On March 22, 2019, ICLR hosted a Friday Forum webinar title 'Hail: Challenges Solutions and Future Prospects', led by Dr. Julian Brimelow of Environment and Climate Change Canada (ECCC). The growth of hail, and especially large hail, requires the alignment of several key ingredients and processes spanning a wide range of scales. Consequently, modelling hail growth and forecasting hail size is challenging. Given that hailstorms inflict billions of dollars in damages, it is important to improve the lead time of warnings. However, predicting the occurrence and size of hail remains problematic. Central to this problem is the lack of skillful short-term forecast guidance. This is in turn partly attributable to the scarcity of reliable surface hail reports for verification. The first half of the talk focuses on the challenges associated with observing hail, and identifying methods for obtaining accurate estimates of hail size by pairing information from social media and weather radar. It also highlights the key challenges and limitations facing users of social media data and provides some potential solutions. The number of days when the environmental conditions favour severe thunderstorms over N. America has been predicted to increase under anthropogenic climate change (ACC). However, how hail might be affected by ACC is unclear. The second half of the talk will speak to the first study to investigate the spatiotemporal response of hail frequency and size to over N. America. Dr. Julian Brimelow is a scientist at Environment and Climate Change Canada and an expert in hazardous convective weather. Julian graduated from the University of Pretoria with a BSc in meteorology in 1993, completed his MSc at the University of Alberta in 1999, and in 2011 finished his PhD at the University of Manitoba. Julian is currently working to improve the detection and prediction of hail using radar products and data from Canada’s weather models. Julian has a broad publication record on deep convection, thunderstorms, hail, flooding and drought. Julian has worked as a meteorologist for the South African Weather Service and the British Antarctic Survey and as a research assistant at the University of Alberta.

1. Talk for ICLR, 22 March 2019
Julian Brimelow
Meteorological Service of Canada
Hail: Challenges, Solutions and Future Prospects
Fir0002/Flagstaffotos

2. Page 2 – March 22, 2019
http://orf.media/
Observing and forecasting hail
is as difficult, if not more
difficult, than is forecasting
tornadoes
Orf.media
http://ccc2.atmos.colostate.edu

3. Page 3 – March 22, 2019
How Big Can Hail Get?

4. Page 4 – March 22, 2019
Background
• Hail science is difficult and challenges abound
Kumjian et al.
(2015, MWR)
• Final size of hail on the ground determined
by myriad of processes occurring across wide
range of spatiotemporal scales
• Imperfect understanding of:
i. Microphysics
ii. Storm-scale processes
iii. Connections between storm environ-
ment and storm processes
• Hail size not unique to a given environment
• Reliable observations sorely lacking
• Lack of coordination and collaboration (most research done in parallel)

5. Page 5 – March 22, 2019
The Elusive Hail Climatology
Allen and Lepore (in review)Mroz et al. (2017)
Prein & Holland
(2018)

6. Page 6 – March 22, 2019
But it is not all bad
• How do we best leverage these developments?
Cray
• Incredible advances in computing
power and availability of remote
sensing data
• Can run CAMs and 3D models using
multi-moment microphysics schemes
Dennis and Kumjian (2017, JAS)
• Dual-polarization radar more common; data
processing techniques improving
Ryzhkov et al. (2017, JAMC)
• Emergence of laser scanners and 3D
printers; miniaturization of sensors
UC Irvine
IBHS
IBHS
• Social media is now an important data source

7. Page 7 – March 22, 2019
Forecasts
Nowcasts
Alerts
Risk mitigation &
Assessment
Observations
Size,amount&
weight
Microphysics
Modelling (NWP
& CAM)
Identification
&discrimination
Remote sensing
Experiments
& in-situdata
Wind tunnel
Aircraft
DATA AND
KNOWLEDGE
GAPS
Coordination
&
Collaboration

