HailstormsEverything you wanted to know

about hail (and more)

ATMO 352Spring 2013

Background: Photorealistic rendering of a WRF simulated hailstorm

Hail ClimatologyHail Climatology

First-order surface observation stations with five-year average number of hail days during 1896-1995, from Changnon and Changnon (2000)

Severe Hail ClimatologySevere Hail Climatology• Hailstorms responsible

for nearly $2.4 billion of damage to crops and property annually in the USA (Changnon 1999)

– Little documented on long-term 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 hailr=5 cm, 50,000 microns

Vivian, SD HailstoneVivian, SD Hailstone

Holds record for size (8” diameter) and mass (1.94 lbs)

In Short, Large Hail Needs…In Short, Large Hail Needs…

(1) (1) Supercooled waterSupercooled water (mass to be (mass to be collected by the hailstone)collected by the hailstone)

(2) (2) An EmbryoAn Embryo (some initial particle to (some initial particle to collect supercooled water, usually a collect supercooled water, usually a frozen raindrop or snow aggregate)frozen raindrop or snow aggregate)

(3) (3) An updraftAn updraft (must be sustained aloft (must be sustained aloft for sufficient time to allow growth)for sufficient time to allow growth)

(4) (4) Cold TemperaturesCold Temperatures

Freezing – Just the factsFreezing – Just the facts• At T < 0°C, water molecules join together to At T < 0°C, water molecules join together to

form a crystalline structureform a crystalline structure

• If the drop is free of impurities, the thermal If the drop is free of impurities, the thermal agitation will prevent freezingagitation will prevent freezing

• Freezing of liquid water in the atmosphere Freezing of liquid water in the atmosphere largely depends upon the presence of foreign largely depends upon the presence of foreign particles called particles called ice nucleiice nuclei

– The nuclei do not have to be inside the drop; water The nuclei do not have to be inside the drop; water can freeze on contact (freezing rain, aircraft icing)can freeze on contact (freezing rain, aircraft icing)

– Typical ice nuclei: desert dust, clay minerals, Typical ice nuclei: desert dust, clay minerals, decaying plant leaf material, ice crystals themselves, decaying plant leaf material, ice crystals themselves, etc.etc.

– Most IN “activate” at temperatures cooler than -10 Most IN “activate” at temperatures cooler than -10

°C°C

Supercooled WaterSupercooled Water• If ice nuclei are If ice nuclei are not not present, liquid water can be present, liquid water can be

super-cooled to temperatures as low as super-cooled to temperatures as low as -40 °C-40 °C

• We normally deal with bulk amounts of water. We normally deal with bulk amounts of water. The presence of one ice nucleus will freeze the The presence of one ice nucleus will freeze the entire amount of liquid water. entire amount of liquid water.

• In a cloud, the liquid water is divided upon In a cloud, the liquid water is divided upon millions of droplets; each must contain or millions of droplets; each must contain or interact with an ice nucleus to freezeinteract with an ice nucleus to freeze

– There are substantially fewer ice nuclei in the There are substantially fewer ice nuclei in the atmosphere than cloud condensation nuclei (CCN)atmosphere than cloud condensation nuclei (CCN)

– Ice multiplication and/or shattering can help generate Ice multiplication and/or shattering can help generate new ice crystalsnew ice crystals

““Warm Rain”Warm Rain”

• You’ll learn more about the warm rain process You’ll learn more about the warm rain process in physical meteorologyin physical meteorology

• Important in tropics, mid-latitude summer, and Important in tropics, mid-latitude summer, and possibly mid-latitude winterpossibly mid-latitude winter

• In short, “warm rain” is a 4 step process:In short, “warm rain” is a 4 step process:

(1)(1)NucleationNucleation of cloud drops on aerosol of cloud drops on aerosol particlesparticles

(2)(2)Additional Additional CondensationCondensation due to due to supersaturationsupersaturation

(3)(3)Drop Growth by Drop Growth by Collision and CoalescenceCollision and Coalescence(4)(4)Drop BreakupDrop Breakup

RH~100%SS=0Drops nucleated

SS>0condensation

Drop D > 45 mcoalescence

Drop breakup

Air cools,RH increases

I

II

III

IV

““Cold Rain” ProcessCold Rain” Process

Further growth by Further growth by accretionaccretion or or aggregationaggregation can also can also occuroccur

