Hailstorms
Everything you wanted to know
about hail (and more)
ATMO 352
Spring 2013
Background: Photorealistic rendering of a WRF simulated hailstorm
Hail Climatology
First-order surface observation stations with five-year average number
of hail days during 1896-1995, from Changnon and Changnon (2000)
Severe Hail Climatology
• Hailstorms responsible
for nearly $2.4 billion of
damage to crops and
property annually in the
USA (Changnon 1999)
– Little documented on longterm or seasonal forecasting
techniques
• Large hailstones (D > 5
cm) often associated with
supercell thunderstorms
– Often accompanied by intense
wind gusts, violent tornadoes,
and extreme precipitation
Softball-sized hail
r=5 cm, 50,000 microns
Vivian, SD Hailstone
Holds record for size (8” diameter)
and mass (1.94 lbs)
In Short, Large Hail Needs…
(1) Supercooled water (mass to be
collected by the hailstone)
(2) An Embryo (some initial particle to
collect supercooled water, usually a
frozen raindrop or snow aggregate)
(3) An updraft (must be sustained aloft
for sufficient time to allow growth)
(4) Cold Temperatures
Freezing – Just the facts
• At T < 0°C, water molecules join together to form a
crystalline structure
• If the drop is free of impurities, the thermal
agitation will prevent freezing
• Freezing of liquid water in the atmosphere largely
depends upon the presence of foreign particles
called ice nuclei
– The nuclei do not have to be inside the drop; water can
freeze on contact (freezing rain, aircraft icing)
– Typical ice nuclei: desert dust, clay minerals, decaying
plant leaf material, ice crystals themselves, etc.
– Most IN “activate” at temperatures cooler than -10 °C
Supercooled Water
• If ice nuclei are not present, liquid water can be
super-cooled to temperatures as low as -40 °C
• We normally deal with bulk amounts of water. The
presence of one ice nucleus will freeze the entire
amount of liquid water.
• In a cloud, the liquid water is divided upon millions
of droplets; each must contain or interact with an
ice nucleus to freeze
– There are substantially fewer ice nuclei in the atmosphere
than cloud condensation nuclei (CCN)
– Ice multiplication and/or shattering can help generate new
ice crystals
“Warm Rain”
• You’ll learn more about the warm rain process in
physical meteorology
• Important in tropics, mid-latitude summer, and
possibly mid-latitude winter
• In short, “warm rain” is a 4 step process:
(1) Nucleation of cloud drops on aerosol particles
(2) Additional Condensation due to
supersaturation
(3) Drop Growth by Collision and Coalescence
(4) Drop Breakup
Drop D > 45 m
coalescence
SS>0
condensation
RH~100%
SS=0
Drops nucleated
III
II
IV
Drop breakup
I
Air cools,
RH increases
“Cold Rain” Process
Vapor diffuses towards the
crystals, growing by
deposition and depleting
water vapor in the air
Further growth by
accretion or aggregation
can also occur
Accretion or riming: growth
by collision with supercooled
drops which freeze on
contact
Aggregation: growth by
collision of ice crystals
Cloud Particle Imager Data
from AIRS II Flights (2004)
“Hole Punch” Clouds
Areas of mid- or high-level, liquid
clouds. Ice grows at the expense of
evaporating liquid drops (Bergeron
process) and creates a cloud free region
Conical Graupel
(Knight and Knight, 1973)
Preferential
collection along
crystal edge
Embryos Summary
• Growth in cloud may form frozen drops
or conical graupel as initial embryo
• Takes 20–30 minutes for either process
• Where do they form?
– Feeder cells upwind (10 – 20 km)
– Upwind flanks of main updraft
– Secondary growth from shedding/melting
– Can NOT be in main updraft: not enough
time to grow
Graupel/Hail Growth
• Primary graupel growth by freezing of raindrops, riming of
ice crystals, aggregation of snow, etc.
• Large hailstones acquire
most mass by accretion
of supercooled drops
(e.g., Knight and Knight 2005)
• Secondary generation
possible during “wet
growth” (shedding)
• 40 – 60 minutes of
growth required to form
large hail
Knight and Knight (2001)
Further Growth
• Latent heat is released when a supercooled water
droplet freezes on a hailstone surface
• Dry Growth: rate of supercooled drop collection is
low, hailstone surface remains below 0 °C, drops
freeze immediately upon impact
– Opaque ice (air bubbles), brittle, small crystals
• Wet Growth: Hailstone surface warms to 0 °C,
freezing does NOT immediately occur, water drops
“spread out” across hailstone and some shed
– Clear ice, larger crystals, drops may fill pores of hailstone
and lead to densification
– Wet surface makes stone “sticky” for collection with ice
Hailstone Thin Sections
(Knight and Knight 2005)
Wet
growth
Dry
growth
Growth Trajectories
“Recycling”
trajectories not
as common as
once thought
Most trajectories
up-and-down
once around
main updraft
Embryos may be
ingested from
other sources
Hail in Multicells
May produce its
own embryos,
but hail may not
grow to very
large sizes
Graupel/Hail Size Distributions
• Early observational work
confirmed an exponential
size distribution for
graupel/hail particles at
the ground
– (Waldvogel 1974; Federer and
Waldvogel 1975; Knight et al.
1982; Cheng and English
1983; Chen et al. 1985; among
others)
• Other observations reveal
a better match to a
gamma distribution
– (e.g., Matson and Huggins
1980; Ziegler et al. 1983)
Matson and Huggins (1980)
Hailstone Terminal Velocity
• Hailstones are assumed to fall
at their terminal velocity, Vtg
p = 500 hPa
– Balance between drag and
gravitational forces
• Larger/more dense particles
have greater fall speeds
• Faster graupel/hail fall speeds
for lower air density
Sea level
 4 g  g Dg 
Vtg  

