Hail: theory and observations

30 November 2006

Supercell schematic

Why is it important to study hail?

• Relatively rare occurrence• Not well understood

– Consequently difficult to forecast

• Potential for destructive property damage– Cars (personal and dealerships)– Livestock– Structures (windows/roofs)– Agriculture– Airline industry

Hail days per year

Large (> 5 cm diameter) hail days per year

Norman, OK

Houston, TX

Hagelgefährdung in Österreich

Mäßig gefährdete Gebiete sind gelb umrandet (praktisch gesamt Österreich exklusive dem Hochgebirge), stärker bedrohte Regionen rot. Besonders gefährdet sind Teile der SO-Stmk (nicht selten mehrere Hageltage an einem Punkt), was schwarz gekennzeichnet ist.

What is hail?

• “Frozen water which accumulates in a thunderstorm and eventually precipitates out”

• DIFFERENT FROM WINTER PRECIP– Hail is NOT sleet or freezing rain!

• Forms in the convective process of a storm– Can you get hail without a thunderstorm? NO

• Size: oblong to spherical– Largest hail often takes irregular sizes– Can be aggregates of other hail stones (collisions)

How does hail form?

• Reconsider the collision-coalesence process• Tiny ice nuclei (dust/aerosols) meet supercooled

water droplets – a “hail embryo” forms (this occurs at ~ -15°C)

• Most of these ice particles are swept up into the anvil part of the storm

• Embryos on the edge of the main updraft fall back into the supercooled cloud droplets– Collision-coalesence process grows them into graupel

How does hail form?

• Most graupel particles end up melting and falling as raindrops– Some, however, go on to become hailstones

• Small hail: (5mm to 2cm)– graupel particle swept into the updraft and up through

mesocyclone– hailstone falls out due to its weight

• Large hail: (2cm to 10+cm)– graupel particle (likely originating near the “hook”) swept into the

turbulent part of the updraft– spirals up through a region very rich with supercooled water– grows tremendously large in this “utopia” of sorts!– finally becomes heavy enough to fall out

Hail location and growth process

• Largest hail is typically found adjacent to the main updraft– It is heaviest and thus falls out first

• Important note: hailstone makes only ONE pass through the updraft!

• Two growth processes: wet and dry– Wet growth: supercooled water does NOT freeze on contact;

coats the hailstone• Leads to more spherical shapes

– Dry growth: supercooled water freezes on contact; air bubbles trapped

Hail growth in a supercell

Typical hail formation region

Hail detection

• Newest RADAR technology can detect presence of hail– Uses “polarization” of different beams

• Hail reflects differently than raindrops– Raindrops resemble “hamburger buns” (NOT classical

teardrop shape!!)– Hail is more spherical than rain– Thus can detect presence of hail by lack of differential

reflectivities

Hail recap….

• Hail measurements are standardized by objects– Dime/penny/nickel/quarter/half-dollar– Baseball/tennis ball– Softball

• Pictures from hail events– 5 April 2003: Woodson, TX (video from Mon.)– 22 June 2003: largest hail ever recorded

An unfortunate casualty….

How can radar detect hail?

Using conventional (horizontal polarization) radar, hail can sometimes be detected with the presence of a “hail spike”.

Radar beam reaches hail stone, is reflected to the surface, then reflected back to the hail stone, and finally back to the radar site. The computer algorithm interprets this “false signal” as precipitation occurring several tens of km beyond the thunderstorm.

How can radar detect hail?

Examples of “hail spikes”, or “three-body scatter (the hail, the ground, and the hail = three bodies) on radar

• http://blaze.ocs.ou.edu/~dcheresn/Video/HailinWoodson_4_5_03.wmv