8. Page 8 – March 22, 2019
Hail datasets have significant limitations, in part
because of high spatiotemporal variability
of hail swaths. Other issues:
• Sparse observation networks
• Population and measurement biases
• Timing and location errors
• Confusion in reporting methods
• Focus on reporting only large hail sizes
• Do reports reflect true storm severity?
• Lack of long-term datasets for climate studies
Gap 1: Observations

9. Page 9 – March 22, 2019
Diverse, yet complementary, tools available:
• Social media and reporting apps
• Public education and outreach
• Weigh hailstones instead of focussing on size
• More intensive field studies to collect hail
HailSTONE, SHAVE, IBHS, CHAT, CoCoRaHS
• Maintain and expand hailpad networks
• Install networks of impact disdrometers
• Use remote sensing data to identify hailswaths
and estimate maximum hail size
Gap 1: Possible Solutions
Social
media
𝑫 = $
𝟔𝑴
𝝅𝝆𝒊
𝟑
IBHS
HailSens

10. Page 10 – March 22, 2019
Leveraging Social Media
• Could images of hail posted on social media provide a source of objective
and accurate hail size measurements?
• Scanned Twitter feeds for #abstorm, #skstorm and #mbstorm for
summers of 2014 through 2016 for Tweets including images of hail
and/or references to hail. Sorted hail images into four categories
Damage Scene No Scale Scalable

11. Page 11 – March 22, 2019
Estimating the Size of Hail
• Calculated equivalent spherical diameter (De) of largest stones
• Diameter important because kinetic energy scales with D4
• Use scaling & assuming stones are spheroids (typically tri-axial ellipsoids)

12. Page 12 – March 22, 2019
Report Statistics for 2016
• Very few images of hail Tweeted by chasers
• Over 70% of images Tweeted from home
• Almost 80% of images of little or no use for our purposes

13. Page 13 – March 22, 2019
So Many Problems…
1) No location provided or vague reference to location.
2) Tweet posted tens of minutes to hours after storm.
3) Uncertainty in event time (disagreement between time stamp on Tweet
and time provided in Tweet).
4) Using objects that don’t have a fixed size or have an indeterminable size.
5) Photos of hail being held in hand is a huge issue.
6) Photos taken after hail experienced significant melting.
7) Photos taken at oblique angles.
8) Showing “wrong” side of coin (i.e., heads).
9) Photos of scalable object and hailstones in different planes.
10) Combinations of the above.

14. Page 14 – March 22, 2019
• Measuring hail is fiddly and difficult:
• Most reports made from home where
people have access to a kitchen scale
• Can also take along when chasing
• Quick & easy to weigh stones (M) accurately calc. D
An Alternative Approach
𝑫 = $
𝟔𝑴
𝝅𝝆𝒊
𝟑

15. Page 15 – March 22, 2019
Incomplete knowledge of key microphysical processes
and outdated parameterization schemes
• Fall mode and terminal velocity of hailstones
• Partitioning of cloud condensate in updrafts
• Collection efficiency of ice during dry growth
• Role of updraft volume in size and amount of
hail; environmental factors controlling volume
• Residence time in the hail-growth zone
• Trajectories of hailstones in storms
Gap 2: Microphysics and Modelling
List (2014, JAMC)NWSChalon et al. (1976, MWR)

16. Page 16 – March 22, 2019
Leverage advances in computing, models, sensor
miniaturization and computation fluid dynamics
•Collection of cloud base to top microphysics data
• Vertical wind tunnel with mixed-phase particles
• Carefully designed numerical experiments using
CM-1 and other 3D cloud models
• Impact of environment & storm-relative flow on
updraft properties and residence time in HGZ
• Improve access to computational fluid dynamics
• Machine learning to improve microphysics schemes
• High-speed photography (> 1000 fps) to analyze
motion of 3D printed hailstones
Gap 2: Possible Solutions
www.dlr.de/IPA
University of Mainz
Dennis and Kumjian (2017, JAS)Dennis and Kumjian (2017, JAS)
Cheng et al. (2014, JAS)