Accretion or riming: Accretion or riming: growth by collision with growth by collision with supercooled drops which supercooled drops which freeze on contactfreeze on contact

Aggregation: growth by Aggregation: growth by collision of ice crystalscollision of ice crystals

Vapor diffuses towards the Vapor diffuses towards the crystals, growing by crystals, growing by deposition and depleting deposition and depleting water vapor in the airwater vapor in the air

Cloud Particle Imager Data from AIRS II Flights (2004)

““Hole Punch” CloudsHole Punch” CloudsAreas 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 GraupelConical Graupel (Knight and Knight, (Knight and Knight, 1973)1973)

Preferential collection along crystal edge

Embryos SummaryEmbryos Summary• Growth in cloud may form frozen drops or Growth in cloud may form frozen drops or

conical graupel as initial embryoconical graupel as initial embryo

• Takes 20–30 minutes for either processTakes 20–30 minutes for either process

• Where do they form?Where do they form?

– Feeder cells upwind (10 – 20 km)Feeder cells upwind (10 – 20 km)

– Upwind flanks of main updraftUpwind flanks of main updraft

– Secondary growth from shedding/meltingSecondary growth from shedding/melting

– Can Can NOTNOT be in main updraft: not enough time be in main updraft: not enough time to growto grow

Graupel/Hail GrowthGraupel/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 GrowthFurther 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 Hailstone Thin Sections (Knight and Knight 2005)(Knight and Knight 2005)

DryDrygrowthgrowth

WetWetgrowthgrowth

Growth TrajectoriesGrowth Trajectories““Recycling” Recycling” trajectories not trajectories not as common as as common as once thoughtonce thought

Most trajectories Most trajectories up-and-down up-and-down once around once around main updraftmain updraft

Embryos may be Embryos may be ingested from ingested from other sourcesother sources

Hail in MulticellsHail in Multicells

May produce its May produce its own embryos, own embryos, but hail may not but hail may not grow to very grow to very large sizeslarge sizes

Graupel/Hail Size Graupel/Hail Size DistributionsDistributions

• 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 VelocityHailstone Terminal Velocity• Hailstones are assumed to fall

at their terminal velocity, Vtg

– Balance between drag and gravitational forces

• Larger/more dense particles have greater fall speeds

• Faster graupel/hail fall speeds for lower air density

• 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.)

0.54

3g g

tgD

g DV

C

Knight and Knight (2001)

p = 500 hPap = 500 hPa

Sea levelSea level

Precipitation MassUpdraft ( +2.5 m s-1)Downdraft ( -2.5 m s-1)

Faster falling graupel/hail

Slower falling graupel/hail

Hailswath MechanicsHailswath Mechanics

0 5 10Nautical miles

Light Rain

Heavy Rain & Small Hail

Anvil Edge

Gust Front

N

Hook echo

WSR-88D Radar Image

Hook echo

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.

Large Hail

Sounding InvestigationSounding Investigation• Identify convective Identify convective

modemode• Total CAPETotal CAPE• CAPE in -30 to -10 °CCAPE in -30 to -10 °C• Rotation/turning Rotation/turning

hodographshodographs• Height of the Height of the

Freezing LevelFreezing Level• Wet Bulb Zero heightWet Bulb Zero height

No one single parameter has been shown to have No one single parameter has been shown to have significant skill when considered alone!significant skill when considered alone!

Impact of CAPEImpact of CAPE• Supercells usually Supercells usually

occur with significant occur with significant CAPE (values 1000-CAPE (values 1000-2000 J kg2000 J kg-1 -1 or more)or more)

• Bulk Richardson Bulk Richardson Number (ratio of Number (ratio of instability to shear) instability to shear) can be used to can be used to predict storm type predict storm type

– Values between 10-50 Values between 10-50 generally associated generally associated with supercell stormswith supercell storms

CAPE ShapeCAPE Shape

Wet Bulb TemperatureWet Bulb Temperature

LCLLCL

Edwards and Edwards and Thompson (1998)Thompson (1998)

Thermodynamic SummaryThermodynamic Summary• Generally, you’d like to see:Generally, you’d like to see:

