The Billion Dollar Calgary Hailstorm of 13 June 2020, Part III: Hail Size and Weather Modification Aspects

– By Paul Joe – 

Abstract

On 13 June 2020, a hailstorm caused $1.2B of damage in a 10-15 km² area in northeast Calgary within a 20 minute period. A radar-based overview of the storm’s initiation, explosive development and merging with a second storm was provided in Part I. Part II described details of the merger and the interaction with a meso-front.  Here, in  Part III, radar-based hail size swath analysis, a description of cloud seeding operations and a synthesis of the observations and unresolved questions are provided.

1 Introduction

On 13 June 2020, a hailstorm caused over $1.2B of insured damages in the City of Calgary. An overview was presented in Part I (Joe et al., 2024a). The storm was extreme and resulted from the merger of an air mass thunderstorm with a second storm which formed on a meso-front associated with a mountain-plains circulation. In Part II, detailed features of the storm evolution and meso-front interactions were presented to better understand the physical factors impacting the storm development (Joe et al., 2024b). The merging and meso-front interactions were complex and several explanations were advanced but further research is needed. 

The objectives of this last part are to document the surface damage patterns based on radar estimated hail sizes, the weather modification seeding operations, and summarise outstanding research questions.

2 Hail Size and Swath

In general terms, the greater the water content, the greater the intensity of the precipitation and the larger the hail; the taller the storm; the stronger the updraft that is able to support the growth of large hail, and the higher the freezing level, the greater the time available to melt resulting in smaller hail. These factors are used to estimate hail size using radar.

The vertically integrated liquid water content (VIL) of the storm was used as a proxy for precipitation intensity. Assuming that reflectivity is totally due to rain (liquid), the Marshall-Palmer drop size distribution relationship (Marshall and Palmer, 1948) is used to estimate liquid water content (LWC).  VIL is computed by integrating the LWC in a column from the reflectivity volume scan. The height of the 50 dBZ echo top is used as a measure of the height of the storm. The freezing level height was based on the 00 UTC 14 June 2020 sounding from Stony Plain, AB and is assumed to be constant for the analysis period. 

These variables were used in an empirical relationship to estimate hail size (Witt et al., 1998; Joe et al., 2004; Ebert et al., 2004). While hydrometeor type including hail (regardless of size) is routinely estimated (Park et al., 2009; Lim et al., 2005; Thompson et al., 2014), techniques to estimate hail size using the polarimetry data are still in the research phase (Ortega et al., 2016).

Figure 1 is a composite of individual radar hail size images that are made every six minutes from 2200 UTC 13 June 2020 to 0600 UTC 14 June 2020. The maximum value is used for overlapping data. During the early stages (0118 to 0136 UTC, 14 June 2020) of the event, Storm B (already merged with A2) moved slowly at ∼ 30 km/h. Later the storm speed almost doubled to ∼ 60 km/h (see Part I). As fast moving storms can result in the appearance of gaps in the hail swath, a semi-Lagrangian linear interpolation using the centroid velocity (see Part I) was used to aggregate individual images. The hail size estimates should be interpreted on a storm basis and not at the pixel or radar data resolution due to the manner in which the  algorithm was developed (Witt et al., 1998; Joe et al., 2004) and so the swath area may be overestimated as the hail may follow trajectories determined by the storm airflow. The observations used to develop the empirical relationship were limited and so radar hail sizes greater than 10 cm are extrapolated, considered uncertain and hence they are limited in Figure 1.

Figure 1 shows that greater than 6 cm (very large hail, NWS 2024) fell in northeast Calgary coinciding with the damage survey. The maximum hail sizes subsequently decreased but, later on, very large hail was observed again. The swath of the very large hail (dashed black line in Figure 1) was episodic, indicating that the thunderstorm was multicellular and not a steady-state single-updraft supercell. The spatial extent of the swath was approximately 15 km by 150 km.