17. Page 17 – March 22, 2019
Identifying hail and hail size with or without radar
•Current operational techniques limited to areas
with radar coverage
• Limited ability to discriminate between hail size
•Few validated operational tools to quantify
mass of hail on ground (kinetic energy)
•Satellite techniques can only identify hail aloft
•Maximum Expected Hail Size (MESH)— needs
refining, could incorporate dual-pol radar data?
Gap 3: Identification of Hail
Ortega et al. (2016, JAMC)
Witt et al. (1998, WAF)

18. Page 18 – March 22, 2019
• Cube sats can identify hail swaths in near-
real time (e.g., Planet Labs)
• GPM data (dual-wavelength space radar)
• New generation geostationary satellites
(e.g., GOES-16, Himawari-9, METEOSAT)
• Identify overshooting tops, indicator of
updraft width?
• Lightning data (lightning mapping arrays)
• Combine above with concomitant storm
environment data from global NWP models
• Ongoing development of conventional radar
and DP radar metrics
Gap 3: Possible Solutions
Mroz et al. (2017, JAMC)
Bedka et al. (2010, JAMC)
Emersic et al.
(2011, MWR)Bedka et al. (2018, JAMC)

19. Page 19 – March 22, 2019
MESH Introduction
Fortunately, the answer is a cautious “YES”:
Maximum Expected Size of Hail (a.k.a. MESH)
• MESH is a radar-derived hail size product based on a combination of
theory and empirical analysis that is used for nowcasting of hail
• IF found to be reliable, MESH could be used as a proxy for hail reports
• Allow high spatiotemporal resolution and accurate information on
location and size of hail in near-real time
• Are there alternative data sources that can help us solve the myriad of
issues facing traditional severe storm reports?

20. Page 20 – March 22, 2019
#1 #2 #3 #4
Dmax 4.8 4.4 4.2 4.1
Dmin 4.2 3.9 3.8 3.7
α 0.88 0.90 0.91 0.89
De 4.6 4.2 4.1 4.0
MESH = 3.5
20-160 km
MESH = 3.8
MESH = 3.0
#2
#3
#4
#1
MESH Verification
This MESH value
(maximum within
10 km radius) used
for verification

21. Page 21 – March 22, 2019
Results 2014-2016
1:1
r = 0.70; p << 0.01

22. Page 22 – March 22, 2019
MESH Data vs. Reports
Unfiltered cell counts 2014-2016
Filtered cell counts 2014-2016
Severe Hail Reports 2014-2016
MESH ≥ 3 cm 2014-2016

23. Page 23 – March 22, 2019
Stagnation of lab experiments and in-situ cloud data
• Incredibly innovative and ingenious icing wind
tunnel experiments in 50s through 80s
• Very little ice tunnel research for hail obtained
since the early 1990s
• Most ice tunnel experiments conducted using static
hail growth and supercooled water only
Gap 4: Experiments
Lesins and List
(1986, JAS)

24. Page 24 – March 22, 2019
• Replacement for T-28 (Thunderbolt?)
• Study hail growth in vertical icing
tunnel with mixture of supercooled
droplets and ice crystals
• Embrace unconventional thinking
• Alternatives to icing tunnels exist
• Fall behaviour of natural and
instrumented 3D printed stones
• Develop instrumented, artificial
hailstones (“smartdust”)
• Release HailPods into storms
Gap 4: Possible Solutions
T-28Thunderbolt II
Iflyworld.com
Xu and Li (2015, Sensors)
Orf.media

25. Page 25 – March 22, 2019
Dual-Polarization Radar Network
Radisson, SK
• Less attenuation
• Superior for observing large hydrometeors
• Allows for precipitation classification
• Network uniformity hardware & software
• 6-min scans (10-min previously), better coverage