– Wet Bulb Zero (WBZ) Heights of 2.2 – 2.8 Wet Bulb Zero (WBZ) Heights of 2.2 – 2.8 kmkm• Too high, too much meltingToo high, too much melting• Too low, low-level air too negatively buoyantToo low, low-level air too negatively buoyant

– Freezing Level heights < 4 km Freezing Level heights < 4 km • Need deep cloud layer for hail to grow withinNeed deep cloud layer for hail to grow within

– CAPE values > 2000 J kgCAPE values > 2000 J kg-1-1

• To first order, wTo first order, wmaxmax = (CAPE) = (CAPE)0.50.5

– 850–500, 500–300 hPa lapse rate > 7 K km850–500, 500–300 hPa lapse rate > 7 K km--

11Don’t forget about “dynamic factors”: Fronts, shortwaves, outflow boundaries, rotation, etc.

Nowcasting HailNowcasting Hail• Radar imagery can be used to determine Radar imagery can be used to determine

the relative strength of an updraft the relative strength of an updraft (ability to grow large hail) and diagnose (ability to grow large hail) and diagnose the presence of hail in clouds the presence of hail in clouds – (Bounded) Weak Echo Regions (Bounded) Weak Echo Regions

(WERs/BWERs)(WERs/BWERs)– ““V-notches”V-notches”– ““Hail spikes”Hail spikes”– Dual polarizationDual polarization– Vertically Integrated Liquid Water (VIL)Vertically Integrated Liquid Water (VIL)

Weak Echo Regions Weak Echo Regions (WERs)(WERs)

• Strong updrafts will Strong updrafts will suspend precipitation suspend precipitation particles aloft particles aloft creating an creating an overhang/WER when overhang/WER when observed on weather observed on weather radarradar

• WERs are good WERs are good indicators of indicators of potentially severe potentially severe stormsstorms

Bounded Weak Echo Bounded Weak Echo RegionsRegions

Bounded WERs can be seen on vertical cross sections or as “doughnuts” of weak reflectivity on horizontal sections

V-NotchV-Notch

• Strong supercells Strong supercells may have a slot may have a slot of weak of weak reflectivity along reflectivity along the downshear the downshear edge edge

• Why?Why?

Vertically Integrated Liquid Vertically Integrated Liquid Water Content (VIL)Water Content (VIL)

Computes total water mass in a vertical columnComputes total water mass in a vertical column

Three Body ScatteringThree Body Scattering

Hail SpikesHail Spikes

Where is the radar in Where is the radar in each case?each case?

Hail Spikes in 3DHail Spikes in 3D

(Pruppacher and Klett, 1997)(Pruppacher and Klett, 1997)

4 mm 3.7 mm 2.9 mm

2.7 mm 1.8 mm 1.4 mm

Differential Reflectivity ZDifferential Reflectivity ZDRDR

ZDR [dB] = 10 log( )– Depends on axis ratioDepends on axis ratio

oblate: Zoblate: ZDRDR > 0 > 0

prolate: Zprolate: ZDRDR < 0 < 0

– For drops: ZFor drops: ZDRDR ~ drop size (0 - 4 ~ drop size (0 - 4 dB)dB)

– Hail: Hail: ZZDRDR ~ 0 – 1 dB ~ 0 – 1 dB

zHH

zVV

ZZZZDRDR

High reflectivity core in purpleZDR minima of near zero co-located with highest Z

Dual Polarization - Dual Polarization - HorizontalHorizontal

““Hail HoleHail Hole”: Large Z”: Large Zhh and near zero or negative Z and near zero or negative ZDRDR

Adapted from Zrnic and Ryzhkov (1999)

Dual Polarization - VerticalDual Polarization - Vertical

NSSL Cimarron Polarimetric Radar viewpoint of 9 June 1993 Squall line

CASE STUDIESCASE STUDIES

Working in groups, review the individual Working in groups, review the individual cases and answer the following questions:cases and answer the following questions:

(1)(1) Is there hail in this storm?Is there hail in this storm?

(2)(2) Is the hail reaching the ground?Is the hail reaching the ground?

(3)(3) Would you warn on this cell?Would you warn on this cell?