At ~0230 UTC 14 June, the storm complex split in an unusual fashion. The original storm moved to the left, slowly weakened and eventually died whereas a new intense storm developed on the right flank and moved in the same direction as the original storm. Typically, when an intense storm splits, two mature storms emerge with the storm on the right flank intensifying (as in this case) and deviating to the right while the storm on the left flank rapidly weakens (e.g., Joe and Dudley, 2000).  Given the pattern of the left swath (red dashed line in Figure 1), it is enticing to make a link with earlier storms that formed on the meso-front. That is, the meso-front continued to sustain a storm with it.

3 Weather Modification

Operational weather modification activities have been conducted by the Alberta Severe Weather Management Society, a consortium of insurance companies, since 1997 to mitigate and protect urban infrastructure from large hail (Gilbert et al., 2016). With current construction standards in Alberta, small hail does not generally damage urban infrastructure except when accompanied by strong winds. The weather modification procedure is carried out by cloud seeding using silver iodide in the feeder or daughter clouds (the precipitation initiation zone of thunderstorms). Gilbert et al. (2016) pointed out that seeding is hypothesized to increase the number of hail embryos that will compete and use up the cloud liquid water, creating many smaller hailstones and thereby reducing the development and number of large hailstones. This is known as beneficial competition. The seeding may also stimulate the production of precipitation so water is removed early on and is not available for the production of hail and there are other hypotheses using the same seeding procedure but are not considered applicable in this case (Flossmann et al., 2019; personal communication, Terry Krauss).

Whenever a storm is detected on radar and anticipated to strike Calgary or any city in hail alley, seeding aircraft are deployed to seed the clouds (see Gilbert et al., 2016 for a description of the seeding procedures). The seeding materials are injected either at the base of the feeder clouds using wing-mounted flares or generators, or into the top of the clouds using flares that drop into the precipitation initiation zone. Seeding ceases when the storm is no longer a threat to municipalities or major industrial complexes or when the storm exits hail alley.

Figure 2 shows a sequence of images (12 minutes apart) to illustrate how the seeding was conducted in this event. The underlying image is a MAXR radar image with extreme reflectivities (>70 dBZ) are highlighted. The new growth areas and updrafts are outside these precipitation cores and the aircraft seeded these in these areas. In the example, below-cloud seeding was carried out on the forward (northern) flank of the storm and cloud top seeding was carried out in the rear (southern flank) of the storm. Note the Strathmore radar data is time stamped by the end of the scan cycle and has a temporal resolution of 6 minutes while the aircraft data have a time resolution of 2 seconds. 

Figure 3 shows an underlying composite MAXR image (2200 UTC 13 June 2020 to 0600 UTC 14 June 2020) with overlaid aircraft tracks, seeding locations and seeding method for the entire data period. Table 1 provides a chronological synthesis of the morphology of Storm B and the seeding. 

In anticipation of severe thunderstorms, one aircraft was deployed from Cochrane at 2348 UTC 13 June 2020 and was directed to Storm A1 and arrived at 0012 UTC 14 June 2020, however no seeding was done. A second aircraft was deployed at 0009 UTC  14 June 2020 and briefly seeded a storm near Okotoks  at ~0029 UTC 14 June 2020. It did not develop and quickly died.  The two aircraft were redirected to Storm B (see Part I and II) and began seeding at 0039 UTC.  One aircraft seeded in the feeder clouds (<2 km AGL) levels) using wingtip generators in front of the storm at low levels and the other dropped flares in the rear of the storm from cloud top (~6 km). Another aircraft began seeding this storm at 0203 UTC 14 June and dropped flares from cloud top (~5 km) ahead of the storm. All cloud top seeding ceased at 0221 UTC.  Low level seeding of Storm B ceased at 0306 UTC when the storm left hail alley.  A fourth aircraft seeded a storm to the northwest of Storm B, near Cochrane, but is not discussed here.

