meteorological Society 101, 8; 10.1175/BAMS-D-19-0012.1
Plan views of WRF simulated minimum UH (m2 s‒1). Green and blue contours show the terrain at 1,000 and 2,000 m, respectively, and the white star shows the location of Villa Carlos Paz.
Citation: Bulletin of the American Meteorological Society 101, 8; 10.1175/BAMS-D-19-0012.1
Hail reports
There were numerous social media posts of photos and videos showing hail as it was falling, and/or retrieved after the storm. Some of these reports were featured by international news agencies. In this study, we focus on three reported instances of giant and gargantuan hail in the storm, the locations and details of which are shown in Fig. 1 and provided in Table 2.
Maximum dimension, mass, and location of the three large hailstones documented in this study.
Maria’s hailstone.
First, we describe a giant (>10 cm) hailstone that was retrieved shortly after it fell, photographed with reference objects, and preserved in a freezer by Maria Navidad Garay (hereafter “Maria’s hailstone,” with its location indicated by the southernmost label in Fig. 1). Maria’s hailstone is shown in Fig. 8. According to Maria, the hail fall lasted approximately 15 min. She experienced mainly smaller stones, and found just the one large stone in the grass. It impacted the ground with substantial force, as it had penetrated 2–3 cm into the ground when she found it. The photographs from shortly after it was retrieved (Figs. 8a,b) show a rather round stone with scalloped lobes (e.g., Knight and Knight 1970) covering much of the surface, and no large icicle lobes. This implies no preferred orientation direction in its final growth layer, presumably owing to random tumbling during its descent. She recounted that many of the stones had similar roundish shapes, with clear outsides and milky insides. During the hail fall, she recalled very little rain. After the hail, she experienced rain, but no wind. Her grass was covered with hail, including some broken pieces; she also reported accumulations of hail near her door, which was likely from runoff from the rooftop gutters. Upon surveying her property, she showed us damage to the carport roof and the hail net covering portions of her driveway. Thus, it is likely that some other large stones fell at her location, but perhaps melted, broke apart, or otherwise were not noticed. Since moving to Villa Carlos Paz in 1986, this was by far the largest hailstone she had ever seen.
(a) Photograph of Maria’s hailstone shortly after it was retrieved from her front yard. (b) Photograph of the giant hailstone next to decorative balls, which measured 7.5 cm in diameter (as measured with digital calipers, not shown). Photos courtesy of Maria Navidad Garay, used with permission. (c) Photograph of preserved hailstone after being in the freezer for 9 months, with ruler for comparison. (d) 3D rendering of the laser-scanned hailstone.
Citation: Bulletin of the American Meteorological Society 101, 8; 10.1175/BAMS-D-19-0012.1
The official measurements of Maria’s hailstone took place during the RELAMPAGO field campaign, after it had been preserved in her freezer for 9 months. The preserved stone had an artificially flattened base (Figs. 8c,d) from melting and refreezing in the freezer. (She was proud of her hailstone and often took it out of the freezer to show guests.) Therefore, some sublimation and melting/refreezing occurred in the intervening time, perhaps negatively biasing the measurements we report here. The hailstone weighed approximately 303 g. We used a 3D infrared laser scanner (e.g., Giammanco et al. 2017; Jiang et al. 2019) to obtain a high-resolution, 3D rendering of the hailstone (Fig. 9). The maximum dimension measured from the laser scan was 11.38 cm (4.48 in.), which was confirmed with digital calipers (not shown).
Photograph of Maria’s hailstone being scanned using the 3D laser scanner, which affords high-resolution maps of the hailstone’s detailed shape. Photograph courtesy of J. Marquis, used with permission.
Citation: Bulletin of the American Meteorological Society 101, 8; 10.1175/BAMS-D-19-0012.1
Victoria’s hailstone.
The most famous hailstone from the 8 February event was photographed by Victoria Druetta, a teenager living in Villa Carlos Paz. This hailstone, which we believe has the largest maximum dimension documented in the Southern Hemisphere, will be referred to as “Victoria’s hailstone,” and it fell at the location indicated by the middle point in Fig. 1. During the RELAMPAGO field campaign, we were able to interview Victoria and her family about their experience, and obtained several additional photos and videos they took during the event.