3

C

D 
Knight and Knight (2001)
0.5
• May have significant impacts
upon precipitation estimates
– Large, faster-falling particles are
less prone to horizontal
advection; more intense
precipitation over a local region
(e.g., McCumber et al. 1991; Gilmore et
al. 2004a; Gilmore et al. 2004b; van den
Heever and Cotton 2004; etc.)
Faster falling
graupel/hail
Slower falling
graupel/hail
Precipitation Mass
Updraft ( +2.5 m s-1)
Downdraft ( -2.5 m s-1)
Hailswath Mechanics
N
Light Rain
Large
Hail
Heavy Rain &
Small Hail
Gust
Front
Hook
echo
Anvil Edge
Nautical miles
0
5
10
Hook
echo
WSR-88D Radar Image
The mesocyclone wraps some of the heaviest precipitation
around the updraft creating a “hook echo” on radar. The largest
hail falls in a narrow swath located near the updraft core.
Sounding Investigation
• Identify convective
mode
• Total CAPE
• CAPE in -30 to -10 °C
• Rotation/turning
hodographs
• Height of the
Freezing Level
• Wet Bulb Zero height
No one single parameter has been shown to have
significant skill when considered alone!
Impact of CAPE
• Supercells usually
occur with significant
CAPE (values 10002000 J kg-1 or more)
• Bulk Richardson
Number (ratio of
instability to shear)
can be used to
predict storm type
– Values between 10-50
generally associated
with supercell storms
CAPE Shape
Wet Bulb Temperature
LCL
Edwards and
Thompson (1998)
Thermodynamic Summary
• Generally, you’d like to see:
– Wet Bulb Zero (WBZ) Heights of 2.2 – 2.8 km
• Too high, too much melting
• Too low, low-level air too negatively buoyant
– Freezing Level heights < 4 km
• Need deep cloud layer for hail to grow within
– CAPE values > 2000 J kg-1
• To first order, wmax = (CAPE)0.5
– 850–500, 500–300 hPa lapse rate > 7 K km-1
Don’t forget about “dynamic factors”: Fronts, shortwaves,
outflow boundaries, rotation, etc.
Nowcasting Hail
• Radar imagery can be used to determine
the relative strength of an updraft
(ability to grow large hail) and diagnose
the presence of hail in clouds
–
–
–
–
–
(Bounded) Weak Echo Regions (WERs/BWERs)
“V-notches”
“Hail spikes”
Dual polarization
Vertically Integrated Liquid Water (VIL)
Weak Echo Regions (WERs)
• Strong updrafts will
suspend precipitation
particles aloft creating
an overhang/WER
when observed on
weather radar
• WERs are good
indicators of
potentially severe
storms
Bounded Weak Echo Regions
Bounded WERs
can be seen on
vertical cross
sections or as
“doughnuts” of
weak reflectivity
on horizontal
sections
V-Notch
• Strong supercells
may have a slot of
weak reflectivity
along the
downshear edge
• Why?
Vertically Integrated Liquid
Water Content (VIL)
Computes total water mass in a vertical column
Three Body Scattering
Hail Spikes
Where is the radar in
each case?
Hail Spikes in 3D
Differential Reflectivity ZDR
4 mm
zHH
ZDR [dB] = 10 log(
)
zVV
2.7 mm
3.7 mm
1.8 mm
2.9 mm
1.4 mm
(Pruppacher and Klett, 1997)
– Depends on axis ratio
oblate: ZDR > 0
prolate: ZDR < 0
– For drops: ZDR ~ drop size (0 - 4 dB)
– Hail: ZDR ~ 0 – 1 dB
Dual Polarization - Horizontal
ZDR minima of near zero
co-located with highest Z
High reflectivity core in purple
ZDR
“Hail Hole”: Large Zh and near zero or negative ZDR
Z
Dual Polarization - Vertical
NSSL Cimarron Polarimetric Radar viewpoint of 9 June 1993 Squall line
Adapted from Zrnic and Ryzhkov (1999)
CASE STUDIES
Working in groups, review the individual
cases and answer the following questions:
(1) Is there hail in this storm?
(2) Is the hail reaching the ground?
(3) Would you warn on this cell?