Given its explosive behaviour, the seeding operation was as timely as could be expected as Storm B was only identifiable on the Strathmore radar at 0030 UTC 14 June 2020, 9 min earlier. The cloud seeding operator (Weather Modification Incorporated) have their own radar operating on a 4 minute cycle and may have identified the storm several minutes earlier (not analysed here). The storm was producing large hail over southern Calgary starting at about 0036-0042 UTC based on radar and the damage survey. It was over northeast Calgary from 0118 UTC to 0142 UTC and at its peak intensity (reflectivity) at 0124 to 0130 UTC. In total, seeding operations on Storm B lasted for two and half hours. Note that no seeding was carried out on any storms directly associated with the meso-front.

Table 1: Event synthesis, comments and open questions.

4 Synthesis and Discussion

The storm was complex and formed from multiple convective cells, developed rapidly and produced large hail within twelve minutes (Table 1 and Figure 4.). The storm merged with a second storm and produced very large and giant hail within an hour. The storm was long-lived and intense for at least 4.5 hours from 0030 UTC 14 June 2020 to 0600 UTC 14 June 2020 (end of analysis) with a multicellular pattern of very large/giant hail on the ground. There are insufficient reports (official observations, from social media or elsewhere) to verify the radar swath analysis. Very dense surface observation networks of sensors capable of observing large hydrometeors and large sampling volumes are needed for quantitative analysis and hail reports through social media are needed and encouraged to provide geographical coverage (Elmore et al., 2014; Ortega et al., 2016).

The seeding information was presented without judgement or attribution to its impact. Within the scientific community, the expert opinion is that the effect and impact of seeding of thunderstorms is inconclusive and results are difficult to prove (WMO, 2016).

In a recent statistical study over Alberta, VIL was used as a surrogate for hailstorm intensity or hail size (Pirani et al., 2023). Changes in VIL between seeded versus unseeded storms were compared for long track storms (>1 hour) using ten years of radar data. A weak but statistically significant relationship was found in which strong storms decreased in intensity and weak storms increased in intensity with seeding. The effect was strongest after a 30 minute lag after seeding. The authors note that caution in attribution to seeding should be exercised as there are many confounding factors. 

Table 1 synthesises the storm and seeding history and identifies some open questions. It was a complex storm that evolved from linear to cellular convection, then from multicellular to single-cellular storm and back again. One of the ongoing scientific challenges is understanding the nature of long-lived Alberta storms and whether they are continuous with a single continuous updraft (supercells) or multicellular/pulse in nature (Strong et al., 2007). In this case the storm cyclically re-organized over several hours.  

Early identification of severe storm initiation to improve weather warnings is a nowcasting challenge (Wilson et al., 1998). Many factors were identified, including the synoptic environment, meso-scale features (diurnal and mountain-plain circulation effects), complex terrain and urban texture (inferred but not directly discussed), and the interaction of merging thunderstorms, amongst others (Strong et al., 2007; Taylor et al., 2011). 

There are also many unresolved scientific issues related to cloud microphysics and precipitation processes related to hail formation (Morrison et al., 2020). These were not addressed here and a deeper investigation using microphysical numerical models and polarization diversity radar is warranted (Lu et al., 2023).

There are also unresolved weather modification questions (Gilbert et al., 2016; Flossmann et al., 2019; Pirani et al., 2023). Given that merged storms lead to enhancement (size and intensity), should a different seeding strategy be adopted. An open question is whether early seeding the weaker storms on the meso-front, the convective rolls or the cellular convection to disrupt their intensification would be an effective strategy.  

5 Summary

The Billion Dollar Hailstorm of 13 June 2020 was presented in three parts. The overall objective of the study was to share with the readers some of the many aspects of this extreme and unique hailstorm that affected the lives of many Calgarians and caused more than $1.2B in damages. It was an awe-inspiring storm and this analysis posed more questions than answers – a researcher’s delight.

6 Acknowledgement

The radar data for this study were provided by Environment and Climate Change Canada. The seeding data were provided by Adam Brainard of Weather Modification, Incorporated on behalf of the Alberta Severe Weather Management Society. Adam’s support is greatly appreciated. The initial investigation was contracted by the Institute of Catastrophic Loss Reduction under the leadership of Paul Kovacs and Gordon McBean. The author is appreciative of the thorough reviews by Ron Stewart and Sudesh Boodoo that significantly improved the manuscript.

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