Various photographs and video stills of Victoria’s hailstone are shown in Fig. 10. Stills from a Snapchat video (Figs. 10a,b) show the hailstone shortly after it landed in the grass, taken at 1642 local time (1942 UTC). Interviews with the family revealed that they watched from their living room window as the hailstone fell. “It was really impressive, we were all in shock,” she recalls. She recounts that it landed near some parked cars and “smashed” when it hit the ground. Her brother told Victoria to go out and find the hailstone, telling her to wear a motorcycle helmet for safety.2 However, she was unable to find the piece they saw break off on impact. Nonetheless, her story suggests the hailstone could have been even more massive before it was retrieved.
(a),(b) Screenshots of a Snapchat video of Victoria’s hailstone shortly after it landed. (c) Photograph of the hailstone in hand just after retrieving it. (d) Official measurement, after noticeable sublimation and melting: 18 cm (7.1 in.) in maximum dimension, 422 g in mass. (e) Viral photo of her mother holding the stone, shortly after it was retrieved. Photogrammetry based on measurements of their hands suggests the maximum size after it was retrieved is close to 19 cm. All imagery provided courtesy of Victoria Druetta, used with permission.
Citation: Bulletin of the American Meteorological Society 101, 8; 10.1175/BAMS-D-19-0012.1
Figures 10c and 10e shows the hailstone shortly after it was retrieved from the yard. The hailstone was preserved in her freezer, and several hours later photographed again with a ruler (Fig. 10d) and weighed. At the time of the measurements (Fig. 10d), the hailstone was 18 cm in maximum dimension, to the authors’ knowledge making it the largest (in terms of maximum dimension) officially documented in the Southern Hemisphere. The gargantuan hailstone weighed 442 g. There are some noticeable differences between the freshly fallen hailstone and after it had sublimated in the freezer, though we estimate the difference in maximum dimension was ≤1 cm between photographs. Unlike Maria’s hailstone, Victoria’s hailstone shows several large (>2–3 cm) icicle lobes extending laterally from the particle’s center of mass. These large protuberances greatly increase the maximum dimension of the particle, whereas the mass is only somewhat larger than Maria’s hailstone. Further, Victoria’s hailstone is only just over half the mass of the Vivian, South Dakota, hailstone (which was ∼879 g), despite being within 1–2 cm of the Vivian stone’s maximum dimension. This demonstrates why cloud physicists typically are more interested in measurements of hailstone mass and volume than maximum dimension (e.g., Knight and Knight 2005).
There were at least two other extremely large hailstones that the family witnessed during this event (though did not measure or preserve). Her father recalled a stone that cracked a neighboring building’s window ledge (translated from Spanish): “But that other hailstone was also something very impactful, very strong.” Further, the father recounted another hailstone damaged a neighbor’s vehicle (translated from Spanish):
Here there was another vehicle parked, and a stone fell and hit between the trunk and the bumper. It made a tremendous dent. Tremendous. [The car’s owner] made a claim for his car insurance because he had insurance for hail damage, but the insurance didn’t want to recognize it because they requested proof. The insurance said it must have been an impact from another metal object or something like that because it couldn’t have been an impact by a hailstone. We sent him proof of the television station interview that they had done with Victoria, but even then, he couldn’t charge the insurance company. He had to pay for the car’s damage himself.
Thus, the stone in Fig. 10 was not the only extreme hailstone to fall at the family’s location during this storm.
Downtown Villa Carlos Paz.
Finally, a high-resolution video posted on YouTube3 features an extremely large hailstone falling in downtown Villa Carlos Paz (northernmost point in Fig. 1). During RELAMPAGO, we visited the location featured in the video and measured reference objects evident in the frame (street light poles, widths of the green awning supports, size of sidewalk tiles, depth of the curb, etc.). We also placed rulers in the street at the estimated impact location of the hailstone, and took photographs from the estimated location and vantage point of the videographer. This allowed us to conduct the frame-by-frame photogrammetric analysis to estimate the hailstone’s size. These estimates suggest the hailstone might have been a world record for maximum dimension, if officially measured. Unfortunately, the stone was not preserved or measured, precluding any official measurements.