26. Page 26 – March 22, 2019
Rollout
Doppler Overlap
Completed
Location to be Confirmed
• 13 weeks to install
• 5 installed to date
• 20 (33) new radars
• 7 installs per year
• Radar relocations
• Completion ~2022
• 2019 installs:
Exeter; Bethune
Marion Bridge
Chipman; Mtl River Hbr
Landrienne
Strathmore

27. Page 27 – March 22, 2019
Specifications
Provider: Leonardo/Selex
Germany
Power: 1MW
Source: Klystron
Beam Width: 0.88º
Sensitivity: -115 dBm
Dish: 9.1 m
Doppler Range: 250 km

28. Page 28 – March 22, 2019
Summary
• Hail presents a significant and costly threat to society
• All aspects of hail science, monitoring and prediction are difficult
• Hail is a complex and multi-faceted natural hazard
BUT
• We have entered a new era of hail research
• New technologies, opportunities and instrumentation are available
• Our scientific understanding has advanced
NEXT
• Need an infusion of long-tem funding
• Need collaboration, including a multi-season field program
• Still much science to be done!

29. Page 29 – March 22, 2019
Thank You – Merci Beaucoup

30. The changing hail threat over
North America in response to
anthropogenic climate change
Julian Brimelow, William Burrows &
John Hanesiak
Meteorological Service Canada

31. Hail: A costly and vexing problem
• Anthropogenic climate change (ACC) predicted to
increase the number of severe storm environments
• Low confidence and uncertainty surrounding how
future hailstorms will respond to ACC
• Uncertainty stems from the limited number of
studies, empirical nature and conflicting results
• How hail frequency and size might respond to ACC
is important, need to address limitations
31

32. Previous Work
32
• Research on the response of severe storms to ACC has focussed on
CAPE-BWD6 phase space
• Greater the CAPE and BWD6 => greater potential for severe storms
• Empirical relationships derived for the current climate between
atmospheric parameters and damaging hail days => applied to future
Diffenbaugh et al. (2013, PNAS)

33. 33
• Explicitly model response of hail to ACC using
HAILCAST and three AOGCM-RCM pairings
HadCM3-MM5; HadCM3-HRM3 & CCSM3-MM5
• 1971-2000 and 2041-2070
• ~50 km grid; ~5000 points (~ 1 billion profiles)
• 1 March to 30 September
• Identified largest hailstone for each day from HAILCAST
• Aggregated exceedances (e.g., D > 2 cm) by month,
season and year for each of 15 ecoregions

34. “Current” Hail Climate
34
MAM
Days D ≥ 2 cm
MAM
AKE(J) Days with HALO
MAMJJAJJAJJAM-S M-SM-S
Accumulated Kinetic energy (AKE)
Function of #hail days and D on hail days

35. Key drivers and processes
35
Instability
Moisture
Suff. CAPE
Suff. BWD
Low CIN
Trigger
HAIL
• Factors do not operate independently
• Complex interplay of drivers and processes
• Instability and moisture expected to increase
• Increases CAPE, but can increase CIN
• HigherT also increases melting potential
• Changes in BWD unclear
• Changes in triggers difficult to simulate
• Use CAM or look at synoptic & meso forcing
Suff. Updraft
LWC
Embryos
Growth time
Vol. updraft
SRW
HAIL
Meso and
Synoptic Scale
Macro and
Microscale

36. 36
Future Changes
Days D ≥ 1 cm Days D ≥ 2 cm Ds (cm) AKE (J)

37. Summary
37
• First study to explicitly model response of hail to warming
• Complex interplay of drivers and processes
• Response of hail to ACC over N. America varies by region and by season
• Fewer hail days expected over most areas in the future
• Marked decrease in hail damage potential over east and southeast
• BUT, increase in hail diameter on days with hail
• Result is greater hail damage potential over most areas in MAM;
higher latitudes and mountains in JJA
• More work to be done (e.g., CORDEX data), expand globally
• Use CAMs and WRF-HAILCAST