The video shows sparse concentrations of giant hailstones falling in downtown Villa Carlos Paz, right at Paseo Central (cf. Fig. 1). There are several noteworthy impacts captured in this video, including one particularly large hailstone hitting a roofing structure (audible, but off camera), eliciting responses from onlookers and the camera operator. As the camera pans to view the impact, a few fragments are seen flying from the impact location. Then, the hailstone can be seen bouncing off an awning, falling into the street, and breaking off additional fragments as it impacts the ground. Fortunately, the initial impact with the awning slowed the hailstone and kept it more-or-less intact until it impacted the pavement, allowing it to be clearly viewed in a sequence of video frames (Fig. 11). Seconds later, another large hailstone lands on the road right next to the one at rest, explodes violently on impact, and sends shards flying several meters away.
Sequence of frames from a YouTube video taken in downtown Villa Carlos Paz, showing a gargantuan hailstone (annotated with cyan arrows in each panel). The frame numbers from the video are included in each panel.
Citation: Bulletin of the American Meteorological Society 101, 8; 10.1175/BAMS-D-19-0012.1
Using the aforementioned measurements of objects in this scene, we photogrammetrically estimated the size of the hailstone from the still video frames. Of course, there are numerous sources of uncertainty and error with this type of technique, including blurred imagery in the video owing to the moving camera and/or the hailstone motion, difficulty with the image processing to distinguish the hailstone from the background, etc. The frames were carefully screened for any possible blurring from panning or hailstone motion, and ensured that enough contrast was present to separate the hailstone from the background. Six frames were found to be useful for photogrammetry (Table 3). For each, we cropped out the rest of the image except for the hailstone and transformed the color images to grayscale using color luminosity (e.g., Soderholm et al. 2020). Doing so allows for a single lightness scale. In these images, the hailstone is bright, or greater “lightness” values. For each frame, a threshold lightness value was applied to identify pixels associated with the hailstone, but not the background. The centroid of the identified hailstone pixels was then identified, as was the maximum dimension. These are shown in Fig. 12. Table 3 lists the frame number, maximum dimension (in pixels), and threshold applied.
Frames used for the photogrammetric analysis, from Fig. 12. Maximum dimension of the hailstone in the image (in pixels) is given, as is the threshold used by the image processing routine, and the minimum and maximum size estimate (in cm). The asterisk indicates the frame is from when the camera was zoomed in on the hailstone.
Grayscale images of the gargantuan hailstone extracted from the YouTube video. The blue “×” marker indicates the hailstone centroid, and the yellow line shows the maximum dimension determined by the image processing. The final image (frame 391) has been manually cropped to eliminate other hail fragments on the road.
Citation: Bulletin of the American Meteorological Society 101, 8; 10.1175/BAMS-D-19-0012.1
The sizes of the reference objects in the video were determined by manually counting pixels across the objects. Given that edges of objects are not sharply resolved in the video, there is a range of sizes for each object. For one extreme, we counted all pixels that deviated from the background color as part of the reference object; for the other, we only counted pixels not substantially different in color from the center or main body of the reference object. In doing so, we obtain a minimum and maximum number of pixels for a given reference object’s length. We took this range into consideration as a measure of uncertainty when converting the hailstone size estimates from pixels to physical units. The distribution of estimates from all six frames is shown in Fig. 13 and included in Table 3. Based on each of these estimates (n = 12), we computed the confidence interval about the mean of the hailstone maximum dimension estimates using the bias-corrected and accelerated bootstrapping technique (e.g., Efron and Tibshirani 1993) with 2000 samples. At 95% significance, the estimated maximum hail dimension is between 18.8 and 23.7 cm (7.4–9.3 in.). This would make the hailstone close to or exceeding the world record for maximum dimension, were it officially measured.
Kernel density estimate (bandwidth = 2 cm) of the distribution of maximum dimension estimates (cm) from the photogrammetric analyses. Overlaid are the confidence intervals about the mean of estimates at 95% significance (bar along abscissa).
Citation: Bulletin of the American Meteorological Society 101, 8; 10.1175/BAMS-D-19-0012.1
Subjectively, we assess frames 293 and 294 to be the best estimates, based on little-to-no horizontal or blurring evident, and based on the reference objects used for the estimates (awning supports) being at the same distance from the camera as the hailstone. These two frames yield maximum dimension estimates of between 22.5 and 28.2 cm (8.9–11.1 in.). Recall that the hailstone impacted a structure and had pieces broken off prior to its appearance in the video. Further, even after impacting the road (frame 319), breaking apart further, and coming to a rest (frame 391), our conservative estimate of the maximum dimension in the plane of view of the camera is still ∼15 cm (i.e., satisfies the criteria for gargantuan size). Despite the uncertainty associated with its true maximum dimension, the hailstone is impressively large.
Discussion and conclusions
A new recommended category for extreme hail of sizes >15 cm in maximum dimension is proposed: “gargantuan hail.” The proposed new category underscores the extreme size and damage potential of such hail, and hopefully encourages its reporting and improved documentation in other cases to better understand the storms and processes capable of producing such a hazard. This study documents such a storm, and the gargantuan hail it produced in Villa Carlos Paz, in the Córdoba province of Argentina. This case occurred over a heavily populated urban area, affording numerous eyewitnesses and social media postings. Multiple other giant hail reports were documented within city limits (not shown); thus, it was not an event with only one “freak” instance of a singular gargantuan hailstone. This indicates that gargantuan hail production can result from multiple pathways/trajectories within a storm. The video and eyewitness testimony suggests the giant and gargantuan hail was sparse in concentration and occurred at a time with little to no rain, suggesting fallout outside of the heavy precipitation core. Radar imagery supports that the gargantuan hail fell in close proximity to the updraft (as inferred from the BWER and ZDR column) as it passed over Villa Carlos Paz, similar to the case analyzed in Witt et al. (2018). This is consistent with earlier arguments of size sorting offered by Browning and Foote (1976) and Browning (1977), among others.
The two photographed hailstones exhibited thick outer layers indicative of wet growth, consistent with other documented very large hail (Knight and Knight 2005). Wet growth can lead to icicle lobes on the hailstone surface, which can greatly increase its maximum dimension relative to its mass. Indeed, though Victoria’s stone was about 58% greater in maximum dimension than Maria’s stone, it was only about 28% greater in mass. Similarly, though only about 2 cm smaller in maximum dimension than the Vivian, South Dakota, record hailstone, its mass was only about half (50.3%) of the Vivian hailstone’s mass. This underscores the need to better characterize hailstones by mass, in addition to maximum dimension.
Unsurprisingly, the storm that produced the gargantuan hail in Villa Carlos Paz was a supercell, consistent with studies by Blair et al. (2011) and Blair et al. (2017) and earlier studies suggesting supercells are capable of large hail (e.g., Nelson 1983; Foote 1984; Rasmussen and Heymsfield 1987; Miller et al. 1990; Tessendorf et al. 2005). Aside from being broadly conducive for supercells and thus hail production, nothing in the environment indicated conditions favorable for such extreme hail production. Further, the storm’s radar presentation as observed with the operational C-band RMA1 radar was not atypical of supercells. The 1-km-grid-spacing WRF simulation of this case, despite closely following the observed storm evolution, also did not indicate anything out of the ordinary for supercellular convection, though it did highlight the rapid environmental evolution leading up to the storm.
The lack of indications of an extreme event in the prestorm environment, numerical model forecasts, or radar imagery collectively demonstrate the challenges associated with forecasting or even detecting extreme hail events. Further research is needed to better understand the environmental conditions leading to storms capable of producing gargantuan hail, radar signatures of gargantuan hail, and the climatology of such events. We encourage local forecast offices, broadcast meteorologists, and emergency managers to interface with and educate the public during severe convective storm episodes to better document the occurrence of gargantuan hail, including accurate time and location of hail fall, and accurate measurements of hailstone size, especially mass. Such documentation will facilitate an improved understanding of the storms capable of producing such hazardous hail.
Acknowledgments
We thank the National Science Foundation Grants AGS-1661679 and AGS-1855063, and an award from the Insurance Institute for Business and Home Safety (IBHS). We thank IBHS for lending us their 3D laser scanner, and particularly Ross Maiden for training the lead author on its use. ERA5 data are from the European Centre for Medium-Range Weather Forecasts (ECMWF), acquired through the Copernicus Climate Change Service Climate Data Store (CDS). RMA1 data were provided by Secretaría de Infraestructura y Política Hídrica, Ministerio del Interior, Obras Públicas y Vivienda of the Argentinean National Government within the SINARAME Project. The National System of Weather Radars (Sistema Nacional de Radares Meteorológicos, SINARAME) project is an Argentinean effort to expand the radar network over the nation. We also thank the citizens of Villa Carlos Paz for their time and accommodation during RELAMPAGO. In particular, we are very grateful to Victoria Druetta and her family, and to Maria Navidad Garay. Finally, we thank the three anonymous reviewers for their criticisms and constructive comments on the manuscript, which helped produce a substantially improved manuscript.
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